Subjects -> ASTRONOMY (Total: 94 journals)
Showing 1 - 46 of 46 Journals sorted alphabetically
Advances in Astronomy     Open Access   (Followers: 51)
Annual Review of Astronomy and Astrophysics     Full-text available via subscription   (Followers: 39)
Annual Review of Earth and Planetary Sciences     Full-text available via subscription   (Followers: 63)
Artificial Satellites     Open Access   (Followers: 23)
Astrobiology     Hybrid Journal   (Followers: 14)
Astronomical & Astrophysical Transactions: The Journal of the Eurasian Astronomical Society     Hybrid Journal   (Followers: 6)
Astronomical Journal     Full-text available via subscription   (Followers: 8)
Astronomical Review     Open Access   (Followers: 4)
Astronomische Nachrichten     Hybrid Journal   (Followers: 4)
Astronomy & Geophysics     Hybrid Journal   (Followers: 48)
Astronomy and Astrophysics     Full-text available via subscription   (Followers: 60)
Astronomy and Astrophysics     Open Access   (Followers: 32)
Astronomy and Computing     Hybrid Journal   (Followers: 2)
Astronomy Letters     Hybrid Journal   (Followers: 22)
Astronomy Reports     Hybrid Journal   (Followers: 15)
Astronomy Studies Development     Open Access   (Followers: 12)
Astroparticle Physics     Hybrid Journal   (Followers: 8)
Astrophysical Bulletin     Hybrid Journal   (Followers: 3)
Astrophysical Journal     Full-text available via subscription   (Followers: 19)
Astrophysical Journal Letters     Full-text available via subscription   (Followers: 14)
Astrophysical Journal Supplement Series     Full-text available via subscription   (Followers: 14)
Astrophysics     Hybrid Journal   (Followers: 29)
Astrophysics and Space Science     Hybrid Journal   (Followers: 46)
Astrophysics and Space Sciences Transactions (ASTRA)     Open Access   (Followers: 56)
Astropolitics: The International Journal of Space Politics & Policy     Hybrid Journal   (Followers: 12)
Celestial Mechanics and Dynamical Astronomy     Hybrid Journal   (Followers: 11)
Chinese Astronomy and Astrophysics     Full-text available via subscription   (Followers: 24)
Colloid Journal     Hybrid Journal   (Followers: 3)
Comptes Rendus Physique     Full-text available via subscription   (Followers: 2)
Computational Astrophysics and Cosmology     Open Access   (Followers: 3)
COSPAR Colloquia Series     Full-text available via subscription   (Followers: 11)
Earth, Moon, and Planets     Hybrid Journal   (Followers: 55)
Earth, Planets and Space     Open Access   (Followers: 74)
EAS Publications Series     Full-text available via subscription   (Followers: 8)
EPL Europhysics Letters     Partially Free   (Followers: 8)
Experimental Astronomy     Hybrid Journal   (Followers: 39)
Expert Opinion on Astronomy and Astrophysics     Open Access   (Followers: 7)
Extreme Life, Biospeology & Astrobiology - International Journal of the Bioflux Society     Full-text available via subscription   (Followers: 6)
Few-Body Systems     Hybrid Journal   (Followers: 1)
Foundations of Physics     Hybrid Journal   (Followers: 41)
Frontiers in Astronomy and Space Sciences     Open Access   (Followers: 12)
Galaxies     Open Access   (Followers: 6)
Globe, The     Full-text available via subscription   (Followers: 4)
Gravitation and Cosmology     Hybrid Journal   (Followers: 4)
Icarus     Hybrid Journal   (Followers: 75)
International Journal of Advanced Astronomy     Open Access   (Followers: 28)
International Journal of Astrobiology     Hybrid Journal   (Followers: 4)
International Journal of Astronomy     Open Access   (Followers: 19)
International Journal of Astronomy and Astrophysics     Open Access   (Followers: 29)
International Journal of Satellite Communications Policy and Management     Hybrid Journal   (Followers: 13)
International Letters of Chemistry, Physics and Astronomy     Open Access   (Followers: 12)
ISRN Astronomy and Astrophysics     Open Access   (Followers: 7)
Journal for the History of Astronomy     Full-text available via subscription   (Followers: 19)
Journal of Astrobiology & Outreach     Open Access   (Followers: 3)
Journal of Astronomical Instrumentation     Open Access   (Followers: 3)
Journal of Astronomical Telescopes, Instruments, and Systems     Hybrid Journal   (Followers: 5)
Journal of Astrophysics     Open Access   (Followers: 26)
Journal of Astrophysics and Astronomy     Open Access   (Followers: 52)
Journal of Atmospheric and Solar-Terrestrial Physics     Hybrid Journal   (Followers: 199)
Journal of Cosmology and Astroparticle Physics     Hybrid Journal   (Followers: 38)
Journal of Geophysical Research : Planets     Full-text available via subscription   (Followers: 179)
Journal of Geophysical Research : Space Physics     Full-text available via subscription   (Followers: 178)
Journal of High Energy Astrophysics     Full-text available via subscription   (Followers: 22)
Kinematics and Physics of Celestial Bodies     Hybrid Journal   (Followers: 10)
KronoScope     Hybrid Journal   (Followers: 1)
Macalester Journal of Physics and Astronomy     Open Access   (Followers: 4)
MNASSA : Monthly Notes of the Astronomical Society of South Africa     Full-text available via subscription   (Followers: 1)
Molecular Astrophysics     Full-text available via subscription   (Followers: 1)
Monthly Notices of the Royal Astronomical Society     Hybrid Journal   (Followers: 14)
Monthly Notices of the Royal Astronomical Society : Letters     Hybrid Journal  
Nature Astronomy     Hybrid Journal   (Followers: 8)
New Astronomy     Hybrid Journal   (Followers: 27)
New Astronomy Reviews     Full-text available via subscription   (Followers: 17)
Nonlinear Dynamics     Hybrid Journal   (Followers: 19)
NRIAG Journal of Astronomy and Geophysics     Open Access   (Followers: 5)
Open Astronomy     Open Access   (Followers: 2)
Physics of the Dark Universe     Open Access   (Followers: 4)
Planetary and Space Science     Hybrid Journal   (Followers: 101)
Planetary Science     Open Access   (Followers: 52)
Proceedings of the International Astronomical Union     Full-text available via subscription   (Followers: 2)
Publications of the Astronomical Society of Australia     Hybrid Journal   (Followers: 2)
Publications of the Astronomical Society of Japan     Hybrid Journal   (Followers: 3)
Publications of the Astronomical Society of the Pacific     Full-text available via subscription   (Followers: 4)
Research & Reviews : Journal of Space Science & Technology     Full-text available via subscription   (Followers: 17)
Research in Astronomy and Astrophysics     Full-text available via subscription   (Followers: 29)
Revista Mexicana de Astronomía y Astrofísica     Open Access   (Followers: 2)
Science China Physics, Mechanics & Astronomy     Hybrid Journal   (Followers: 4)
Solar Physics     Hybrid Journal   (Followers: 34)
Solar System Research     Hybrid Journal   (Followers: 14)
Space Science International     Open Access   (Followers: 192)
Space Science Reviews     Hybrid Journal   (Followers: 97)
Space Weather     Full-text available via subscription   (Followers: 24)
Transport and Aerospace Engineering     Open Access   (Followers: 13)
Universe     Open Access   (Followers: 5)
Similar Journals
Journal Cover
Galaxies
Number of Followers: 6  

  This is an Open Access Journal Open Access journal
ISSN (Online) 2075-4434
Published by MDPI Homepage  [233 journals]
  • Galaxies, Vol. 9, Pages 3: A Broadband Signal Recycling Scheme for
           Approaching the Quantum Limit from Optical Losses

    • Authors: Teng Zhang, Joe Bentley, Haixing Miao
      First page: 3
      Abstract: Quantum noise limits the sensitivity of laser interferometric gravitational-wave detectors. Given the state-of-the-art optics, the optical losses define the lower bound of the best possible quantum-limited detector sensitivity. In this work, we come up with a broadband signal recycling scheme which gives a potential solution to approaching this lower bound by converting the signal recycling cavity to be a broadband signal amplifier using an active optomechanical filter. We will show the difference and advantage of such a scheme compared with the previous white light cavity scheme using the optomechanical filter in [Phys.Rev.Lett.115.211104 (2015)]. The drawback is that the new scheme is more susceptible to the thermal noise of the mechanical oscillator.
      Citation: Galaxies
      PubDate: 2021-01-01
      DOI: 10.3390/galaxies9010003
      Issue No: Vol. 9, No. 1 (2021)
       
  • Galaxies, Vol. 9, Pages 4: Gravitational Measurements in Higher Dimensions

    • Authors: Davood Mahdavian Yekta, Seyed Aliasghar Alavi, Majid Karimabadi
      First page: 4
      Abstract: We attempt to study three significant tests of general relativity in higher dimensions, both in commutative and non-commutative spaces. In the context of non-commutative geometry, we will consider a solution of Einstein’s equation in higher dimensions, with a source given by a static, spherically symmetric Gaussian distribution of mass. The resulting metric would describe a regular or curvature singularity free black hole in higher dimensions. The metric should smoothly interpolate between Schwarzschild geometry at large distance, and de-Sitter spacetime at short distance. We will consider gravitational redshift, lensing, and time delay in each sector. It will be shown that, compared to the four-dimensional spacetime, there can be significant modifications due to the presence of extra dimensions and the non-commutative corrected black holes. Finally, we shall attempt to obtain a lower bound on the size of the extra dimensions and on the mass needed to form a black hole in different dimensions.
      Citation: Galaxies
      PubDate: 2021-01-11
      DOI: 10.3390/galaxies9010004
      Issue No: Vol. 9, No. 1 (2021)
       
  • Galaxies, Vol. 9, Pages 1: A Modified Kwee–Van Woerden Method for
           Eclipse Minimum Timing with Reliable Error Estimates

    • Authors: Hans J. Deeg
      First page: 1
      Abstract: The Kwee–van Woerden (KvW) method used for the determination of eclipse minimum times has been a staple in eclipsing binary research for decades, due its simplicity and the independence of external input parameters, which also makes it well-suited to obtaining timings of exoplanet transits. However, its estimates of the timing error have been known to have a low reliability. During the analysis of very precise photometry of CM Draconis eclipses from TESS space mission data, KvW’s original equation for the timing error estimate produced numerical errors, which evidenced a fundamental problem in this equation. This contribution introduces an improved approach for calculating the timing error with the KvW method. A code that implements this improved method, together with several further updates of the original method, are presented. An example of the application to CM Draconis light curves from TESS is given. The eclipse minimum times are derived with the KvW method’s three original light curve folds, but also with five and seven folds. The use of five or more folds produces minimum timings with a substantially better precision. The improved method of error calculation delivers consistent timing errors which are in excellent agreement with error estimates obtained by other means. In the case of TESS data from CM Draconis, minimum times with an average precision of 1.1 s are obtained. Reliable timing errors are also a valuable indicator for evaluating if a given scatter in an O-C diagram is caused by measurement errors or by a physical period variation.
      Citation: Galaxies
      PubDate: 2020-12-22
      DOI: 10.3390/galaxies9010001
      Issue No: Vol. 9, No. 1 (2020)
       
  • Galaxies, Vol. 9, Pages 2: Messengers of the Universe-Cosmic Rays
           Exploring Supermassive Black Holes

    • Authors: Anna Uryson
      First page: 2
      Abstract: Cosmic rays were discovered over one hundred years ago but there are still unsolved problems. One of the hot problems is the origin of cosmic rays of the highest energies. Sources are still unclear and it is neither clear how particles gain ultra-high energies. Possible sources of cosmic rays at the highest energies are supermassive black holes. From this perspective we discuss in a popular form some recent developments in cosmic ray studies along with author’s recent results. The paper also offers materials for further reading.
      Citation: Galaxies
      PubDate: 2020-12-29
      DOI: 10.3390/galaxies9010002
      Issue No: Vol. 9, No. 1 (2020)
       
  • Galaxies, Vol. 8, Pages 70: Variable Magellanic HMXB Sources versus
           Variable ULX Sources: Nothing to Brag about the ULX Sources

    • Authors: Dimitris M. Christodoulou, Silas G. T. Laycock, Rigel Cappallo, Ankur Roy, Sayantan Bhattacharya , Demosthenes Kazanas
      First page: 70
      Abstract: We carry out a meta-analysis of ultraluminous X-ray (ULX) sources that show large variabilities (by factors of >10) between their highest and lowest emission states in the X-ray energy range of 0.3–10 keV. We are guided by a recent stringent compilation of 25 such X-ray sources by Song et al. We examine the relation of logN versus logSmax, where N is the number of sources radiating above the maximum-flux level Smax. We find a strong deviation from all previously determined slopes in various high-mass X-ray binary (HMXB) samples. In fact, the ULX data clearly show a slope of −0.91. Thus, ULX sources do not appear to be uniform and isotropic in our Universe. We compare the ULX results against the local X-ray luminosity function of HMXBs in the Small Magellanic Cloud (SMC) constructed from our latest library that includes 41 Chandra 0.3–8 keV sources and 56 XMM-Newton 0.2–12 keV sources. The ULX data are not drawn from the same continuous distribution as the SMC data (the ULX data peak at the low tails of the SMC distributions), and none of our data sets is drawn from a normal distribution or from a log-normal distribution (they all show marked excesses at both tails). At a significance level of α=0.05 (2σ), the two-sample p-value of the Kolmogorov–Smirnov (KS) test gives p=4.7×10−3<α for the ULX versus the small Chandra sample and p=1.1×10−5<<α for the ULX versus the larger XMM-Newton sample, respectively. This adds to the evidence that ULX sources are not simply the higher end of the known local Be/X-ray pulsar distribution, but they represent a class of X-ray sources different from the young sources found in the SMC and in individual starburst galaxies. On the other hand, our two main SMC data sets are found to be statistically consistent, as they are drawn from the same continuous parent distribution (null hypothesis H0): at the α=0.05 significance level, the two-sample KS test shows an asymptotic p-value of 0.308>α, which tells us to accept H0.
      Citation: Galaxies
      PubDate: 2020-09-24
      DOI: 10.3390/galaxies8040070
      Issue No: Vol. 8, No. 4 (2020)
       
  • Galaxies, Vol. 8, Pages 71: Unraveling the Physics of Quasar Jets: Optical
           Polarimetry and Implications for the X-ray Emission Process

