for Journals by Title or ISSN for Articles by Keywords help

Publisher: Springer-Verlag (Total: 2574 journals)

Similar Journals
 Astrophysics and Space ScienceJournal Prestige (SJR): 0.616 Citation Impact (citeScore): 1Number of Followers: 43      Hybrid journal (It can contain Open Access articles) ISSN (Print) 1572-946X - ISSN (Online) 0004-640X Published by Springer-Verlag  [2574 journals]
• The 27-day periodicity and its sub harmonics
• Abstract: Significant short-term variations of ≈1.7, ≈2.4, ≈2.8, ≈3.3, ≈3.8 days periods are found when high resolution data (five minutes average) of interplanetary electric field (Ey), south ward component of magnetic field (Bz) and geomagnetic index (AE) is subjected to wavelet analysis. The fourth, fifth, sixth sub harmonics of ≈27-day period, and few other short-term periods are significantly reported by Singh and Badruddin (Astrophys. Space Sci. 359:60, 2015a; Solar Phys. 290:3071, 2015b and Solar Phys. 294:27, 2019) in cosmic ray intensity, geomagnetic activity index and various solar wind plasma and field parameters. The reported and observed periods ≈6.8 day, ≈5.5 day, ≈4.4 day, ≈3.9 day, ≈3.4 day, ≈3.0 day, ≈2.8 day, ≈2.4 day, ≈2.2 day, ≈2.0 day, ≈1.9 day, ≈1.8 day; and ≈1.7 day are close to integral multiples of 1/4, 1/5, 1/6, 1/7, 1/8, 1/9, 1/10, 1/11, 1/12, 1/13, 1/14, 1/15 and 1/16 of one solar rotation period (≈27.0 day), hence these periods could be sub harmonics of fundamental period (≈27 day). If so, then these results lead to infer some sort of continuous simultaneous oscillations of active regions having integral multiple frequencies and varying amplitudes occurring on the solar disc.
PubDate: 2019-11-15

• Evolution implications of neutron star magnetic fields: inferred from
pulsars and cyclotron lines of HMXBs
• Abstract: The evolution of neutron star (NS) magnetic field (B-field) has long been an important topic, which is still not yet settled down. Here, we analyze the NS B-fields inferred by the cyclotron resonance scattering features (CRSFs) for the high mass X-ray binaries (HMXBs) and by the magnetic dipole model for the spin-down pulsars. We find that the B-fields of both the 32 NS-HMXBs and 28 young pulsars with the supernova remnants follow the log-normal distributions, with the average values of $$3.4\times 10^{12}$$ G and $$4.1\times 10^{12}$$ G respectively, which are further verified to come from the same continuous distribution by the statistical tests. These results declaim that the two methods of measuring NS B-fields are reliable for the above two groups of samples. In addition, since the NS-HMXBs have experienced the spin-down phase as the normal pulsars without accretion and then the spin-up phase by accretion, their ages should be about million years (Myrs). Our statistical facts imply that the B-fields of NS-HMXBs have little decayed in their non-accretion spin-down phases of ∼ Myrs, as well as in their accretion phases of ∼0.1 Myrs.
PubDate: 2019-11-14

• Acoustic α$\alpha$ -disk
• Abstract: It is shown that the turbulent flow of acoustic waves propagating outward from the inner edge of the disk causes the accretion of the matter onto the center. The exponential amplification of waves takes place in the resonance region, $$\omega = (n\pm 1)\Omega$$. Here $$\omega$$ is the frequency of the acoustic wave, $$n$$ is its azimuthal wave number, $$\Omega (r)$$ is the angular frequency of rotation of the disk. The effect is similar to the inverse Landau damping in a collisionless plasma. Energy comes from the energy of rotation of the disk. That leads to decrease of the disk angular momentum and to accretion of the matter. The value of the accretion rate $$dM/dt$$ is $${\dot{M}} = \pi rc_{s} \Sigma _{0} (c_{s} / v_{\phi 0})^{2} W$$. Here $$c_{s}$$ is the speed of sound of the disk gas, $$v_{\phi 0}$$ is the Keplerian rotation velocity, $$\Sigma _{0}$$ is the surface density of the disk, $$W$$ is total power of the acoustic turbulence, $$W \simeq \int _{0}^{\infty }d \omega \sum_{n\geq 0} \frac{\Sigma'}{\Sigma _{0}} ^{2} (\omega , n)$$, $$\Sigma' ^{2} (\omega , n)$$ is the spectral power of turbulence. The presented picture of accretion is consistent with the observed variations of X-ray and optical radiation from objects whose activity is associated with accretion of gas onto them.
PubDate: 2019-11-14