    • Authors: Eric S. Perlman, Devon Clautice, Sayali Avachat, Mihai Cara, William B. Sparks, Markos Georganopoulos, Eileen Meyer
      First page: 71
      Abstract: Since the launch of Chandra twenty years ago, one of the greatest mysteries surrounding Quasar Jets is the production mechanism for their extremely high X-ray luminosity. Two mechanisms have been proposed. In the first view, the X-ray emission is inverse-Comptonized CMB photons. This view requires a jet that is highly relativistic (bulk Lorentz factor >20–40) on scales of hundreds of kiloparsecs, and a jet that is comparably or more powerful than the black hole’s Eddington luminosity. The second possibility is synchrotron emission from a high-energy population of electrons. This requires a much less powerful jet that does not need to be relativistically beamed, but it imposes other extreme requirements, namely the need to accelerate particles to >100 TeV energies at distances of hundreds of kiloparsecs from the active nucleus. We are exploring these questions using a suite of observations from a diverse group of telescopes, including the Hubble Space Telescope (HST), Chandra X-ray Observatory (CXO), Fermi Gamma-ray Space Telescope and various radio telescope arrays. Our results strongly favor the hypothesis that the X-ray emission is synchrotron radiation from a separate, high-energy electron population. We discuss the observations, results and new questions brought up by these surprising results. We investigate the physical processes and magnetic field structure that may help to accelerate particles to such extreme energies.
      Citation: Galaxies
      PubDate: 2020-09-27
      DOI: 10.3390/galaxies8040071
      Issue No: Vol. 8, No. 4 (2020)
       
  • Galaxies, Vol. 8, Pages 72: Leptonic and Hadronic Radiative Processes in
           Supermassive-Black-Hole Jets

    • Authors: Matteo Cerruti
      First page: 72
      Abstract: Supermassive black holes lying in the center of galaxies can launch relativistic jets of plasma along their polar axis. The physics of black-hole jets is a very active research topic in astrophysics, owing to the fact that many questions remain open on the physical mechanisms of jet launching, of particle acceleration in the jet, and on the radiative processes. In this work I focus on the last item, and present a review of the current understanding of radiative emission processes in supermassive-black-hole jets.
      Citation: Galaxies
      PubDate: 2020-10-01
      DOI: 10.3390/galaxies8040072
      Issue No: Vol. 8, No. 4 (2020)
       
  • Galaxies, Vol. 8, Pages 73: Equiaffine Braneworld

    • Authors: Fan Zhang
      First page: 73
      Abstract: Higher dimensional theories, wherein our four dimensional universe is immersed into a bulk ambient, have received much attention recently, and the directions of investigation had, as far as we can discern, all followed the ordinary Euclidean hypersurface theory’s isometric immersion recipe, with the spacetime metric being induced by an ambient parent. We note, in this paper, that the indefinite signature of the Lorentzian metric perhaps hints at the lesser known equiaffine hypersurface theory as being a possibly more natural, i.e., less customized beyond minimal mathematical formalism, description of our universe’s extrinsic geometry. In this alternative, the ambient is deprived of a metric, and the spacetime metric becomes conformal to the second fundamental form of the ordinary theory, therefore is automatically indefinite for hyperbolic shapes. Herein, we advocate investigations in this direction by identifying some potential physical benefits to enlisting the help of equiaffine differential geometry. In particular, we show that a geometric origin for dark energy can be proposed within this framework.
      Citation: Galaxies
      PubDate: 2020-10-21
      DOI: 10.3390/galaxies8040073
      Issue No: Vol. 8, No. 4 (2020)
       
  • Galaxies, Vol. 8, Pages 74: Imprint of Pressure on Characteristic Dark
           Matter Profiles: The Case of ESO0140040

    • Authors: Kuantay Boshkayev, Talgar Konysbayev, Ergali Kurmanov, Orlando Luongo, Marco Muccino
      First page: 74
      Abstract: We investigate the dark matter distribution in the spiral galaxy ESO0140040, employing the most widely used density profiles: the pseudo-isothermal, exponential sphere, Burkert, Navarro-Frenk-White, Moore and Einasto profiles. We infer the model parameters and estimate the total dark matter content from the rotation curve data. For simplicity, we assume that dark matter distribution is spherically symmetric without accounting for the complex structure of the galaxy. Our predictions are compared with previous results and the fitted parameters are statistically confronted for each profile. We thus show that although one does not include the galaxy structure it is possible to account for the same dynamics assuming that dark matter provides a non-zero pressure in the Newtonian approximation. In this respect, we solve the hydrostatic equilibrium equation and construct the dark matter pressure as a function for each profile. Consequently, we discuss the dark matter equation of state and calculate the speed of sound in dark matter. Furthermore, we interpret our results in view of our approach and we discuss the role of the refractive index as an observational signature to discriminate between our approach and the standard one.
      Citation: Galaxies
      PubDate: 2020-10-22
      DOI: 10.3390/galaxies8040074
      Issue No: Vol. 8, No. 4 (2020)
       
  • Galaxies, Vol. 8, Pages 75: Asteroseismic Analysis of δ Scuti Components
           of Binary Systems: The Case of KIC 8504570

    • Authors: Alexios Liakos, Panagiotis Niarchos
      First page: 75
      Abstract: The present work concerns the Asteroseismology of the Kepler-detached eclipsing binary KIC 8504570. Particularly, it focuses on the pulsational behaviour of the oscillating component of this system and the estimation of its physical parameters in order to enrich the so far poor sample of systems of this kind. Using spectroscopic observations, the spectral type of the primary component was determined and used to create accurate light curve models and estimate its absolute parameters. The light curve residuals were subsequently analysed using Fourier transformation techniques to obtain the pulsation models. Theoretical models of δ Scuti stars were employed to identify the oscillation modes of the six detected independent frequencies of the pulsator. In addition, more than 385 combination frequencies were also detected. The absolute and the pulsational properties of the δ Scuti star of this system are discussed and compared with all the currently known similar cases. Moreover, using a recent(empirical) luminosity–pulsation period relationship for δ Scuti stars, the distance of the system was estimated.
      Citation: Galaxies
      PubDate: 2020-10-28
      DOI: 10.3390/galaxies8040075
      Issue No: Vol. 8, No. 4 (2020)
       
  • Galaxies, Vol. 8, Pages 76: Magnetized Particle Motion in γ-Spacetime
           in a Magnetic Field

    • Authors: Ahmadjon Abdujabbarov, Javlon Rayimbaev, Farruh Atamurotov, Bobomurat Ahmedov
      First page: 76
      Abstract: In the present work we explored the dynamics of magnetized particles around the compact object in γ-spacetime in the presence of an external asymptotically-uniform magnetic field. The analysis of the circular orbits of magnetized particles around the compact object in the spacetime of a γ-object immersed in the external magnetic field has shown that the area of stable circular orbits of magnetized particles increases with the increase of γ-parameter. We have also investigated the acceleration of the magnetized particles near the γ-object and shown that the center-of-mass energy of colliding magnetized particles increases with the increase of γ-parameter. Finally, we have applied the obtained results to the astrophysical scenario and shown that the values of γ-parameter in the range of γ∈(0.5,1) can mimic the spin of Kerr black hole up to a≃0.85, while the magnetic interaction can mimic the γ-parameter at γ∈(0.8,1) and spin of a Kerr black hole up to a≃0.3.
      Citation: Galaxies
      PubDate: 2020-10-29
      DOI: 10.3390/galaxies8040076
      Issue No: Vol. 8, No. 4 (2020)
       
  • Galaxies, Vol. 8, Pages 77: Thermodynamic Constraints on the Non-Baryonic
           Dark Matter Gas Composing Galactic Halos

    • Authors: Anne M. Hofmeister
      First page: 77
      Abstract: To explain rotation curves of spiral galaxies through Newtonian orbital models, massive halos of non-baryonic dark matter (NBDM) are commonly invoked. The postulated properties are that NBDM interacts gravitationally with baryonic matter, yet negligibly interacts with photons. Since halos are large, low-density gaseous bodies, their postulated attributes can be tested against classical thermodynamics and the kinetic theory of gas. Macroscopic models are appropriate because these make few assumptions. NBDM–NBDM collisions must be elastic to avoid the generation of light, but this does not permit halo gas temperature to evolve. If no such collisions exist, then the impossible limit of absolute zero would be attainable since the other available energy source, radiation, does not provide energy to NBDM. The alternative possibility, an undefined temperature, is also inconsistent with basic thermodynamic principles. However, a definable temperature could be attained via collisions with baryons in the intergalactic medium since these deliver kinetic energy to NBDM. In this case, light would be produced since some proportion of baryon collisions are inelastic, thereby rendering the halo detectable. Collisions with baryons are unavoidable, even if NBDM particles are essentially point masses. Note that <0.0001 × the size of a proton is needed to avoid scattering with γ-rays, the shortest wavelength used to study halos. If only elastic collisions exist, NBDM gas would collapse to a tiny, dense volume (zero volume for point masses) during a disturbance—e.g., cosmic rays. NBDM gas should occupy central galactic regions, not halos, since self-gravitating objects are density stratified. In summary, properties of NBDM halos as postulated would result in violations of thermodynamic laws and in a universe unlike that observed.
      Citation: Galaxies
      PubDate: 2020-11-08
      DOI: 10.3390/galaxies8040077
      Issue No: Vol. 8, No. 4 (2020)
       
  • Galaxies, Vol. 8, Pages 78: Study of Eclipsing Binaries: Light Curves
           & O-C Diagrams Interpretation

    • Authors: Helen Rovithis-Livaniou
      First page: 78
      Abstract: The continuous improvement in observational methods of eclipsing binaries, EBs, yield more accurate data, while the development of their light curves, that is magnitude versus time, analysis yield more precise results. Even so, and in spite the large number of EBs and the huge amount of observational data obtained mainly by space missions, the ways of getting the appropriate information for their physical parameters etc. is either from their light curves and/or from their period variations via the study of their (O-C) diagrams. The latter express the differences between the observed, O, and the calculated, C, times of minimum light. Thus, old and new light curves analysis methods of EBs to obtain their principal parameters will be considered, with examples mainly from our own observational material, and their subsequent light curves analysis using either old or new methods. Similarly, the orbital period changes of EBs via their (O-C) diagrams are referred to with emphasis on the use of continuous methods for their treatment in absence of sudden or abrupt events. Finally, a general discussion is given concerning these two topics as well as to a few related subjects.
      Citation: Galaxies
      PubDate: 2020-11-13
      DOI: 10.3390/galaxies8040078
      Issue No: Vol. 8, No. 4 (2020)
       
  • Galaxies, Vol. 8, Pages 79: The Squeezed Light Source for the Advanced
           Virgo Detector in the Observation Run O3

    • Authors: Moritz Mehmet, Henning Vahlbruch
      First page: 79
      Abstract: From 1 April 2019 to 27 March 2020, the Advanced Virgo detector, together with the two Advanced LIGO detectors, conducted the third joint scientific observation run O3, aiming for further detections of gravitational wave signals from astrophysical sources. One of the upgrades to the Virgo detector for O3 was the implementation of the squeezed light technology to improve the detector sensitivity beyond its classical quantum shot noise limit. In this paper, we present a detailed description of the optical setup and performance of the employed squeezed light source. The squeezer was constructed as an independent, stand-alone sub-system operated in air. The generated squeezed states are tailored to exhibit high purity at intermediate squeezing levels in order to significantly reduce the interferometer shot noise level while keeping the correlated enhancement of quantum radiation pressure noise just below the actual remaining technical noise in the Advanced Virgo detector.
      Citation: Galaxies
      PubDate: 2020-11-26
      DOI: 10.3390/galaxies8040079
      Issue No: Vol. 8, No. 4 (2020)
       
  • Galaxies, Vol. 8, Pages 80: Temperature Control for an Intra-Mirror Etalon
           in Interferometric Gravitational Wave Detector Fabry–Perot Cavities

    • Authors: Jonathan Brooks, Maddalena Mantovani, Annalisa Allocca, Julia Casanueva Diaz, Vincenzo Dattilo, Alain Masserot, Paolo Ruggi
      First page: 80
      Abstract: The sensitivity of interferometric gravitational wave detectors is optimized, in part, by balanced finesse in the long Fabry–Perot arm cavities. The input test mass mirrors of Advanced Virgo feature parallel faces, which creates an etalon within the substrate, adding variability in the total mirror reflectivity, in order to correct imbalanced finesse due to manufacturing tolerances. Temperature variations in mirror substrate change the optical path length primarily through varying the index of refraction and are tuned to correct for a finesse imbalance of up to 2.8% by a full etalon fringe of 0.257 K. A negative feedback control system was designed to control the mirror temperature by using an electrical resistive heating belt actuator for a heat transfer process modeled as a two-pole plant. A zero controller filter was designed which achieves temperature control within 2.3% of the etalon fringe and recovers to within 10% of the working point within 32 hours after a step input of one etalon fringe. A preliminary unlock condition control designed to compensate when the interferometer unlocks shows that the control remains stable even after a drastic change in the plant due to the absence of the laser heating. Further improvements to the control must also consider the full heat transfer mechanisms by using modern control state space models.
      Citation: Galaxies
      PubDate: 2020-12-01
      DOI: 10.3390/galaxies8040080
      Issue No: Vol. 8, No. 4 (2020)
       
  • Galaxies, Vol. 8, Pages 81: Modulated Differential Wavefront Sensing:
           Alignment Scheme for Beams with Large Higher Order Mode Content

    • Authors: A. Bisht, M. Prijatelj, J. Leong, E. Schreiber, C. Affeldt, M. Brinkmann, S. Doravari, H. Grote, V. Kringel, J. Lough, H. Lueck, K. Strain, M. Weinert, H. Wittel, K. Danzmann
      First page: 81
      Abstract: Modulated differential wavefront sensing (MDWS) is an alignment control scheme in the regime of beams with strong higher order transversal modes (HOMs). It is based on the differential wavefront sensing (DWS) technique. MDWS represents a significant upgrade over conventional techniques used in the presence of high HOM content as it allows for higher control bandwidths while eliminating the need of auxiliary alignment modulations, that otherwise cause loss of applied squeezing. The output port of gravitational wave (GW) interferometers (IFO) is one such place where a lot of HOMs are present. These are filtered out by a cavity called the output mode cleaner (OMC), whose alignment gets challenging due to the presence of HOMs. In this paper, we present the first demonstration of the MDWS scheme for aligning the fundamental mode from the IFO to the OMC at the gravitational wave detector-GEO 600.
      Citation: Galaxies
      PubDate: 2020-12-03
      DOI: 10.3390/galaxies8040081
      Issue No: Vol. 8, No. 4 (2020)
       
  • Galaxies, Vol. 8, Pages 82: The Hunt for Environmental Noise in Virgo
           during the Third Observing Run

    • Authors: Irene Fiori, Federico Paoletti, Maria Concetta Tringali, Kamiel Janssens, Christos Karathanasis, Alexis Menéndez-Vázquez, Alba Romero-Rodríguez, Ryosuke Sugimoto, Tatsuki Washimi, Valerio Boschi, Antonino Chiummo, Marek Cieślar, Rosario De Rosa, Camilla De Rossi, Francesco Di Renzo, Ilaria Nardecchia, Antonio Pasqualetti, Barbara Patricelli, Paolo Ruggi, Neha Singh
      First page: 82
      Abstract: The first twenty years of operation of gravitational-wave interferometers have shown that these detectors are affected by physical disturbances from the surrounding environment. These are seismic, acoustic, or electromagnetic disturbances that are mainly produced by the experiment infrastructure itself. Ambient noise can limit the interferometer sensitivity or potentially generate transients of non-astrophysical origin. Between 1 April 2019 and 27 March 2020, the network of second generation interferometers—LIGO, Virgo and GEO—performed the third joined observing run, named O3, searching for gravitational signals from the deep universe. A thorough investigation has been done on each detector before and during data taking in order to optimize its sensitivity and duty cycle. In this paper, we first revisit typical sources of environmental noise and their coupling paths, and we then describe investigation methods and tools. Finally, we illustrate applications of these methods in the hunt for environmental noise at the Virgo interferometer during the O3 run and its preparation phase. In particular, we highlight investigation techniques that might be useful for the next observing runs and the future generation of terrestrial interferometers.
      Citation: Galaxies
      PubDate: 2020-12-07
      DOI: 10.3390/galaxies8040082
      Issue No: Vol. 8, No. 4 (2020)
       
  • Galaxies, Vol. 8, Pages 83: Anomalous Sun Flyby of 1I/2017 U1 (`Oumuamua)

    • Authors: Klaus Wilhelm, Bhola Dwivedi
      First page: 83
      Abstract: The findings of Micheli et al. (Nature2018, 559, 223–226) that 1I/2017 U1 (`Oumuamua) showed anomalous orbital accelerations have motivated us to apply an impact model of gravity in search for an explanation. A small deviation from the 1/r potential, where r is the heliocentric distance, is expected for the gravitational interaction of extended bodies as a consequence of this model. This modification of the potential results from an offset of the effective gravitational centre from the geometric centre of a spherically symmetric body. Applied to anomalous Earth flybys, the model accounts for energy gains relative to an exact Kepler orbit and an increased speed of several spacecraft. In addition, the flat rotation profiles of eight disk galaxies could be explained, as well as the anomalous perihelion advances of the inner planets and the asteroid Icarus. The solution in the case of `Oumuamua is also based on the proposal that the offset leads to an approach and flyby trajectory different from a Kepler orbit without postulating cometary activity. As a consequence, an adjustment of the potential and centrifugal orbital energies can be envisaged outside the narrow uncertainty ranges of the published post-perihelion data without a need to re-analyse the original data. The observed anomalous acceleration has been modelled with respect to the orbit solutions JPL 16 and “Pseudo-MPEC” for 1I/`Oumuamua.
      Citation: Galaxies
      PubDate: 2020-12-07
      DOI: 10.3390/galaxies8040083
      Issue No: Vol. 8, No. 4 (2020)
       
  • Galaxies, Vol. 8, Pages 84: Advanced LIGO Laser Systems for O3 and Future
           Observation Runs

    • Authors: Nina Bode, Joseph Briggs, Xu Chen, Maik Frede, Peter Fritschel, Michael Fyffe, Eric Gustafson, Matthew Heintze, Peter King, Jian Liu, Jason Oberling, Richard L. Savage, Andrew Spencer, Benno Willke
      First page: 84
      Abstract: The advanced LIGO gravitational wave detectors need high power laser sources with excellent beam quality and low-noise behavior. We present a pre-stabilized laser system with 70 W of output power that was used in the third observing run of the advanced LIGO detectors. Furthermore, the prototype of a 140 W pre-stabilized laser system for future use in the LIGO observatories is described and characterized.
      Citation: Galaxies
      PubDate: 2020-12-08
      DOI: 10.3390/galaxies8040084
      Issue No: Vol. 8, No. 4 (2020)
       
  • Galaxies, Vol. 8, Pages 85: Interferometer Sensing and Control for the
           Advanced Virgo Experiment in the O3 Scientific Run

    • Authors: Annalisa Allocca, Diego Bersanetti, Julia Casanueva Diaz, Camilla De Rossi, Maddalena Mantovani, Alain Masserot, Loïc Rolland, Paolo Ruggi, Bas Swinkels, Enzo Nicolas Tapia San Martin, Marco Vardaro, Michal Was
      First page: 85
      Abstract: Advanced Virgo is a 2nd-generation laser interferometer based in Cascina (Italy) aimed at the detection of gravitational waves (GW) from astrophysical sources. Together with the two USA-based LIGO interferometers they constitute a network which operates in coincidence. The three detectors observed the sky simultaneously during the last part of the second Observing Run (O2) in August 2017, and this led to two paramount discoveries: the first three-detector observation of gravitational waves emitted from the coalescence of a binary black hole system (GW170814), and the first detection ever of gravitational waves emitted from the coalescence of a binary neutron star system (GW170817). Coincident data taking was re-started for the third Observing Run (O3), which started on 1st April 2019 and lasted almost one year. This paper will describe the new techniques implemented for the longitudinal controls with respect to the ones already in use during O2. Then, it will present an extensive description of the full scheme of the angular controls of the interferometer, focusing on the different control strategies that are in place in the different stages of the lock acquisition procedure, which is the complex sequence of operations by which an uncontrolled, “free” laser interferometer is brought to the final working point, which allows the detector to reach the best sensitivity.
      Citation: Galaxies
      PubDate: 2020-12-08
      DOI: 10.3390/galaxies8040085
      Issue No: Vol. 8, No. 4 (2020)
       
  • Galaxies, Vol. 8, Pages 86: Fine-Tuning the Optical Design of the Advanced
           Virgo+ Gravitational-Wave Detector Using Binary-Neutron Star Signals

    • Authors: Jonathon Baird, Matteo Barsuglia
      First page: 86
      Abstract: Advanced Virgo+ is a major upgrade of the Advanced Virgo gravitational-wave detector aiming to increase sensitivity in terms of binary neutron star (BNS) range by a factor 3–5 in the next few years. In this work, we present an optimization of the mirror transmittances for the second phase of the project (to be implemented for the O5 observation run) using a random walk algorithm implemented with the advGWINC software. In addition to BNS range, a post merger (PM) SNR is also used as a figure of merit to identify configurations that fine-tune the sensitivity curve, as a function of arm-cavity round trip losses.
      Citation: Galaxies
      PubDate: 2020-12-08
      DOI: 10.3390/galaxies8040086
      Issue No: Vol. 8, No. 4 (2020)
       
  • Galaxies, Vol. 8, Pages 87: Development of a Frequency Tunable Green Laser
           Source for Advanced Virgo+ Gravitational Waves Detector

    • Authors: Camilla De De Rossi, Jonathan Brooks, Julia Casanueva Casanueva Diaz, Antonino Chiummo, Eric Genin, Matthieu Gosselin, Nicolas Leroy, Maddalena Mantovani, Beatrice Montanari, Flavio Nocera, Gabriel Pillant
      First page: 87
      Abstract: After one year of data taking, the observing run three (O3), we are preparing for an improved version of the system, named the Advanced Virgo plus. One of the major upgrades will be the installation of the signal recycling mirror to form an additional optical cavity and improve the sensitivity of the interferometer. This also requires a change in the lock acquisition strategy. In particular, the arms will be locked at the beginning with lasers at a different wavelength from the main one. Such a strategy has already been implemented and tested in LIGO and KAGRA, and in this paper we will present how it has been conceived in Virgo.
      Citation: Galaxies
      PubDate: 2020-12-09
      DOI: 10.3390/galaxies8040087
      Issue No: Vol. 8, No. 4 (2020)
       
  • Galaxies, Vol. 8, Pages 88: Classification of Planetary Nebulae through
           Deep Transfer Learning

    • Authors: Dayang N. F. Awang Iskandar, Albert A. Zijlstra, Iain McDonald, Rosni Abdullah, Gary A. Fuller, Ahmad H. Fauzi, Johari Abdullah
      First page: 88
      Abstract: This study investigate the effectiveness of using Deep Learning (DL) for the classification of planetary nebulae (PNe). It focusses on distinguishing PNe from other types of objects, as well as their morphological classification. We adopted the deep transfer learning approach using three ImageNet pre-trained algorithms. This study was conducted using images from the Hong Kong/Australian Astronomical Observatory/Strasbourg Observatory H-alpha Planetary Nebula research platform database (HASH DB) and the Panoramic Survey Telescope and Rapid Response System (Pan-STARRS). We found that the algorithm has high success in distinguishing True PNe from other types of objects even without any parameter tuning. The Matthews correlation coefficient is 0.9. Our analysis shows that DenseNet201 is the most effective DL algorithm. For the morphological classification, we found for three classes, Bipolar, Elliptical and Round, half of objects are correctly classified. Further improvement may require more data and/or training. We discuss the trade-offs and potential avenues for future work and conclude that deep transfer learning can be utilized to classify wide-field astronomical images.
      Citation: Galaxies
      PubDate: 2020-12-11
      DOI: 10.3390/galaxies8040088
      Issue No: Vol. 8, No. 4 (2020)
       
  • Galaxies, Vol. 8, Pages 52: Rotational Disruption of Astrophysical Dust
           and Ice—Theory and Applications

    • Authors: Thiem Hoang
      First page: 52
      Abstract: Dust is an essential component of the interstellar medium (ISM) and plays an important role in many different astrophysical processes and phenomena. Traditionally, dust grains are known to be destroyed by thermal sublimation, Coulomb explosions, sputtering, and shattering. The first two mechanisms arise from the interaction of dust with intense radiation fields and high-energy photons (extreme UV), which work in a limited astrophysical environment. The present review is focused on a new destruction mechanism present in the dust-radiation interaction that is effective in a wide range of radiation fields and has ubiquitous applications in astrophysics. We first describe this new mechanism of grain destruction, namely rotational disruption induced by Radiative Torques (RATs) or RAdiative Torque Disruption (RATD). We then discuss rotational disruption of nanoparticles by mechanical torques due to supersonic motion of grains relative to the ambient gas, which is termed MEchanical Torque Disruption (METD). These two new mechanisms modify properties of dust and ice (e.g., size distribution and mass), which affects observational properties, including dust extinction, thermal and nonthermal emission, and polarization. We present various applications of the RATD and METD mechanisms for different environments, including the ISM, star-forming regions, astrophysical transients, and surface astrochemistry.
      Citation: Galaxies
      PubDate: 2020-07-06
      DOI: 10.3390/galaxies8030052
      Issue No: Vol. 8, No. 3 (2020)
       
  • Galaxies, Vol. 8, Pages 53: Magnetism Science with the Square Kilometre
           Array

    • Authors: George Heald, Sui Mao, Valentina Vacca, Takuya Akahori, Ancor Damas-Segovia, Bryan Gaensler, Matthias Hoeft, Ivan Agudo, Aritra Basu, Rainer Beck, Mark Birkinshaw, Annalisa Bonafede, Tyler Bourke, Andrea Bracco, Ettore Carretti, Luigina Feretti, J. Girart, Federica Govoni, James Green, JinLin Han, Marijke Haverkorn, Cathy Horellou, Melanie Johnston-Hollitt, Roland Kothes, Tom Landecker, Błażej Nikiel-Wroczyński, Shane O'Sullivan, Marco Padovani, Frédérick Poidevin, Luke Pratley, Marco Regis, Christopher Riseley, Tim Robishaw, Lawrence Rudnick, Charlotte Sobey, Jeroen Stil, Xiaohui Sun, Sharanya Sur, A. Taylor, Alec Thomson, Cameron Van Eck, Franco Vazza, Jennifer West, the SKA Magnetism Science Working
      First page: 53
      Abstract: The Square Kilometre Array (SKA) will answer fundamental questions about the origin, evolution, properties, and influence of magnetic fields throughout the Universe. Magnetic fields can illuminate and influence phenomena as diverse as star formation, galactic dynamics, fast radio bursts, active galactic nuclei, large-scale structure, and dark matter annihilation. Preparations for the SKA are swiftly continuing worldwide, and the community is making tremendous observational progress in the field of cosmic magnetism using data from a powerful international suite of SKA pathfinder and precursor telescopes. In this contribution, we revisit community plans for magnetism research using the SKA, in light of these recent rapid developments. We focus in particular on the impact that new radio telescope instrumentation is generating, thus advancing our understanding of key SKA magnetism science areas, as well as the new techniques that are required for processing and interpreting the data. We discuss these recent developments in the context of the ultimate scientific goals for the SKA era.
      Citation: Galaxies
      PubDate: 2020-07-06
      DOI: 10.3390/galaxies8030053
      Issue No: Vol. 8, No. 3 (2020)
       
  • Galaxies, Vol. 8, Pages 54: Debate on the Physics of Galactic Rotation and
           the Existence of Dark Matter

    • Authors: Anne M. Hofmeister, Robert E. Criss
      First page: 54
      Abstract: This Special Issue was motivated by the disparate explanations of galactic dynamics promulgated by different philosophical camps [...]
      Citation: Galaxies
      PubDate: 2020-07-15
      DOI: 10.3390/galaxies8030054
      Issue No: Vol. 8, No. 3 (2020)
       
  • Galaxies, Vol. 8, Pages 55: NuSTAR View of TeV Blazar Mrk 501

    • Authors: Ashwani Pandey
      First page: 55
      Abstract: We report the results of flux and spectral variability studies of all seven Nuclear Spectroscopic Telescope Array (NuSTAR) observations of TeV γ − ray emitting blazar Markarian (or Mrk) 501. We found strong evidence of intraday variability in 3–79 keV X-ray light curves (LCs) of Mrk 501 during four out of these seven observations. We examined spectral variability using a model-independent hardness-ratio analysis and found a general “harder-when-brighter” behaviour in two observations. We also investigated the nature of 3–79 keV X-ray spectra of TeV blazar Mrk 501 and found that five out of seven spectra are well described by the curved log-parabola models with photon indices (at 10 keV) α ∼ 2.12–2.32 and a curvature β ∼ 0.15–0.28. The two other spectra are somewhat better represented by simple power-law models with photon indices 2.70 and 2.75. We briefly discuss available physical models to explain our results.
      Citation: Galaxies
      PubDate: 2020-07-25
      DOI: 10.3390/galaxies8030055
      Issue No: Vol. 8, No. 3 (2020)
       
  • Galaxies, Vol. 8, Pages 56: Parameters of the Supernova-Driven
           Interstellar Turbulence