• Complexity factor for a class of compact stars in f(R,T)$f(R,T)$ gravity
• Abstract: We investigate the concept of complexity factor for a class of compact star in the framework of modified $$f(R,T)$$ gravity. We obtain a generic form of hydrostatic equilibrium equation, express the Einstein field equations, mass function and also physical observation for linear form of function $$f(R,T)=R+2\lambda T$$, where $$\lambda$$ is the coupling parameter, $$R$$ is Ricci scalar and $$T$$ is trace of energy momentum tensor. We have analyzed the properties of compact astrophysical objects like energy density and anisotropic pressure are affected by changing the values of coupling parameter $$\lambda$$. We obtained numerical outputs of some physical variables for different chosen values of coupling parameter $$\lambda$$ to observe the effect of $$\lambda$$ on these quantities and show these in tabular form for different compact stars $$4U 1820\mbox{-}30$$, $$\mathit{Her} X\mbox{-}1$$, $$\mathit{SAX} J 1808.4\mbox{-}3658$$ and $$\mathit{VelaX}\mbox{-}12$$ with radii 10, 7.7, 7.07 and 9.99 respectively. We determine structure scalars with orthogonal splitting of the Riemann tensor and with the help of these scalars the complexity factor can be determined. Furthermore, we have checked some astrophysical sources for vanishing complexity factor.
PubDate: 2019-11-08

• The eruptive latitude of the solar flares during the Carrington rotations
(CR1986-CR2195)
• Abstract: In this study, we are investigating the solar flare events during each Carrington rotation in the period 2002–2017. We studied the relationship between solar flare event location and solar cycle progress (phases). The solar flare events are tending to accumulate around a specific latitude line in the southern and northern hemispheres of the solar disk, which we call the “eruptive latitude”. The eruptive latitude is migrating towards the solar equator during the declining phase and away from the solar equator during the ascending phase. The eruptive latitude is consistent with the sunspot butterfly diagram. We found the suitable equation describing the relationship between the eruptive latitude $$\varphi$$ and the Carrington rotation number $$\mathit{CR}$$ using sinusoidal summation function.
PubDate: 2019-11-08

• Bulk viscous string cosmological models in Saez-Ballester theory of
gravity
• Abstract: In this communication, a new class of bulk viscous string cosmological models has been constructed in Saez-Ballester theory of gravitation. To obtain the deterministic solution of the field equations, we have considered deceleration parameter as a bilinear function of cosmic time $$t$$ for model I and special parametrization of Hubble parameter for model II. The presented class of the cosmological models indicate phase conversion from early decelerated expansion phase to present accelerated expansion phase. To discuss the dynamicity of the universe, the behaviour of various physical parameters has also been studied and presented graphically. For stability analysis, the nature of various energy conditions is investigated. Statefinder pair analysis is used to discriminate the constructed models with other dark energy models and it is noticed that the proposed models are in good agreement with recent observational data.
PubDate: 2019-11-08