    • Authors: Luke Chamandy, Anvar Shukurov
      First page: 56
      Abstract: Galactic dynamo models take as input certain parameters of the interstellar turbulence, most essentially the correlation time τ, root-mean-square turbulent speed u, and correlation scale l. However, these quantities are difficult, or, in the case of τ, impossible, to directly observe, and theorists have mostly relied on order of magnitude estimates. Here we present an analytic model to derive these quantities in terms of a small set of more accessible parameters. in our model, turbulence is assumed to be driven concurrently by isolated supernovae (SNe) and superbubbles (SBs), but clustering of SNe to form SBs can be turned off if desired, which reduces the number of model parameters by about half. In general, we find that isolated SNe and SBs can inject comparable amounts of turbulent energy into the interstellar medium, but SBs do so less efficiently. this results in rather low overall conversion rates of SN energy into turbulent energy of ∼1–3%. the results obtained for l, u and τ for model parameter values representative of the Solar neighbourhood are consistent with those determined from direct numerical simulations. Our analytic model can be combined with existing dynamo models to predict more directly the magnetic field properties for nearby galaxies or for statistical populations of galaxies in cosmological models.
      Citation: Galaxies
      PubDate: 2020-07-31
      DOI: 10.3390/galaxies8030056
      Issue No: Vol. 8, No. 3 (2020)
       
  • Galaxies, Vol. 8, Pages 57: Close Binary Perspectives

    • Authors: R.E. Wilson
      First page: 57
      Abstract: Development of analytic binary star models is discussed in historical and on-going perspective, beginning with an overview of paradigm shifts, the merits of direct (rectification-free) models, and fundamental four-type binary system morphology. Attention is called to the likelihood that many or even most cataclysmic variables may be of the double contact morphological type. Eclipsing binary distance estimates differ from those of standard candles in being individually measurable—without reliance on (usually nearby) objects that are assumed similar. Recent progress on circumstellar accretion disk models is briefly summarized, with emphasis on the separate roles of fluid dynamic, structural, and analytic models. Time-related parameters (ephemeris, apsidal motion, and light travel time) now can be found with a unified algorithm that processes light curves, velocity curves, and pre-existing eclipse timings together, without need to compute any new timings. Changes in data publication practices are recommended and logical errors and inconsistencies in terminology are noted. Parameter estimation strategies are discussed.
      Citation: Galaxies
      PubDate: 2020-08-03
      DOI: 10.3390/galaxies8030057
      Issue No: Vol. 8, No. 3 (2020)
       
  • Galaxies, Vol. 8, Pages 58: Temporal and Spectral Variability of OJ 287
           before the April–June 2020 Outburst

    • Authors: Nibedita Kalita, Alok C. Gupta, Minfeng Gu
      First page: 58
      Abstract: We present the results of a temporal and spectral study of the BL Lacertae object OJ 287 in optical, UV, and X-ray bands with observations performed by Swift satellite during September 2019–March 2020. In this period, the source showed moderate variability characterized by variability amplitude of ∼22–31% in all the wavelengths on a short timescale, except the hard X-ray band which was variable by only ∼8%. We observed that the X-ray flux of the source was significantly dominated by the soft photons below 2 keV. Soft lags of ∼45 days were detected between the optical/UV and soft X-ray emissions, while there is no correlation between the hard X-rays and the lower energy bands indicating the presence of two emission components or electron populations. Although two components contribute to the X-ray emission, most of the 0.3–10 keV spectra were well fitted with an absorbed power-law model which outlines the dominance of synchrotron over inverse Compton (IC) mechanism. The X-ray spectra follow a weak “softer when brighter” trend.
      Citation: Galaxies
      PubDate: 2020-08-06
      DOI: 10.3390/galaxies8030058
      Issue No: Vol. 8, No. 3 (2020)
       
  • Galaxies, Vol. 8, Pages 59: X-ray Flux and Spectral Variability of Blazar
           H 2356-309

    • Authors: Kiran A. Wani, Haritma Gaur
      First page: 59
      Abstract: We present the results of timing and spectral analysis of the blazar H 2356-309 using XMM-Newton observations. This blazar is observed during 13 June 2005–24 December 2013 in total nine observations. Five of the observations show moderate flux variability with amplitude 1.7–2.2%. We search for the intra-day variability timescales in these five light curves, but did not find in any of them. The fractional variability amplitude is generally lower in the soft bands than in the hard bands, which is attributed to the energy dependent synchrotron emission. Using the hardness ratio analysis, we search for the X-ray spectral variability along with flux variability in this source. However, we did not find any significant spectral variability on intra-day timescales. We also investigate the X-ray spectral curvature of blazar H 2356-309 and found that six of our observations are well described by the log parabolic model with α = 1.99–2.15 and β = 0.03–0.18. Three of our observations are well described by power law model. The break energy of the X-ray spectra varies between 1.97–2.31 keV. We investigate the correlation between various parameters that are derived from log parabolic model and their implications are discussed.
      Citation: Galaxies
      PubDate: 2020-08-10
      DOI: 10.3390/galaxies8030059
      Issue No: Vol. 8, No. 3 (2020)
       
  • Galaxies, Vol. 8, Pages 60: UV Spectroscopy of Massive Stars

    • Authors: D. John Hillier
      First page: 60
      Abstract: We present a review of UV observations of massive stars and their analysis. We discuss O stars, luminous blue variables, and Wolf–Rayet stars. Because of their effective temperature, the UV (912−3200 Å) provides invaluable diagnostics not available at other wavebands. Enormous progress has been made in interpreting and analysing UV data, but much work remains. To facilitate the review, we provide a brief discussion on the structure of stellar winds, and on the different techniques used to model and interpret UV spectra. We discuss several important results that have arisen from UV studies including weak-wind stars and the importance of clumping and porosity. We also discuss errors in determining wind terminal velocities and mass-loss rates.
      Citation: Galaxies
      PubDate: 2020-08-12
      DOI: 10.3390/galaxies8030060
      Issue No: Vol. 8, No. 3 (2020)
       
  • Galaxies, Vol. 8, Pages 61: Observing Planetary and Pre-Planetary Nebulae
           with the James Webb Space Telescope

    • Authors: Raghvendra Sahai
      First page: 61
      Abstract: Most stars in the Universe that leave the main sequence in a Hubble time will end their lives evolving through the Planetary Nebula (PN) evolutionary phase. The heavy mass loss which occurs during the preceding Asymptotic Giant Branch (AGB) phase is important across astrophysics, dramatically changing the course of stellar evolution, contributing to the dust content of the interstellar medium, and influencing its chemical composition. The evolution from the AGB phase to the PN phases remains poorly understood, especially the dramatic transformation that occurs in the morphology of the mass-ejecta as AGB stars enter the post-AGB phase and their round circumstellar envelopes evolve into pre-PNe (PPNe) and then to PNe. The majority of PPNe and PNe deviate strongly from spherical symmetry. Strong binary interactions most likely play a fundamental role in influencing this evolutionary phase, but the details of these interactions remain shrouded in mystery. Thus, understanding the formation and evolution of these objects is of wide astrophysical importance. PNe have long been known to emit across a very large span of wavelengths, from the radio to X-rays. Extensive use of space-based observatories at X-ray (Chandra/ XMM-Newton), optical (HST) and mid- to far-infrared (Spitzer, Herschel) wavelengths in recent years has produced significant new advances in our knowledge of these objects. Given the expected advent of the James Webb Space Telescope (JWST) in the near future, we focus on future high-angular-resolution, high-sensitivity observations at near and mid-IR wavelengths with JWST that can help in addressing the major unsolved problems in the study of PNe and their progenitors.
      Citation: Galaxies
      PubDate: 2020-08-12
      DOI: 10.3390/galaxies8030061
      Issue No: Vol. 8, No. 3 (2020)
       
  • Galaxies, Vol. 8, Pages 62: X-ray Spectral Evolution of High Energy Peaked
           Blazars

    • Authors: Haritma Gaur
      First page: 62
      Abstract: The synchrotron hump of the high energy peaked blazars generally lies in the 0.1–10 keV range and such sources show extreme flux and spectral variability in X-ray bands. Various spectral studies showed that the X-ray spectra of high energy peaked blazars are curved and better described by the log-parabolic model. The curvature is attributed to the energy dependent statistical acceleration mechanism. In this work, we review the X-ray spectral studies of high energy peaked blazars. It is found that the log-parabolic model well describes the spectra in a wide energy interval around the peak. The log-parabolic model provides the possibility of investigating the correlation between the spectral parameters derived from it. Therefore, we compiled the studies of correlations between the various parameters derived from the log-parabolic model and their implications to describe the variability mechanism of blazars.
      Citation: Galaxies
      PubDate: 2020-08-25
      DOI: 10.3390/galaxies8030062
      Issue No: Vol. 8, No. 3 (2020)
       
  • Galaxies, Vol. 8, Pages 63: X-ray and Gamma-ray Variability of NGC 1275

    • Authors: Varsha Chitnis, Amit Shukla, K. P. Singh, Jayashree Roy, Sudip Bhattacharyya, Sunil Chandra, Gordon Stewart
      First page: 63
      Abstract: Gamma-ray emission from the bright radio source 3C 84, associated with the Perseus cluster, is ascribed to the radio galaxy NGC 1275 residing at the centre of the cluster. Study of the correlated X-ray/gamma-ray emission from this active galaxy, and investigation of the possible disk-jet connection, are hampered because the X-ray emission, particularly in the soft X-ray band (2–10 keV), is overwhelmed by the cluster emission. Here we present a method to spectrally decouple the cluster and active galactic nucleus (AGN) emission in imaging X-ray detectors. We use three sets of simultaneous Niel Gehrels Swift XRT and NuStar data. These observations were made during the period 2015 November to 2017 February, when a huge increase in the gamma-ray emission was observed. We find that the gamma-ray emission shows a very high degree of variability (40%–50%) on time scales of 1–10 days, whereas the hard X-ray emission, associated with the AGN, shows a low variability (∼15%–30%), on various time scales in the range of 0.01–60 days.
      Citation: Galaxies
      PubDate: 2020-08-28
      DOI: 10.3390/galaxies8030063
      Issue No: Vol. 8, No. 3 (2020)
       
  • Galaxies, Vol. 8, Pages 64: X-ray Flux and Spectral Variability of the TeV
           Blazars Mrk 421 and PKS 2155-304

    • Authors: Alok C. Gupta
      First page: 64
      Abstract: We reviewed X-ray flux and spectral variability properties studied to date by various X-ray satellites for Mrk 421 and PKS 2155-304, which are TeV emitting blazars. Mrk 421 and PKS 2155-304 are the most X-ray luminous blazars in the northern and southern hemispheres, respectively. Blazars show flux and spectral variabilities in the complete electromagnetic spectrum on diverse timescales ranging from a few minutes to hours, days, weeks, months and even several years. The flux and spectral variability on different timescales can be used to constrain the size of the emitting region, estimate the super massive black hole mass, find the dominant emission mechanism in the close vicinity of the super massive black hole, search for quasi-periodic oscillations in time series data and several other physical parameters of blazars. Flux and spectral variability is also a dominant tool to explain jet as well as disk emission from blazars at different epochs of observations.
      Citation: Galaxies
      PubDate: 2020-09-04
      DOI: 10.3390/galaxies8030064
      Issue No: Vol. 8, No. 3 (2020)
       
  • Galaxies, Vol. 8, Pages 65: Habitable Zones in Binary Star Systems: A
           Zoology

    • Authors: Siegfried Eggl, Nikolaos Georgakarakos, Elke Pilat-Lohinger
      First page: 65
      Abstract: Several concepts have been brought forward to determine where terrestrial planets are likely to remain habitable in multi-stellar environments. Isophote-based habitable zones, for instance, rely on insolation geometry to predict habitability, whereas radiative habitable zones take the orbital motion of a potentially habitable planet into account. Dynamically informed habitable zones include gravitational perturbations on planetary orbits, and full scale, self consistent simulations promise detailed insights into the evolution of select terrestrial worlds. All of the above approaches agree that stellar multiplicity does not preclude habitability. Predictions on where to look for habitable worlds in such environments can differ between concepts. The aim of this article is to provide an overview of current approaches and present simple analytic estimates for the various types of habitable zones in binary star systems.
      Citation: Galaxies
      PubDate: 2020-09-04
      DOI: 10.3390/galaxies8030065
      Issue No: Vol. 8, No. 3 (2020)
       
  • Galaxies, Vol. 8, Pages 66: Short-Term X-ray Variability during Different
           Activity Phases of Blazars S5 0716+714 and PKS 2155-304

    • Authors: Pankaj Kushwaha, Main Pal
      First page: 66
      Abstract: We explored the statistical properties of short-term X-ray variability using long-exposure XMM-Newton data during high X-ray variability phases of blazars S5 0716+714 and PKS 2155-304. In general, the hardness ratio shows correlated variations with the source flux state (count rate), but in a few cases, mainly the bright phases, the trend is complex with both correlation and anti-correlation, indicating spectral evolution. Stationarity tests suggest the time series are non-stationarity or have trend stationarity. Except for one, none of the histogram fits resulted in a reduced-χ2∼1 for a normal and log-normal profile but a normal profile is favored in general. On the contrary, the Anderson–Darling test favors log-normal with a test-statistic value lower for log-normal over normal for all the observations, even if out of significance limits. None of the IDs show linear RMS-flux relation. The contrary inferences from different widely used statistical methods indicate that a careful analysis is needed while the complex behavior of count rate with hardness ratio suggests spectral evolution over a few 10 s of kilo-seconds during bright phases of the sources. In these cases, the spectrum extracted from whole observation may not be meaningful for spectral studies and certainly not a true representation of the spectral state of the source.
      Citation: Galaxies
      PubDate: 2020-09-06
      DOI: 10.3390/galaxies8030066
      Issue No: Vol. 8, No. 3 (2020)
       
  • Galaxies, Vol. 8, Pages 67: A Relativistic Orbit Model for Temporal
           Properties of AGN

    • Authors: Prerna Rana, A. Mangalam
      First page: 67
      Abstract: We present a unified model for X-ray quasi-periodic oscillations (QPOs) seen in Narrow-line Seyfert 1 (NLSy1) galaxies, γ-ray and optical band QPOs that are seen in Blazars. The origin of these QPOs is attributed to the plasma motion in corona or jets of these AGN. In the case of X-ray QPOs, we applied the general relativistic precession model for the two simultaneous QPOs seen in NLSy1 1H 0707-945 and deduce orbital parameters, such the radius of the emission region, and spin parameter a for a circular orbit; we obtained the Carter’s constant Q, a, and the radius in the case of a spherical orbit solution. In other cases where only one X-ray QPO is seen, we localized the orbital parameters for NLSy1 galaxies REJ 1034+396, 2XMM J123103.2+110648, MS 2254.9-3712, Mrk 766, and MCG-06-30-15. By applying the lighthouse model, we found that a kinematic origin of the jet based γ-ray and optical QPOs, in a relativistic MHD framework, is possible. Based on the inbuilt Hamiltonian formulation with a power-law distribution in the orbital energy of the plasma consisting of only circular or spherical trajectories, we show that the resulting Fourier power spectral density (PSD) has a break corresponding to the energy at ISCO. Further, we derive connection formulae between the slopes in the PSD and that of the energy distribution. Overall, given the preliminary but promising results of these relativistic orbit models to match the observed QPO frequencies and PSD at diverse scales in the inner corona and the jet, it motivates us to build detailed models, including a transfer function for the energy spectrum in the corona and relativistic MHD jet models for plasma flow and its polarization properties.
      Citation: Galaxies
      PubDate: 2020-09-11
      DOI: 10.3390/galaxies8030067
      Issue No: Vol. 8, No. 3 (2020)
       
  • Galaxies, Vol. 8, Pages 68: Mean-Field Dynamo Model in Anisotropic Uniform
           Turbulent Flow with Short-Time Correlations

    • Authors: E. V. Yushkov, R. Allahverdiyev, D. D. Sokoloff
      First page: 68
      Abstract: The mean-field model is one of the basic models of the dynamo theory, which describes the magnetic field generation in a turbulent astrophysical plasma. The first mean-field equations were obtained by Steenbeck, Krause and Rädler for two-scale turbulence under isotropy and uniformity assumptions. In this article we develop the path integral approach to obtain mean-field equations for a short-correlated random velocity field in anisotropic streams. By this model we analyse effects of anisotropy and show the relation between dynamo growth and anisotropic tensors of helicity/turbulent diffusivity. Considering particular examples and comparing results with isotropic cases we demonstrate several mean-field effects: super-exponential growth at initial times, complex dependence of harmonics growth on the helicity tensor structure, when generation is possible for near-zero component or near-zero helicity trace, increase of the averaged magnetic field inclined to the initial current density that leads to effective Lorentz back-reaction and violation of force-free conditions.
      Citation: Galaxies
      PubDate: 2020-09-19
      DOI: 10.3390/galaxies8030068
      Issue No: Vol. 8, No. 3 (2020)
       
  • Galaxies, Vol. 8, Pages 69: Program and Results of Investigations Rapid
           Variability of the BL Lac Object 3C 371 in Radio and Optical Ranges

    • Authors: Artem Sukharev, Michail Ryabov, Vladislavs Bezrukovs, Arturs Orbidans, Marcis Bleiders, Sergei Udovichenko, Leonid Keir, Ilgmars Eglitis, Igor Kudsey, Pavol Dubovsky
      First page: 69
      Abstract: BL Lac object 3C 371 is variable in optical and radio range, according to long-term monitoring data, for example AAVSO (American Association of Variable Star Observers) and OVRO (Owens Valley Radio Observatory). In addition, some authors note intra-night variability. However, in terms of access, just a few works are devoted to this topic, and questions remain about intra-day variability in radio range. The purpose of the work is to search for fast variability in radio (5, 6.1, 6.7 GHz) and optical bands (V, R, I) using international cooperation in 2019 and 2020 observation sessions. The 16-m radio telescope VIRAC, in Latvia, as well as optical telescopes AZT-3 (Mayaki, Ukraine), VNT (Vihorlat, Slovakia), and Schmidt camera (Baldone, Latvia) were used. To analyze variability, the STFT method of spectrograms and Lomb–Scargle periodograms for non-uniform time series were used. As result of the work, there is no correlation between optical and radio observations, and no significant quasi-harmonic variability was detected in radio range, but there is irregular low amplitude variability. In the optical range, there is variability with a characteristic time of about seven days and possibly less. Cyclical variability of 3C 371 was found in the optical range, and intra-day variability in radio range is most likely absent, as there are irregular variations and noise. It is planned to continue joint radio-optical observations 3C 371 to accumulate the necessary statistics.
      Citation: Galaxies
      PubDate: 2020-09-21
      DOI: 10.3390/galaxies8030069
      Issue No: Vol. 8, No. 3 (2020)
       
  • Galaxies, Vol. 8, Pages 28: Binary Central Stars of Planetary Nebulae

    • Authors: David Jones
      First page: 28
      Abstract: It is now clear that a vast majority of intermediate-mass stars have stellar and/or sub-stellar companions, therefore it is no longer appropriate to consider planetary nebulae as a single-star phenomenon, although some single, isolated stars may well lead to planetary nebulae. As such, while understanding binary evolution is critical for furthering our knowledge of planetary nebulae, the converse is also true: planetary nebulae can be valuable tools with which to probe binary evolution. In this brief review, I attempt to summarise some of our current understanding with regards to the role of binarity in the formation of planetary nebulae, and the areas in which continued study of planetary nebulae may have wider ramifications for our grasp on the fundaments of binary evolution.
      Citation: Galaxies
      PubDate: 2020-04-01
      DOI: 10.3390/galaxies8020028
      Issue No: Vol. 8, No. 2 (2020)
       
  • Galaxies, Vol. 8, Pages 29: Gaia DR2 Distances to Planetary Nebulae

    • Authors: Iker González-Santamaría, Minia Manteiga, Arturo Manchado, Ana Ulla, Carlos Dafonte
      First page: 29
      Abstract: The aim of this work is to examine distances to planetary nebulae (PNe) together with other properties that were derived from them, using the astrometry of Gaia Data Release 2 (DR2). We were able to identify 1571 objects classified as PNe, for which we assumed distances calculated following a Bayesian statistical approach. From those objects, we selected a sample of PNe with good quality parallax measurements and distance derivations, which we called Golden Astrometry PNe sample (GAPN). In this paper we will review the physical properties of the stars and nebulae in this subsample of PNe.
      Citation: Galaxies
      PubDate: 2020-04-01
      DOI: 10.3390/galaxies8020029
      Issue No: Vol. 8, No. 2 (2020)
       
  • Galaxies, Vol. 8, Pages 30: Molecular Hydrogen Microstructures in
           Planetary Nebulae

    • Authors: Stavros Akras, Denise R. Gonçalves, Gerardo Ramos-Larios, Isabel Aleman
      First page: 30
      Abstract: Molecular hydrogen (H 2 ) emission is commonly detected in planetary nebulae (PNe), specially in objects with bipolar morphologies. New studies showed that H 2 gas is also packed in microstructures embedded in PNe of any morphological type. Despite the presence of H 2 in cometary knots being known for years, only in the last five years, much deeper imagery of PNe have revealed that H 2 also exists in other types of low-ionisation microstructures (LISs). Significant differences are found between the host PNe of cometary knots and other types of LISs, such as nebula age, central star temperature (evolutionary stage) and the absolute sizes of the microstructure itself.
      Citation: Galaxies
      PubDate: 2020-04-01
      DOI: 10.3390/galaxies8020030
      Issue No: Vol. 8, No. 2 (2020)
       
  • Galaxies, Vol. 8, Pages 31: Integral Field Spectroscopy of Planetary
           Nebulae with MUSE

    • Authors: Jeremy R. Walsh, Ana Monreal-Ibero
      First page: 31
      Abstract: The Multi-Unit Spectroscopic Explorer (MUSE) is a large integral field unit mounted on the ESO Very Large Telescope. Its spatial (60 arcsecond field) and wavelength (4800–9300Å) coverage is well suited to detailed imaging spectroscopy of extended planetary nebulae, such as in the Galaxy. An overview of the capabilities of MUSE applied to Planetary Nebulae (PNe) is provided together with the specific advantages and disadvantages. Some examples of archival MUSE observations of PNe are provided. MUSE datacubes for two targets (NGC 3132 and NGC 7009) are analyzed in detail, and they are used to show the advances achievable for planetary nebulastudies. Prospects for further MUSE observations of PNe and a broader analysis of existing datasets are outlined.
      Citation: Galaxies
      PubDate: 2020-04-03
      DOI: 10.3390/galaxies8020031
      Issue No: Vol. 8, No. 2 (2020)
       
  • Galaxies, Vol. 8, Pages 32: Irradiation Investigation: Exploring the
           Molecular Gas in NGC 7293

    • Authors: Jesse Bublitz, Joel Kastner, Pierre Hily-Blant, Thierry Forveille, Miguel Santander-García, Valentin Bujarrabal, Javier Alcolea, Rodolfo Montez
      First page: 32
      Abstract: Background: Many planetary nebulae retain significant quantities of molecular gas and dust despite their signature hostile radiation environments and energetic shocks. Photoionization and dissociation by extreme UV and (often) X-ray emission from their central stars drive the chemical processing of this material. Their well-defined geometries make planetary nebulae ideal testbeds for modeling the effects of radiation-driven heating and chemistry on molecular gas in photodissociation regions. Methods: We have carried out IRAM 30m/APEX 12m/ALMA radio studies of the Helix Nebula and its molecule-rich globules, exploiting the unique properties of the Helix to follow up our discovery of an anti-correlation between HNC/HCN line intensity ratio and central star UV Luminosity. Results: Analysis of HNC/HCN across the Helix Nebula reveals the line ratio increases with distance from the central star, and thus decreasing incident UV flux, indicative of the utility of the HNC/HCN ratio as a tracer of UV irradiation in photodissociation environments. However, modeling of the observed regions suggests HNC/HCN should decrease with greater distance, contrary to the observed trend. Conclusion: HNC/HCN acts as an effective tracer of UV irradiation of cold molecular gas. Further model studies are required.
      Citation: Galaxies
      PubDate: 2020-04-08
      DOI: 10.3390/galaxies8020032
      Issue No: Vol. 8, No. 2 (2020)
       
  • Galaxies, Vol. 8, Pages 33: Physics and Phenomenology of Weakly
           Magnetized, Relativistic Astrophysical Shock Waves

    • Authors: Arno Vanthieghem, Martin Lemoine, Illya Plotnikov, Anna Grassi, Mickael Grech, Laurent Gremillet, Guy Pelletier
      First page: 33
      Abstract: Weakly magnetized, relativistic collisionless shock waves are not only the natural offsprings of relativistic jets in high-energy astrophysical sources, they are also associated with some of the most outstanding displays of energy dissipation through particle acceleration and radiation. Perhaps their most peculiar and exciting feature is that the magnetized turbulence that sustains the acceleration process, and (possibly) the secondary radiation itself, is self-excited by the accelerated particles themselves, so that the phenomenology of these shock waves hinges strongly on the microphysics of the shock. In this review, we draw a status report of this microphysics, benchmarking analytical arguments with particle-in-cell simulations, and extract consequences of direct interest to the phenomenology, regarding, in particular, the so-called microphysical parameters used in phenomenological studies.
      Citation: Galaxies
      PubDate: 2020-04-10
      DOI: 10.3390/galaxies8020033
      Issue No: Vol. 8, No. 2 (2020)
       
  • Galaxies, Vol. 8, Pages 34: A Preferred Orientation Angle for Bipolar
           Planetary Nebulae

    • Authors: Andreas Ritter, Quentin A. Parker
      First page: 34
      Abstract: We present preliminary results from measuring Galactic orientation angles of 766 elliptical and bipolar Planetary Nebulae (PNe) in the Hong Kong/Australian Astronomical Observatory/Strasbourg Observatory H-alpha Planetary Nebula research platform and database (HASH DB). For elliptical PNe the distribution of orientation angles is found to be more or less uniform. However, for bipolar PNe there is statistically significant evidence for preferred orientation angles (as tentatively reported before with smaller samples) across the whole Galaxy.
      Citation: Galaxies
      PubDate: 2020-04-20
      DOI: 10.3390/galaxies8020034
      Issue No: Vol. 8, No. 2 (2020)
       
  • Galaxies, Vol. 8, Pages 35: Predictions and Outcomes for the Dynamics of
           Rotating Galaxies

    • Authors: Stacy McGaugh
      First page: 35
      Abstract: A review is given of a priori predictions made for the dynamics of rotating galaxies. One theory—MOND—has had many predictions corroborated by subsequent observations. While it is sometimes possible to offer post hoc explanations for these observations in terms of dark matter, it is seldom possible to use dark matter to predict the same phenomena.
      Citation: Galaxies
      PubDate: 2020-04-24
      DOI: 10.3390/galaxies8020035
      Issue No: Vol. 8, No. 2 (2020)
       
  • Galaxies, Vol. 8, Pages 36: Dark Matter Dogma: A Study of 214 Galaxies

    • Authors: Alan Sipols, Alex Pavlovich
      First page: 36
      Abstract: The aim of this paper is to test the need for non-baryonic dark matter in the context of galactic rotation and the apparent difference between distributions of galactic mass and luminosity. We present a set of rotation curves and 3.6 μm surface brightness profiles for a diverse sample of 214 galaxies. Using rotation curves as the sole input into our Newtonian disk model, we compute non-parametric radial profiles of surface mass density. All profiles exhibit lower density than parametric models with dark halos and provide a superior fit with observed rotation curves. Assuming all dynamical mass is in main-sequence stars, we estimate radial distributions of characteristic star mass implied by the corresponding pairs of density and brightness profiles. We find that for 132 galaxies or 62% of the sample, the relation between density and brightness can be fully explained by a radially declining stellar mass gradient. Such idealized stellar population fitting can also largely address density and brightness distributions of the remaining 82 galaxies, but their periphery shows, on average, 14 M⊙/pc2 difference between total density and light-constrained stellar density. We discuss how this density gap can be interpreted, by considering a low-luminosity baryonic matter, observational uncertainties, and visibility cutoffs for red dwarf populations. Lastly, we report tight correlation between radial density and brightness trends, and the discovered flattening of surface brightness profiles—both being evidence against dark matter. Our findings make non-baryonic dark matter unnecessary in the context of galactic rotation.
      Citation: Galaxies
      PubDate: 2020-04-28
      DOI: 10.3390/galaxies8020036
      Issue No: Vol. 8, No. 2 (2020)
       
  • Galaxies, Vol. 8, Pages 37: Rotation Curve of the Milky Way and the Dark
           Matter Density

    • Authors: Yoshiaki Sofue
      First page: 37
      Abstract: We review the current status of the study of rotation curve (RC) of the Milky Way, and present a unified RC from the Galactic Center to the galacto-centric distance of about 100 kpc. The RC is used to directly calculate the distribution of the surface mass density (SMD). We then propose a method to derive the distribution of dark matter (DM) density in the in the Milky Way using the SMD distribution. The best-fit dark halo profile yielded a local DM density of ρ ⊙ = 0.36 ± 0.02 GeV cm − 3 . We also review the estimations of the local DM density in the last decade, and show that the value is converging to a value at ρ ⊙ = 0.39 ± 0.09 GeV cm − 3 .
      Citation: Galaxies
      PubDate: 2020-04-29
      DOI: 10.3390/galaxies8020037
      Issue No: Vol. 8, No. 2 (2020)
       