• Analysis of signal strength, satellite visibility, position accuracy and
ionospheric TEC estimation of IRNSS
• Abstract: The Indian Regional Navigation Satellite System (IRNSS) is an indigenously developed satellite navigation system to meet practical needs. The system, comprising a constellation of seven satellites in GEO orbit, has been fully operational since 2018. It is essential to evaluate the performance of the IRNSS continuously for various applications. As a part of ISRO field trial and data collection program an IRNSS Standard Positioning Service User Receiver (UR) placed at ACS College of Engineering (ACSCE), Bangalore, for independent field trial and data collection. The receiver is operational on a $$24\times 7$$ basis. A MATLAB Graphical User Interface has been developed to analyze and plot the data variation of signal strength, elevation angle, visibility of satellites, user position, position error, geometric dilution of precision (GDOP) and find the availability of the number of satellites plotted for every second based on the received data. From the results, it is observed that the signal strength (C/No) is good, i.e. above 40 dBHz, visibility of satellites at receiver location is good. The mean position at user location is found to be $$X=1349700~\mbox{m}$$, $$Y = 6070902~\mbox{m}$$, $$Z = 1413860~\mbox{m}$$ and latitude of 12.8914 degree, longitude of 77.465 degree and altitude of 739 m. The mean position from IRNSS is compared with Google map results showing a good match. The geometry distance of receiver location with respect to Earth center is estimated and observed. The RMS of position error for L1, L5 and dual frequency (L5+S) at ACSCE, Bangalore, is 9.7444 m, 6.6873 m and 5.6667 m, respectively. Hence, as expected the dual-frequency (L5+S) receiver gives an accurate position rather than the single-frequency signals. The IRNSS TEC is measured using Ionospheric group delay and the pseudo-range of L5 and S band which is collected from ACSCE receiver. A third order Savitzky-Golay-Filtered technique is used for TEC smoothing. RMSE between IRNSS TEC from pseudo-range and GPS TEC is 0.6482 TECU and the correlation coefficient is 0.9981. RMSE between IRNSS TEC from ionospheric delay of L5/S and GPS TEC is 1.971 TECU and the correlation coefficient is 0.9966. Finally, smoothed TEC values derived from pseudo-range measurements give a good result and can be used to generate daily TEC maps. In order to analyze the performance of IRNSS over the Indian region, we have chosen three other receiver stations located at Osmania University (Hyderabad), University of Burdwan (Bardhaman, West Bengal) and Shri Mata Vaishno Devi University (Katra, Jammu and Kashmir). Based on the results, we conclude that the IRNSS constellation is performing well and providing good signals for accurate user position determination and ionospheric data analysis.
PubDate: 2019-11-08

• Strongly screening γ$\gamma$ -ray heating rates by β−$\beta^{-}$ decay
of 59 Co, 55 Co, 56 Ni, and 56 Fe in presupernova
• Abstract: Based on the Shell-Model, and the linear response theory, we discuss the influence of strong electron screening (SES) on the $$\gamma$$-ray heating rates (GRHR) by beta decay for several typical iron group nuclei. We calculate the screening potential caused by the relativistically degenerate electron. We also investigate the effect of SES on beta decay Q-value, electron chemical potential, as well as the shell and pair effects. We find that the GRHR increase by several orders magnitude and the screening enhancement factor can get to 76.83 due to SES.
PubDate: 2019-11-07

• An investigation on time dependency of K index-based geomagnetic storm
conditions observed over different locations of North America
• Abstract: Today, the scientists and researchers from almost every corner of the world are well aware of the adverse effects of geomagnetic storms on our highly expensive electrical and electromagnetic systems such as power grid transmission, satellite communication, RF communication systems, etc. The severe damaging effects of these storms on human health and on other inhabitants of the biosphere are also not unknown. A survey of literature on space science reveals that location or space dependency of these geomagnetic storms has already been established. Now the question may arise—Are geomagnetic storms time-dependent' To answer this question, a laborious exercise has been performed. K-index data obtained from the US Department of Commerce, NOAA, Space Weather Prediction Center, for the year, started from 1994 to 2014 (21 years) over three different locations namely Fredericksburg, College, and Estimated Planetary, are analyzed statistically for the eight different time periods namely 00:00–03:00, 03:00–06:00, 06:00–09:00, 09:00–12:00, 12:00–15:00, 15:00–18:00, 18:00–21:00, 21:00–24:00 hours respectively. Results thus obtained have concluded that apart from space or location dependency the geomagnetic storms are time-dependent also.
PubDate: 2019-11-07