  • Galaxies, Vol. 8, Pages 38: The Interaction of Type Ia Supernovae with
           Planetary Nebulae: The Case of Kepler’s Supernova Remnant

    • Authors: Alexandros Chiotellis, Panayotis Boumis, Zoi-Tzoyia Spetsieri
      First page: 38
      Abstract: One of the key methods for determining the unknown nature of Type Ia supernovae (SNe Ia) is the search for traces of interaction between the SN ejecta and the circumstellar structures at the resulting supernova remnants (SNRs Ia). So far, the observables that we receive from well-studied SNRs Ia cannot be explained self-consistently by any model presented in the literature. In this study, we suggest that the circumstellar medium (CSM) being observed to surround several SNRs Ia was mainly shaped by planetary nebulae (PNe) that originated from one or both progenitor stars. Performing two-dimensional hydrodynamic simulations, we show that the ambient medium shaped by PNe can account for several properties of the CSM that have been found to surround SNe Ia and their remnants. Finally, we model Kepler’s SNR considering that the SN explosion occurred inside a bipolar PN. Our simulations show good agreement with the observed morphological and kinematic properties of Kepler’s SNR. In particular, our model reproduces the current expansion parameter of Kepler’s SNR, the partial interaction of the remnant with a dense CSM at its northern region and finally the existence of two opposite protrusions (‘ears’) at the equatorial plane of the SNR.
      Citation: Galaxies
      PubDate: 2020-05-04
      DOI: 10.3390/galaxies8020038
      Issue No: Vol. 8, No. 2 (2020)
       
  • Galaxies, Vol. 8, Pages 39: On the Mass Accretion Rates of Herbig Ae/Be
           Stars. Magnetospheric Accretion or Boundary Layer'

    • Authors: Ignacio Mendigutía
      First page: 39
      Abstract: Understanding how young stars gain their masses through disk-to-star accretion is of paramount importance in astrophysics. It affects our knowledge about the early stellar evolution, the disk lifetime and dissipation processes, the way the planets form on the smallest scales, or the connection to macroscopic parameters characterizing star-forming regions on the largest ones, among others. In turn, mass accretion rate estimates depend on the accretion paradigm assumed. For low-mass T Tauri stars with strong magnetic fields there is consensus that magnetospheric accretion (MA) is the driving mechanism, but the transfer of mass in massive young stellar objects with weak or negligible magnetic fields probably occurs directly from the disk to the star through a hot boundary layer (BL). The intermediate-mass Herbig Ae/Be (HAeBe) stars bridge the gap between both previous regimes and are still optically visible during the pre-main sequence phase, thus constituting a unique opportunity to test a possible change of accretion mode from MA to BL. This review deals with our estimates of accretion rates in HAeBes, critically discussing the different accretion paradigms. It shows that although mounting evidence supports that MA may extend to late-type HAes but not to early-type HBes, there is not yet a consensus on the validity of this scenario versus the BL one. Based on MA and BL shock modeling, it is argued that the ultraviolet regime could significantly contribute in the future to discriminating between these competing accretion scenarios.
      Citation: Galaxies
      PubDate: 2020-05-05
      DOI: 10.3390/galaxies8020039
      Issue No: Vol. 8, No. 2 (2020)
       
  • Galaxies, Vol. 8, Pages 40: Water on Mars—A Literature Review

    • Authors: Mohammad Nazari-Sharabian, Mohammad Aghababaei, Moses Karakouzian, Mehrdad Karami
      First page: 40
      Abstract: To assess Mars’ potential for both harboring life and providing useable resources for future human exploration, it is of paramount importance to comprehend the water situation on the planet. Therefore, studies have been conducted to determine any evidence of past or present water existence on Mars. While the presence of abundant water on Mars very early in its history is widely accepted, on its modern form, only a fraction of this water can be found, as either ice or locked into the structure of Mars’ plentiful water-rich materials. Water on the planet is evaluated through various evidence such as rocks and minerals, Martian achondrites, low volume transient briny outflows (e.g., dune flows, reactivated gullies, slope streaks, etc.), diurnal shallow soil moisture (e.g., measurements by Curiosity and Phoenix Lander), geomorphic representation (possibly from lakes and river valleys), and groundwater, along with further evidence obtained by probe and rover discoveries. One of the most significant lines of evidence is for an ancient streambed in Gale Crater, implying ancient amounts of “vigorous” water on Mars. Long ago, hospitable conditions for microbial life existed on the surface of Mars, as it was likely periodically wet. However, its current dry surface makes it almost impossible as an appropriate environment for living organisms; therefore, scientists have recognized the planet’s subsurface environments as the best potential locations for exploring life on Mars. As a result, modern research has aimed towards discovering underground water, leading to the discovery of a large amount of underground ice in 2016 by NASA, and a subglacial lake in 2018 by Italian scientists. Nevertheless, the presence of life in Mars’ history is still an open question. In this unifying context, the current review summarizes results from a wide variety of studies and reports related to the history of water on Mars, as well as any related discussions on the possibility of living organism existence on the planet.
      Citation: Galaxies
      PubDate: 2020-05-09
      DOI: 10.3390/galaxies8020040
      Issue No: Vol. 8, No. 2 (2020)
       
  • Galaxies, Vol. 8, Pages 41: Shaping of Planetary Nebulae by Exoplanets

    • Authors: Nicola Keaveney, Laura Boyle, Matt Redman
      First page: 41
      Abstract: (1) Background: We investigate the hypothesis that exoplanet engulfment can help explain the observed non-spherical planetary nebula population, as a complementary shaping mechanism to the binary hypothesis. The aim is to investigate the extent to which massive planets can explain the population of non-spherical planetary nebulae; (2) Methods: This research utilises a new tool to calculate the planet-fraction of planetary nebulae progenitor stars called SIMSPLASH; (3) Results: we conclude that ∼15%–30% of non-spherical planetary nebulae around single stars will have a history in which they engulfed a massive planet on the AGB; and (4) Conclusions: Engulfment of massive exoplanets may contribute significantly to the formation of non-spherical planetary nebulae around single stars, yet appears to be insufficient to explain them all.
      Citation: Galaxies
      PubDate: 2020-05-14
      DOI: 10.3390/galaxies8020041
      Issue No: Vol. 8, No. 2 (2020)
       
  • Galaxies, Vol. 8, Pages 42: Gravitational Focusing of Low-Velocity Dark
           Matter on the Earth’s Surface

    • Authors: Yoshiaki Sofue
      First page: 42
      Abstract: We show that the Earth acts as a high-efficiency gravitational collector of low-velocity flow of dark matter (DM). The focal point appears on the Earth’s surface, when the DM flow speed is about 17 km/s with respect to the geo-center. We discuss diurnal modulation of the local DM density influenced by the Earth’s gravity. We also touch upon similar effects on galactic and solar system objects.
      Citation: Galaxies
      PubDate: 2020-05-16
      DOI: 10.3390/galaxies8020042
      Issue No: Vol. 8, No. 2 (2020)
       
  • Galaxies, Vol. 8, Pages 43: Star Formation in the Ultraviolet

    • Authors: Jorick S. Vink
      First page: 43
      Abstract: With the launch of JWST and the upcoming installation of extremely large telescopes, the first galaxies in our Universe will finally be revealed. Their light will be dominated by massive stars, which peak in in the ultra-violet (UV) part of the electromagnetic spectrum. Star formation is the key driver of the evolution of our Universe. At young ages, within 10 Million years, both high and low mass stars generate complex UV emission processes which are poorly understood yet are vital for interpreting high red-shift line emission. For these reasons, the Hubble Space Telescope (HST) will devote 1000 orbits to obtaining a UV Legacy Library of Young Stars as Essential Standards (ULLYSES). The purpose of this Overview is to outline the basic physical principles driving UV emission processes from local (within 100 parsecs of) star formation, ranging from huge star-forming complexes containing hundreds of massive and very-massive stars (VMS), such as 30 Doradus (the Tarantula Nebula) in the neighboring Magellanic Clouds (only 50 kpc away), to galaxies near and far, out to the epoch of Cosmic Reionization.
      Citation: Galaxies
      PubDate: 2020-05-17
      DOI: 10.3390/galaxies8020043
      Issue No: Vol. 8, No. 2 (2020)
       
  • Galaxies, Vol. 8, Pages 44: Infrared Observations of Planetary Nebulae and
           Related Objects

    • Authors: Eric Lagadec
      First page: 44
      Abstract: In this paper, I present how near and mid-infrared observations can be used for the study of planetary nebulae and related objects. I present the main observing techniques, from the ground and space, highlighting main differences and how they can be complementary. I also highlight some new observing facilities and present the infrared observatories of the future to show that the future of infrared observations of planetary nebulae is bright.
      Citation: Galaxies
      PubDate: 2020-05-18
      DOI: 10.3390/galaxies8020044
      Issue No: Vol. 8, No. 2 (2020)
       
  • Galaxies, Vol. 8, Pages 45: Oscillating Cosmological Force Modifies
           Newtonian Dynamics

    • Authors: Igor I. Smolyaninov
      First page: 45
      Abstract: In the Newtonian limit of general relativity a force acting on a test mass in a central gravitational field is conventionally defined by the attractive Newtonian gravity (inverse square) term plus a small repulsive cosmological force, which is proportional to the slow acceleration of the universe expansion. In this paper we considered the cosmological-force correction due to fast quantum oscillations of the universe scale factor, which were suggested recently by Wang et al. (Phys. Rev. D 95, 103504 (2017)) as a potential solution of the cosmological constant problem. These fast fluctuations of the cosmological scale factor violate Lorentz invariance at the Planck scale, and they induce strong changes to the current sign and magnitude of the average cosmological force, thus making it one of the potential probable causes for the modification of Newtonian dynamics in galaxy-scale systems. The modified cosmological force may be responsible for the recently discovered “cosmic-clock” behavior of disk galaxies in the low-redshift universe. The obtained results have strong implications for astroparticle physics since they demonstrate that typical galaxy rotation curves may be obtained without (or almost without) dark-matter particles.
      Citation: Galaxies
      PubDate: 2020-05-22
      DOI: 10.3390/galaxies8020045
      Issue No: Vol. 8, No. 2 (2020)
       
  • Galaxies, Vol. 8, Pages 46: The Web of Micro-Structures in IC 4593

    • Authors: M. Belén Mari, Denise R. Gonçalves, Stavros Akras
      First page: 46
      Abstract: In addition to large-scale structures, planetary nebulae (PNe) show small-scale structures that emit mainly in low-ionization species such as [N ii], [S ii], [O ii], and [O i], known as LISs. Here, we present the analysis of optical long-slit spectra, for three slit positions, of the PN IC 4593, which possesses a pair of knots and an isolated low-ionization knot. The motivation for this work is the need to characterize LISs completely to evaluate their impact on the PNe studies. These data allow us to derive the physical properties and ionization state for each morphological component of the nebula, including its pair of knots and individual knot. Due to the large uncertainties in the [S ii] derived electron densities, we cannot confirm any contrast between the LISs’ electron densities and the surrounding nebula, found in numerous other LISs. Though the lack of spatially-resolved physical parameters in the literature prevents further comparisons, in general, our results derived for the entire nebula agree with previous studies.
      Citation: Galaxies
      PubDate: 2020-05-30
      DOI: 10.3390/galaxies8020046
      Issue No: Vol. 8, No. 2 (2020)
       
  • Galaxies, Vol. 8, Pages 47: Debated Models for Galactic Rotation Curves: A
           Review and Mathematical Assessment

    • Authors: Anne M. Hofmeister, Robert E. Criss
      First page: 47
      Abstract: Proposed explanations of galactic rotation curves (RC = tangential velocity vs. equatorial radius, determined from Doppler measurements) involve dramatically different assumptions. A dominant, original camp invoked huge amounts of unknown, non-baryonic dark matter (NBDM) in surrounding haloes to reconcile RC simulated using their Newtonian orbital models (NOMs) for billions of stars in spiral galaxies with the familiar Keplerian orbital patterns of the few, tiny planets in our Solar System. A competing minority proposed that hypothetical, non-relativistic, non-Newtonian forces govern the internal motions of galaxies. More than 40 years of controversy has followed. Other smaller groups, unsatisfied by explanations rooted in unknown matter or undocumented forces, have variously employed force summations, spin models, or relativistic adaptations to explain galactic rotation curves. Some small groups have pursued inverse models and found no need for NBDM. The successes, failures, and underlying assumptions of the above models are reviewed in this paper, focusing on their mathematical underpinnings. We also show that extractions of RC from Doppler measurements need revising to account for the effect of galaxy shape on flux-velocity profiles and for the possible presence of a secondary spin axis. The latter is indicated by complex Doppler shift patterns. Our findings, combined with independent evidence such as hadron collider experiments failing to produce non-baryonic matter, suggest that a paradigm shift is unfolding.
      Citation: Galaxies
      PubDate: 2020-06-01
      DOI: 10.3390/galaxies8020047
      Issue No: Vol. 8, No. 2 (2020)
       
  • Galaxies, Vol. 8, Pages 48: Surface Brightness Plateau in S4G Galaxies

    • Authors: Sipols, Pavlovich
      First page: 48
      Abstract: Using 3.6-μm data from 2112 galaxies, we show that, contrary to widely held expectations of a continuous steep decline, radial surface brightness profiles of galaxies tend to flatten and form extended plateaus beyond 27–28 magAB/arcsec². This phenomenon could be explained by the presence of extended stellar populations dominated by low-mass stars in galactic outskirts. The flattening of radial brightness profiles questions the artificial exponential extrapolations of brightness data and the automatic assumption that light always declines considerably faster than mass density, presenting an empirical challenge for the dark matter hypothesis.
      Citation: Galaxies
      PubDate: 2020-06-06
      DOI: 10.3390/galaxies8020048
      Issue No: Vol. 8, No. 2 (2020)
       
  • Galaxies, Vol. 8, Pages 49: First Results from a Panchromatic HST/WFC3
           Imaging Study of the Young, Rapidly Evolving Planetary Nebulae NGC 7027
           and NGC 6302

    • Authors: Joel H. Kastner, Jesse Bublitz, Bruce Balick, Rodolfo Montez, Adam Frank, Eric Blackman
      First page: 49
      Abstract: We present the first results from comprehensive, near-UV-to-near-IR Hubble Space Telescope Wide Field Camera 3 (WFC3) emission-line imaging studies of two young planetary nebulae (PNe), NGC 7027 and NGC 6302. These two objects represent key sources for purposes of understanding PNe shaping processes. Both nebulae feature axisymmetric and point-symmetric (bipolar) structures and, despite hot central stars and high nebular excitation states, both harbor large masses of molecular gas and dust. The sweeping wavelength coverage of our Cycle 27 Hubble Space Telescope (HST)/WFC3 imaging surveys targeting these two rapidly evolving PNe will provide a battery of essential tests for theories describing the structural and chemical evolution of evolved star ejecta. Here, we present initial color overlays for selected images, and we highlight some of the first results gleaned from the surveys.
      Citation: Galaxies
      PubDate: 2020-06-15
      DOI: 10.3390/galaxies8020049
      Issue No: Vol. 8, No. 2 (2020)
       
  • Galaxies, Vol. 8, Pages 50: Neutron-Capture Element Abundances in
           Planetary Nebulae

    • Authors: N. C. Sterling
      First page: 50
      Abstract: Nebular spectroscopy is a valuable tool for assessing the production of heavy elements by slow neutron(n)-capture nucleosynthesis (the s-process). Several transitions of n-capture elements have been identified in planetary nebulae (PNe) in the last few years, with the aid of sensitive, high-resolution, near-infrared spectrometers. Combined with optical spectroscopy, the newly discovered near-infrared lines enable more accurate abundance determinations than previously possible, and provide access to elements that had not previously been studied in PNe or their progenitors. Neutron-capture elements have also been detected in PNe in the Sagittarius Dwarf galaxy and in the Magellanic Clouds. In this brief review, I discuss developments in observational studies of s-process enrichments in PNe, with an emphasis on the last five years, and note some open questions and preliminary trends.
      Citation: Galaxies
      PubDate: 2020-06-17
      DOI: 10.3390/galaxies8020050
      Issue No: Vol. 8, No. 2 (2020)
       
  • Galaxies, Vol. 8, Pages 51: On the Age of Galactic Bulge CSPNe: Too Young
           and Complicated'

    • Authors: Foteini Lykou, Albert Zijlstra, Quentin A. Parker
      First page: 51
      Abstract: We present preliminary results of our study of a small sample of planetary nebulae in the Galactic Bulge for which high-angular resolution Hubble Space Telescope imaging is available. From this and from archival spectroscopy, we were able to calculate temperatures and luminosities for their central stars. These were then correlated to up-to-date evolutionary tracks found in the literature to help us estimate stellar masses and therefore ages for the central stars. Our current analysis indicates that our sample appears to represent a somewhat mixed population of planetary nebulae central stars, while at least one of the nebulae might have been formed by a more massive progenitor (i.e., M ZAMS ∼ 4 M ⊙ ).
      Citation: Galaxies
      PubDate: 2020-06-18
      DOI: 10.3390/galaxies8020051
      Issue No: Vol. 8, No. 2 (2020)
       
  • Galaxies, Vol. 8, Pages 6: Applications of Stellar Population Synthesis in
           the Distant Universe

    • Authors: Elizabeth R. Stanway
      First page: 6
      Abstract: Comparison with artificial galaxy models is essential for translating the incomplete and low signal-to-noise data we can obtain on astrophysical stellar populations to physical interpretations which describe their composition, physical properties, histories and internal conditions. In particular, this is true for distant galaxies, whose unresolved light embeds clues to their formations and evolutions, and their impacts on their wider environs. Stellar population synthesis models are now used as the foundation of analysis at all redshifts, but are not without their problems. Here we review the use of stellar population synthesis models, with a focus on applications in the distant Universe.
      Citation: Galaxies
      PubDate: 2020-01-08
      DOI: 10.3390/galaxies8010006
      Issue No: Vol. 8, No. 1 (2020)
       
  • Galaxies, Vol. 8, Pages 7: Investigating Multiwavelength Lognormality with
           Simulations—Case of Mrk 421

    • Authors: Nachiketa Chakraborty
      First page: 7
      Abstract: Blazars are highly variable and display complex characteristics. A key characteristic is the flux probability distribution function or flux PDF whose shape depends upon the form of the underlying physical process driving variability. The BL Lacertae Mrk 421 is one of the brightest and most variable blazars across the electromagnetic spectrum. It has been reported to show hints of lognormality across the spectrum from radio to gamma-ray histograms of observed fluxes. This would imply that the underlying mechanisms may not conform to the “standard” additive, multi-zone picture, but could potentially have multiplicative processes. This is investigated by testing the observed lightcurves at different wavelengths with time-series simulations. We find that the simulations reveal a more complex scenario, than a single lognormal distribution explaining the multiwavelength lightcurves of Mrk 421.
      Citation: Galaxies
      PubDate: 2020-01-16
      DOI: 10.3390/galaxies8010007
      Issue No: Vol. 8, No. 1 (2020)
       
  • Galaxies, Vol. 8, Pages 8: Acknowledgement to Reviewers of Galaxies in
           2019

    • Authors: Galaxies Editorial Office
      First page: 8
      Abstract: The editorial team greatly appreciates the reviewers who have dedicated their considerable time and expertise to the journal’s rigorous editorial process over the past 12 months, regardless of whether the papers are finally published or not [...]
      Citation: Galaxies
      PubDate: 2020-01-27
      DOI: 10.3390/galaxies8010008
      Issue No: Vol. 8, No. 1 (2020)
       
  • Galaxies, Vol. 8, Pages 9: Rotating Disk Galaxies without Dark Matter
           Based on Scientific Reasoning

    • Authors: Feng
      First page: 9
      Abstract: The most cited evidence for (non-baryonic) dark matter has been an apparent lack of visible mass to gravitationally support the observed orbital velocity of matter in rotating disk galaxies, yet measurement of the mass of celestial objects cannot be straightforward, requiring theories derived from the known physical laws along with some empirically established semi-quantitative relationship. The most reliable means for determining the mass distribution in rotating disk galaxies is to solve a force balance equation according to Newton’s laws from measured rotation curves, similar to calculating the Sun’s mass from the Earth’s orbital velocity. Another common method to estimate galactic mass distribution is to convert measured brightness from surface photometry based on empirically established mass-to-light ratio. For convenience, most astronomers commonly assumed a constant mass-to-light ratio for estimation of the so-called “luminous” or “visible” mass, which would not likely be accurate. The mass determined from a rotation curve typically exhibits an exponential-like decline with galactrocentric distance, qualitatively consistent with observed surface brightness but often with a larger disk radial scale length. This fact scientifically suggests variable mass-to-light ratio of baryonic matter in galaxies without the need for dark matter.
      Citation: Galaxies
      PubDate: 2020-02-01
      DOI: 10.3390/galaxies8010009
      Issue No: Vol. 8, No. 1 (2020)
       
  • Galaxies, Vol. 8, Pages 10: Radiation-Driven Stellar Eruptions

    • Authors: Kris Davidson
      First page: 10
      Abstract: Very massive stars occasionally expel material in colossal eruptions, driven by continuum radiation pressure rather than blast waves. Some of them rival supernovae in total radiative output, and the mass loss is crucial for subsequent evolution. Some are supernova impostors, including SN precursor outbursts, while others are true SN events shrouded by material that was ejected earlier. Luminous Blue Variable stars (LBV’s) are traditionally cited in relation with giant eruptions, though this connection is not well established. After four decades of research, the fundamental causes of giant eruptions and LBV events remain elusive. This review outlines the basic relevant physics, with a brief summary of essential observational facts. Reasons are described for the spectrum and emergent radiation temperature of an opaque outflow. Proposed mechanisms are noted for instabilities in the star’s photosphere, in its iron opacity peak zones, and in its central region. Various remarks and conjectures are mentioned, some of them relatively unfamiliar in the published literature.
      Citation: Galaxies
      PubDate: 2020-02-05
      DOI: 10.3390/galaxies8010010
      Issue No: Vol. 8, No. 1 (2020)
       
  • Galaxies, Vol. 8, Pages 11: The Microvariable Activity of BL Lacertae

    • Authors: Alberto C. Sadun, Masoud Asadi-Zeydabadi, Lauren Hindman, J. Ward Moody
      First page: 11
      Abstract: We report on seven nights of optical observation taken over a two-week period, and the resultant analysis of the intermediate-frequency peaked BL Lac object (IBL), BL Lac itself, at redshift z = 0.069. The microvariable behavior can be confirmed over the course of minutes for each night. A relativistic beaming model was used in our analysis, to infer changes to the line of sight angles for the motion of the different relativistic components. This model has very few free parameters. The light curves we generated show both high and moderate frequency cadence to the variable behavior of BL Lac itself, in addition to the well documented long-term variability.
      Citation: Galaxies
      PubDate: 2020-02-07
      DOI: 10.3390/galaxies8010011
      Issue No: Vol. 8, No. 1 (2020)
       
  • Galaxies, Vol. 8, Pages 12: Entropy and Mass Distribution in Disc Galaxies

    • Authors: John Herbert Marr
      First page: 12
      Abstract: The relaxed motion of stars and gas in galactic discs is well approximated by a rotational velocity that is a function of radial position only, implying that individual components have lost any information about their prior states. Thermodynamically, such an equilibrium state is a microcanonical ensemble with maximum entropy, characterised by a lognormal probability distribution. Assuming this for the surface density distribution yields rotation curves that closely match observational data across a wide range of disc masses and galaxy types and provides a useful tool for modelling the theoretical density distribution in the disc. A universal disc spin parameter emerges from the model, giving a tight virial mass estimator with strong correlation between angular momentum and disc mass, suggesting a mechanism by which the proto-disc developed by dumping excess mass to the core or excess angular momentum to a satellite galaxy. The baryonic-to-dynamic mass ratio for the model approaches unity for high mass galaxies, but is generally <1 for low mass discs, and this discrepancy appears to follow a similar relationship to that shown in recent work on the Radial Acceleration Relation (RAR). Although this may support Modified Newtonian Dynamics (MOND) in preference to a Dark Matter (DM) halo, it does not exclude undetected baryonic mass or a gravitational DM component in the disc.
      Citation: Galaxies
      PubDate: 2020-02-08
      DOI: 10.3390/galaxies8010012
      Issue No: Vol. 8, No. 1 (2020)
       
  • Galaxies, Vol. 8, Pages 13: Massive Star Formation in the Ultraviolet
           Observed with the Hubble Space Telescope

    • Authors: Claus Leitherer
      First page: 13
      Abstract: Spectroscopic observations of a massive star formation in the ultraviolet and their interpretation are reviewed. After a brief historical retrospective, two well-studied resolved star clusters and the surrounding H II regions are introduced: NGC 2070 in the Large Magellanic Cloud and NGC 604 in M33. These regions serve as a training set for studies of more distant clusters, which can no longer be resolved into individual stars. Observations of recently formed star clusters and extended regions in star-forming galaxies in the nearby universe beyond the Local Group are presented. Their interpretation relies on spectral synthesis models. The successes and failures of such models are discussed, and future directions are highlighted. I present a case study of the extraordinary star cluster and giant H II region in the blue compact galaxy II Zw 40. The review concludes with a preview of two upcoming Hubble Space Telescope programs: ULLYSES, a survey of massive stars in nearby galaxies, and CLASSY, a study of massive star clusters in star-forming galaxies.
      Citation: Galaxies
      PubDate: 2020-02-09
      DOI: 10.3390/galaxies8010013
      Issue No: Vol. 8, No. 1 (2020)
       
  • Galaxies, Vol. 8, Pages 14: Circular Geodesics Stability in a Static Black
           Hole in New Massive Gravity

    • Authors: Andrés Aceña, Ericson López, Franklin Aldás
      First page: 14
      Abstract: We study the existence and stability of circular geodesics in a family of asymptotically AdS static black holes in New Massive Gravity theory. We show that the mathematical sign of the hair parameter determines the existence of such geodesics. For a positive hair parameter, the stability regions follow the usual pattern, with the innermost geodesic being null, unstable, and separated from the horizon, followed by a region of unstable timelike geodesics and then a region of stable timelike geodesics, which extends in the asymptotic region.
      Citation: Galaxies
      PubDate: 2020-02-14
      DOI: 10.3390/galaxies8010014
      Issue No: Vol. 8, No. 1 (2020)
       
  • Galaxies, Vol. 8, Pages 15: A Multi-Wavelength View of OJ 287 Activity in
           2015–2017: Implications of Spectral Changes on Central-Engine Models and
           MeV-GeV Emission Mechanism

    • Authors: Pankaj Kushwaha
      First page: 15
      Abstract: A diverse range of observational results and peculiar properties across the domains of observation have made OJ 287 one of the best-explored BL Lac objects on the issues of relativistic jets and accretion physics as well as the strong theory of gravity. We here present a brief compilation of observational results from the literature and inferences/insights from the extensive studies but focus on the interpretation of its ∼12-yr quasi-periodic optical outbursts (QPOOs) and high energy emission mechanisms. The QPOOs in one model are attributed to the disk-impact related to dynamics of the binary SMBHs while alternative models attribute it to the geometrical effect related to the precession of a single jet or double jets. We discuss implications of the new spectral features reported during the 2015–2017 multi-wavelength high activity of the source—a break in the NIR-optical spectrum and hardening of the MeV-GeV emission accompanied by a shift in the location of its peak, in the context of the two. The reported NIR-optical break nicely fits the description of a standard accretion disk emission from an SMBH of mass ∼ 10 10 M ⊙ while the time of its first appearance at the end of May, 2013 (MJD 56439) is in close coincidence with the time of impact predicted by the disk-impact binary SMBH model. This spectral and temporal coincidence with the model parameters of the disk-impact binary SMBH model provides independent evidence in favor of the model over the geometrical models which argue for a total central-engine mass in the range of 10 7 - 9 M ⊙ . On the other hand, the MeV-GeV spectral change is naturally reproduced by the inverse Compton scattering of photons from the broad-line region and is consistent with the detection of broad emission lines during the previous cycles of quasi-periodic outbursts. Combining this with previous SED studies suggests that in, OJ 287, the MeV-GeV emission results from external Comptonization.
      Citation: Galaxies
      PubDate: 2020-02-14
      DOI: 10.3390/galaxies8010015
      Issue No: Vol. 8, No. 1 (2020)
       