• Life on Venus and the interplanetary transfer of biota from Earth
• Abstract: Evidence and observations favoring the hypothesis that Venus is habitable, and the celestial mechanisms promoting the interplanetary transfer of life, are reviewed. Venus may have been contaminated with Earthly life early in its history via interplanetary transfer of microbe-laden bolide ejecta; and this seeding with life may have continued into the present via spacecraft and due to radiation pressure and galactic winds blowing microbial-laden dust ejected from the stratosphere via powerful solar winds, into the orbit and atmosphere of Venus. Venus may have had oceans and rivers early in its history until 750 mya, and, hypothetically, some of those species which, theoretically, colonized the planet during that time, may have adapted and evolved when those oceans evaporated and temperatures rose. Venus may be inhabited by a variety of extremophiles which could flourish within the lower cloud layers, whereas others may dwell 10 m below the surface where temperature may be as low as 200 ∘C—which is within the tolerance level of some hyperthermophiles. Speculation as to the identity of mushroom-shaped specimens photographed on the surface of Venus by the Russian probe, Venera 13 support these hypotheses.
PubDate: 2019-11-06

• Charged gravastars with conformal motion in f ( R , T ) $f(R,T)$ gravity
• Abstract: This paper studies the effects of charge on a peculiar stellar object, recognized as gravastar, under the influence of $$f(R,T)$$ gravity by considering the conjecture of Mazur and Mottola in general relativity. The gravastar is also known as an alternative to a black hole and is expressed by three distinct domains named as (i) the interior domain, (ii) the intermediate shell and (iii) the exterior domain. We analyze these domains for a specific $$f(R,T)$$ gravity model conceding the conformal Killing vectors. In the interior domain, we assume that pressure is equal to negative energy density which leads to the existence of repulsive force on the spherical shell. The intermediate shell consists of ultra-relativistic plasma and pressure which shows a direct relation with energy density and counterbalances the repulsive force applied by the interior domain. The exterior vacuum spherical domain is taken to be the de Sitter spacetime illustrated by the Reissner-Nordström metric. We conclude that non-singular solutions of charged gravastar with various physical properties such as length, energy, entropy and equation of state parameter are physically consistent.
PubDate: 2019-11-05

• Kaluza-Klein dark energy model in Lyra manifold in the presence of massive
scalar field
• Abstract: In this investigation we intend to study the dynamics of an anisotropic dark energy cosmological model in the presence of a massive scalar field in a modified Riemannian manifold proposed by Lyra (Math. Z. 54:52, 1951) in the background of a five dimensional Kaluza-Klein space time. We solve the Einstein field equations using some physically significant conditions and present a deterministic dark energy cosmological model. We use here the time dependent displacement vector field of the Lyra manifold. All the dynamical parameters of the model, namely, average Hubble parameter, anisotropy parameter, equation of state parameter, dark energy density, deceleration parameter and statefinders are evaluated for our model and their physical relevance to modern cosmology is discussed in detail.
PubDate: 2019-11-05

• Possible distribution of mass inside a black hole. Is there any upper
limit on mass density'
• Abstract: The maximum mass of a neutron star is about three solar masses. In this case the radius of such neutron star is approximately equal to the Schwarzschild radius. Adding a small amount of matter to this star, a black hole arises. Thus its interior could contain a star with neutron or quark density just below the event horizon instead of the proposed point singularity. We also show that the Hawking miniature black hole evaporation is improbable, since it would yield unrealistic mean mass densities.
PubDate: 2019-11-04