  • Galaxies, Vol. 8, Pages 16: Frequency of Planets in Binaries

    • Authors: Mariangela Bonavita, Silvano Desidera
      First page: 16
      Abstract: The frequency of planets in binaries is an important issue in the field of extrasolar planet studies because of its relevance in the estimation of the global planet population of our galaxy and the clues it can give to our understanding of planet formation and evolution. Multiple stars have often been excluded from exoplanet searches, especially those performed using the radial velocity technique, due to the technical challenges posed by such targets. As a consequence and despite recent efforts, our knowledge of the frequency of planets in multiple stellar systems is still rather incomplete. On the other hand, the lack of knowledge about the binarity at the time of the compilation of the target samples means that our estimate of the planet frequency around single stars could be tainted by the presence of unknown binaries, especially if these objects have a different behavior in terms of planet occurrence. In a previous work we investigated the binarity of the objects included in the Uniform Detectability sample defined by Fisher and Valenti (2005), showing how more than 20% of their targets were, in fact, not single stars. Here, we present an update of this census, made possible mainly by the information now available thanks to the second Gaia Data Release. The new binary sample includes a total of 313 systems, of which 114 were added through this work. We were also able to significantly improve the estimates of masses and orbital parameters for most of the pairs in the original list, especially those at close separations. A few new systems with white dwarf companions were also identified. The results of the new analysis are in good agreement with the findings of our previous work, confirming the lack of difference in the overall planet frequency between binaries and single stars but suggesting a decrease in the planet frequency for very close pairs.
      Citation: Galaxies
      PubDate: 2020-02-18
      DOI: 10.3390/galaxies8010016
      Issue No: Vol. 8, No. 1 (2020)
       
  • Galaxies, Vol. 8, Pages 17: From SN 2010da to NGC 300 ULX-1: Ten Years of
           Observations of an Unusual High Mass X-Ray Binary in NGC 300

    • Authors: Breanna A. Binder, Stefania Carpano, Marianne Heida, Ryan Lau
      First page: 17
      Abstract: In May 2010, an intermediate luminosity optical transient was discovered in the nearby galaxy NGC 300 by a South African amateur astronomer. In the decade since its discovery, multi-wavelength observations of the misnamed “SN 2010da” have continually reshaped our understanding of this high mass X-ray binary system. In this review, we present an overview of the multi-wavelength observations and attempt to understand the 2010 transient event, and later, the reclassification of this system as NGC 300 ULX-1: a red supergiant + neutron star ultraluminous X-ray source.
      Citation: Galaxies
      PubDate: 2020-02-18
      DOI: 10.3390/galaxies8010017
      Issue No: Vol. 8, No. 1 (2020)
       
  • Galaxies, Vol. 8, Pages 18: Radio Galaxies at TeV Energies: Preface

    • Authors: Dorit Glawion
      First page: 18
      Abstract: The majority of the known extragalactic sky from TeV gamma-ray energies consists of blazars having plasma jets pointing in the direction of the line-of-sight, which results in a large Doppler boosting of their emission. Up to now, only six galaxies with a larger viewing angle have been detected in the TeV range. These objects also show fascinating properties, such as fast variability or spectral features and are called “radio galaxies”. The TeV radio galaxies provide a unique laboratory for studying key aspects of active galactic nuclei. This Special Issue of Galaxies targets these exciting objects.
      Citation: Galaxies
      PubDate: 2020-02-22
      DOI: 10.3390/galaxies8010018
      Issue No: Vol. 8, No. 1 (2020)
       
  • Galaxies, Vol. 8, Pages 19: Density Profiles of 51 Galaxies from
           Parameter-Free Inverse Models of Their Measured Rotation Curves

    • Authors: Robert E. Criss, Anne M. Hofmeister
      First page: 19
      Abstract: Spiral galaxies and their rotation curves have key characteristics of differentially spinning objects. Oblate spheroid shapes are a consequence of spin and reasonably describe galaxies, indicating that their matter is distributed in gravitationally interacting homeoidal shells. Here, previously published equations describing differentially spinning oblate spheroids with radially varying density are applied to 51 galaxies, mostly spirals. A constant volumetric density (r, kg m−3) is assumed for each thin homeoid in these formulae, after Newton, which is consistent with RCs being reported simply as a function of equatorial radius r. We construct parameter-free inverse models that uniquely specify mass inside any given r, and thus directly constrain r vs. r solely from velocity v (r) and galactic aspect ratios (assumed as 1:10 for spirals when data are unavailable). Except for their innermost zones, r is proven to be closely proportional to rn, where the statistical average of n for all 36 spirals studied is −1.80 ± 0.40. Our values for interior densities compare closely with independently measured baryon density in appropriate astronomical environments: for example, calculated r at galactic edges agrees with independently estimated r of intergalactic media (IGM). Our finding that central densities increase with galaxy size is consistent with behavior exhibited by diverse self-gravitating entities. Our calculated mass distributions are consistent with visible luminosity and require no non-baryonic component.
      Citation: Galaxies
      PubDate: 2020-02-26
      DOI: 10.3390/galaxies8010019
      Issue No: Vol. 8, No. 1 (2020)
       
  • Galaxies, Vol. 8, Pages 20: Luminous Blue Variables

    • Authors: Kerstin Weis, Dominik J. Bomans
      First page: 20
      Abstract: Luminous Blue Variables are massive evolved stars, here we introduce this outstanding class of objects. Described are the specific characteristics, the evolutionary state and what they are connected to other phases and types of massive stars. Our current knowledge of LBVs is limited by the fact that in comparison to other stellar classes and phases only a few “true” LBVs are known. This results from the lack of a unique, fast and always reliable identification scheme for LBVs. It literally takes time to get a true classification of a LBV. In addition the short duration of the LBV phase makes it even harder to catch and identify a star as LBV. We summarize here what is known so far, give an overview of the LBV population and the list of LBV host galaxies. LBV are clearly an important and still not fully understood phase in the live of (very) massive stars, especially due to the large and time variable mass loss during the LBV phase. We like to emphasize again the problem how to clearly identify LBV and that there are more than just one type of LBVs: The giant eruption LBVs or η Car analogs and the S Dor cycle LBVs.
      Citation: Galaxies
      PubDate: 2020-02-29
      DOI: 10.3390/galaxies8010020
      Issue No: Vol. 8, No. 1 (2020)
       
  • Galaxies, Vol. 8, Pages 21: (Sub)mm-Wavelength Observations of
           Pre-Planetary Nebulae and Young Planetary Nebulae

    • Authors: Carmen Sánchez Contreras
      First page: 21
      Abstract: This is a non-comprehensive review of observations of pre-Planetary Nebulae (pPNe) and young Planetary Nebulae (yPNe) at (sub)mm-wavelengths, a valuable window for probing multi-phased gas and dust in these objects. This contribution focuses on observations of molecular lines (from carbon monoxide—CO—and other species), and briefly at the end, on hydrogen radio recombination lines from the emerging H ii regions at the center of yPNe. The main goal of this contribution is to show the potential of (sub)mm-wavelength observations of pPNe/yPNe to help the community to devise and develop new observational projects that will bring us closer to a better understanding of these latest stages of the evolution of low-to-intermediate (∼0.8–8 M ⊙ ) mass stars.
      Citation: Galaxies
      PubDate: 2020-03-10
      DOI: 10.3390/galaxies8010021
      Issue No: Vol. 8, No. 1 (2020)
       
  • Galaxies, Vol. 8, Pages 22: Diagnosing Magnetic Field Geometry in Blazar
           Jets Using Multi-Frequency, Centimeter-Band Polarimetry and Radiative
           Transfer Modeling

    • Authors: Margo Aller, Philip Hughes, Hugh Aller, Talvikki Hovatta
      First page: 22
      Abstract: We use multi-frequency linear polarization observations from the University of Michigan blazar program (UMRAO), in combination with radiative transfer simulations of emission from a relativistic jet, to investigate the time-dependent flow conditions, including magnetic field geometry, in an example blazar OT 081. We adopt a scenario incorporating relativistic shocks during flaring, and both ordered axial and helical magnetic field components and magnetic turbulence in the underlying flow; these constituents are consistent with the observed periods of ordered behavior in the polarization intermixed with stochastic variations. The simulations are able to reproduce the global features of the observed light curves, including amplitude and spectral evolution of the linear polarization, during four time periods spanning 25 years. From the simulations, we identify the signature of a weak-to-strong helical magnetic field on the polarization, but conclude that a dominant helical magnetic field is not consistent with the UMRAO polarization data. The modeling identifies time-dependent changes in the ratio of the ordered-to-turbulent magnetic field, and changes in the flow direction and Lorentz factor. These suggest the presence of jet-like structures within a broad envelope seen at different orientations.
      Citation: Galaxies
      PubDate: 2020-03-10
      DOI: 10.3390/galaxies8010022
      Issue No: Vol. 8, No. 1 (2020)
       
  • Galaxies, Vol. 8, Pages 23: WORKPLANS: Workshop on Planetary Nebula
           Observations

    • Authors: Isabel Aleman, Jeronimo Bernard-Salas, Joel H. Kastner, Toshiya Ueta, Eva Villaver
      First page: 23
      Abstract: This workshop is the second of the WORKPLANS series, which we started in 2016. The main goal of WORKPLANS is to build up a network of planetary nebulae (PNe) experts to address the main open questions in the field of PNe research. The specific aims of the WORKPLANS workshop series are (i) to discuss and prioritize the most important topics to be investigated by the PN community in the following years; (ii) to establish a network of excellent researchers with complementary expertise; (iii) to formulate ambitious observing proposals for the most advanced telescopes and instrumentation presently available (ALMA, SOFIA, VLT, GTC, HST, etc.), addressing those topics; and (iv) to develop strategies for major proposals to future observatories (JWST, ELT, SPICA, Athena, etc.). To achieve these goals, WORKPLANS II brought together experts in all key sub-areas of the PNe research field, namely: analysis and interpretation of PNe observational data; theoretical modeling of gas and dust emission; evolution from Asymptotic Giant Branch stars (PNe progenitors) to PNe; and the instrumentation and technical characteristics of the relevant observatories.
      Citation: Galaxies
      PubDate: 2020-03-16
      DOI: 10.3390/galaxies8010023
      Issue No: Vol. 8, No. 1 (2020)
       
  • Galaxies, Vol. 8, Pages 24: X-ray Observations of Planetary Nebulae since
           WORKPLANS I and Beyond

    • Authors: Martín A. Guerrero
      First page: 24
      Abstract: Planetary nebulae (PNe) were expected to be filled with hot pressurized gas driving their expansion. ROSAT hinted at the presence of diffuse X-ray emission from these hot bubbles and detected the first sources of hard X-ray emission from their central stars, but it was not until the advent of Chandra and XMM-Newton that we became able to study in detail their occurrence and physical properties. Here I review the progress in the X-ray observations of PNe since the first WORKshop for PLAnetary Nebulae observationS (WORKPLANS) and present the perspective for future X-ray missions with particular emphasis on eROSITA.
      Citation: Galaxies
      PubDate: 2020-03-17
      DOI: 10.3390/galaxies8010024
      Issue No: Vol. 8, No. 1 (2020)
       
  • Galaxies, Vol. 8, Pages 25: Gamma-Ray Dark Matter Searches in Milky Way
           Satellites—A Comparative Review of Data Analysis Methods and Current
           Results

    • Authors: Javier Rico
      First page: 25
      Abstract: If dark matter is composed of weakly interacting particles with mass in the GeV-TeV range, their annihilation or decay may produce gamma rays that could be detected by gamma-ray telescopes. Observations of dwarf spheroidal satellite galaxies of the Milky Way (dSphs) benefit from the relatively accurate predictions of dSph dark matter content to produce robust constraints to the dark matter properties. The sensitivity of these observations for the search for dark matter signals can be optimized thanks to the use of advanced statistical techniques able to exploit the spectral and morphological peculiarities of the expected signal. In this paper, I review the status of the dark matter searches from observations of dSphs with the current generation of gamma-ray telescopes: Fermi-LAT, H.E.S.S, MAGIC, VERITAS and HAWC. I will describe in detail the general statistical analysis framework used by these instruments, putting in context the most recent experimental results and pointing out the most relevant differences among the different particular implementations. This will facilitate the comparison of the current and future results, as well as their eventual integration in a multi-instrument and multi-target dark matter search.
      Citation: Galaxies
      PubDate: 2020-03-17
      DOI: 10.3390/galaxies8010025
      Issue No: Vol. 8, No. 1 (2020)
       
  • Galaxies, Vol. 8, Pages 26: Shaping Planetary Nebulae with Jets and the
           Grazing Envelope Evolution

    • Authors: Noam Soker
      First page: 26
      Abstract: I argue that the high percentage of planetary nebulae (PNe) that are shaped by jets show that main sequence stars in binary systems can accrete mass at a high rate from an accretion disk and launch jets. Not only does this allow jets to shape PNe, but this also points to the importance of jets in other types of binary systems and in other processes. These processes include the grazing envelope evolution (GEE), the common envelope evolution (CEE), and the efficient conversion of kinetic energy to radiation in outflows. Additionally, the jets point to the possibility that many systems launch jets as they enter the CEE, possibly through a GEE phase. The other binary systems in which jets might play significant roles include intermediate-luminosity optical transients (ILOTs), supernova impostors (including pre-explosion outbursts), post-CEE binary systems, post-GEE binary systems, and progenitors of neutron star binary systems and black hole binary systems. One of the immediate consequences is that the outflow of these systems is highly-non-spherical, including bipolar lobes, jets, and rings.
      Citation: Galaxies
      PubDate: 2020-03-18
      DOI: 10.3390/galaxies8010026
      Issue No: Vol. 8, No. 1 (2020)
       
  • Galaxies, Vol. 8, Pages 27: The UV Perspective of Low-Mass Star Formation

    • Authors: P. Christian Schneider, H. Moritz Günther, Kevin France
      First page: 27
      Abstract: The formation of low-mass ( M ★ ≲ 2 M ⊙ ) stars in molecular clouds involves accretion disks and jets, which are of broad astrophysical interest. Accreting stars represent the closest examples of these phenomena. Star and planet formation are also intimately connected, setting the starting point for planetary systems like our own. The ultraviolet (UV) spectral range is particularly suited for studying star formation, because virtually all relevant processes radiate at temperatures associated with UV emission processes or have strong observational signatures in the UV range. In this review, we describe how UV observations provide unique diagnostics for the accretion process, the physical properties of the protoplanetary disk, and jets and outflows.
      Citation: Galaxies
      PubDate: 2020-03-21
      DOI: 10.3390/galaxies8010027
      Issue No: Vol. 8, No. 1 (2020)
       
 
JournalTOCs
School of Mathematical and Computer Sciences
Heriot-Watt University
Edinburgh, EH14 4AS, UK
Email: journaltocs@hw.ac.uk
Tel: +00 44 (0)131 4513762
 


Your IP address: 3.232.96.22
 
Home (Search)
API
About JournalTOCs
News (blog, publications)
JournalTOCs on Twitter   JournalTOCs on Facebook

JournalTOCs © 2009-