• Statistical analysis of interplanetary coronal mass ejections and their
geoeffectiveness during the solar cycles 23 and 24
• Abstract: Interplanetary coronal mass ejections (ICMEs) are believed to be the most common and important drivers of the strongest geomagnetic storms. In this work, the geoeffective characteristics of the ICMEs occurred during the last solar cycles 23 and 24 (years 1996–2017) have been studied in detail. The maximum velocity $$V$$ max, either of the ICME’s Sheath region or of the ICME itself, the mean velocity of the ICME, the minimum value of the southward component of the Interplanetary magnetic field Bs and the $$y$$ -component of the solar wind convective electric field $$E= -V \times B$$ observed at L1 point during the pass of the ICME, were used. It was found that, in accordance to past similar studies, the most dominant characteristic of ICMEs in the generation of geomagnetic storms is the Bs component along with the Ey parameter, while the maximum velocity seems to be of less importance. Nevertheless the maximum speed is an good forecasting factor due to the fact that it is much easier to estimate the velocity of an ICME-structure many hours before it arrives at Earth compared with the observations of Bs and Ey that can only be done, for the time being, at the L1 point. That means we can use the velocity of ICME-structure to forecast the possible generation and magnitude of the geomagnetic storms. From a comparison of the ICME-generated geomagnetic storms with the total number of geomagnetic storms generated during the last two solar cycles, it seems that approximately half of the ICMEs (49% for Dst index and 53% for Kp index) produced geomagnetic storms during the solar cycles 23 and 24. Moreover the velocities of ICMEs are more in accordance with the rising and maximum phases of solar cycles 23 and 24 than the geomagnetic activity (storms) are, as well as during the first stages of the declining phases of these cycles, especially during solar cycle 23.
PubDate: 2019-10-31

• Charged compact star model in Einstein-Maxwell-Gauss-Bonnet gravity
• Abstract: In this paper we present a model of a static charged anisotropic fluid sphere in the Einstein-Maxwell-Gauss-Bonnet (EMGB) theory of gravitation. We utilize the Krori-Barua (KB) ansatz together with a linear equation of state of the form $$p_{r}=\beta \rho -\gamma$$ to generate exact solutions of the EMGB field equations describing compact objects. The model obtained here is found to satisfy the elementary physical requirements necessary for a physically realizable stellar object. We demonstrate that contributions from the Gauss-Bonnet terms have a nonzero impact on the density, pressure and anisotropy profiles. The vanishing of the electromagnetic field at the center of the stellar configuration leads to a relation between the equation of state parameter and the Gauss-Bonnet term. Our model reveals a direct connection between the nature of the matter configuration and higher dimensional effects.
PubDate: 2019-10-30

• Particle acceleration in interstellar shocks
• Abstract: This review presents the fundamentals of the particle acceleration processes active in interstellar medium (ISM), which are essentially based on the so-called Fermi mechanism theory. More specifically, the review presents here in more details the first order Fermi acceleration process—also known as diffusive shock acceleration (DSA) mechanism. In this case, acceleration is induced by the interstellar (IS) shock waves. These IS shocks are mainly associated with emission nebulae (H ii regions, planetary nebulae and supernova remnants). Among all types of emission nebulae, the strongest shocks are associated with supernova remnants (SNRs). Due to this fact they also provide the most efficient manner to accelerate ISM particles to become high energy particles, i.e. cosmic-rays (CRs). The review therefore focuses on the particle acceleration at the strong shock waves of supernova remnants.
PubDate: 2019-10-28

• On the nature of the motion of a test particle in the pseudo-Newtonian
Hill system
• Abstract: The scope of this work is to perform a numerical investigation of the orbital dynamics for a test particle in the pseudo-Newtonian Hill problem. Large two-dimensional sets of initial conditions of prograde and retrograde orbits are investigated. The orbits are classified as bounded (chaotic, sticky or regular), escaping and collision orbits. The smaller alignment index (SALI) method is used to identify chaotic orbits. Additionally, the influence of the energy (or equivalently the value of the Jacobi constant) and of the Schwarzschild radius on the orbital structure of the system are determined. Our numerical results are compared with related previous ones, corresponding to the classical version of the Hill problem.
PubDate: 2019-10-25

• Existence and stability of equilibrium points in the photogravitational
restricted four-body problem with Stokes drag effect
• Abstract: The restricted four-body problem consists of an infinitesimal particle which is moving under the Newtonian gravitational attraction of three finite bodies, $$m_{1}$$ , $$m_{2}$$ and $$m_{3}$$ . The three bodies (called primaries) are moving in circular orbits around their common centre of mass fixed at the origin of the coordinate system. Moreover, according to the solution of Lagrange, these primaries are fixed at the vertices of an equilateral triangle. The fourth body does not affect the motion of the three bodies. In this paper, we deal with the photogravitational version of the problem with Stokes drag acting as a dissipative force. We consider the case where all the primaries are sources of radiation and that two of the bodies, $$m_{2}$$ and $$m_{3}$$ , have equal masses ( $$m _{2} = m_{3} = \mu$$ ) and equal radiation factors ( $$q_{2} = q_{3} = q$$ ) while the dominant primary body $$m_{1}$$ is of mass $$1 - 2\mu$$ . We investigate the dynamical behaviour of an infinitesimal mass in the gravitational field of radiating primaries coupled with the Stokes drag effect. It is found that under constant dissipative force, collinear equilibrium points do not exist (numerically and of course analytically) whereas the existence and positions of the non-collinear equilibrium points depend on the parameters values. The linear stability of the non-collinear equilibrium points ( $$L_{i},i = 1,2, \ldots ,8$$ ) is also studied and it is found that they are all unstable except $$L_{1}$$ , $$L _{7}$$ and $$L_{8}$$ which may be stable for a range of values of $$\mu$$ and various values of radiation factors. Finally, we justify the relevance of the model in astronomy by applying it to a stellar system (Ross 104-Ross775a-Ross775b), for which all the equilibrium points have been seen to be unstable.
PubDate: 2019-10-24

• Bifurcation of ion-acoustic superperiodic waves in auroral zone of
Earth’s magnetosphere
• Abstract: Ion-acoustic (IA) superperiodic waves are explored in an unmagnetized plasma composing of mobile ions, Maxwell distributed cold electrons and non-thermal hot electrons. Implementing the phase portrait concept of the Hamiltonian systems, all distinct phase plots containing superperiodic and superhomoclinic trajectories are displayed for distinct data values of the physical parameters, such as, nonthermal parameter ( $$\beta$$ ), number density ratio ( $$\mu$$ ) of cold electrons and ions at equilibrium, temperature ratio ( $$\sigma$$ ) of cold electrons to hot electrons and of traveling wave velocity ( $$v$$ ). The IA superperiodic wave features are examined for various conditions of physical parameters by symbolic computations. The outcomes of this study may be beneficial in discerning IA superperiodic wave in auroral zone of the Earth’s magnetosphere.
PubDate: 2019-10-21

• A catalogue of 108 extended planetary nebulae observed by GALEX
• Abstract: We present the ultraviolet (UV) imaging observation of planetary nebulae (PNe) using archival data of Galaxy Evolution Explorer (GALEX). We found 358 PNe detected by GALEX in near-UV (NUV). We have compiled a catalogue of 108 extended PNe with sizes greater than $$8''$$ and provided the angular diameters for all the 108 extended PNe in NUV and 28 in FUV from the GALEX images considering $$3\sigma$$ surface brightness level above the background. Of the 108 PNe, 74 are elliptical, 24 are circular and 10 are bipolar in NUV with most being larger in the UV than in the radio, H $$\alpha$$ or optical. We derived luminosities for 33 PNe in FUV ( $$\mathrm{L}_{\text{FUV}}$$ ) and 89 PNe in NUV ( $$\mathrm{L}_{\text{NUV}}$$ ) and found that most of the sources are very bright in UV. The FUV emission of the GALEX band includes contribution from prominent emission lines N IV] (1487 Å), C IV (1550 Å), and O III] (1661 Å) whereas the NUV emission includes C III] (1907 Å) and C II (2325 Å) for PNe of all excitation classes. The other emission lines seen in low excitation PNe are O IV] (1403 Å) and N III (1892 Å) in FUV, and O II (2470 Å) and Mg II (2830 Å) in NUV. Similarly the emission lines O V (1371 Å) and He II (1666 Å) strongly contribute in FUV for high and medium excitation PNe but not for low excitation PNe. A mixture of other emission lines seen in all excitation PNe. We have also provided images of 34 PNe in NUV and 9 PNe in FUV.
PubDate: 2019-10-21

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
Fax: +00 44 (0)131 4513327

Home (Search)
Subjects A-Z
Publishers A-Z
Customise
APIs