Abstract: Abstract This study aims to examine the dynamics of flat Friedmann-Lemaitre-Robertson-Walker (FLRW) model of universe with time varying cosmological constant \(\Lambda (t)\) in Brans-Dicke theory of gravity. The authors find a solution of the field equations using simple parametrization of scale factor \(a(t)\) i.e. \(a(t)=\exp \{(\alpha t+\beta )^{p}\}\) , and power law relation between \(a(t)\) and Brans-Dicke scalar field \(\phi \) . The derived cosmological model shows transition from early cosmic deceleration to present cosmic acceleration. Such feature of cosmic expansion is in agreement with recent empirical observations. The behaviour of cosmographic parameters such as Hubble parameter, deceleration parameter, jerk, snap and lerk parameters is examined graphically. It is observed that model behaves like \(\Lambda CDM\) model in late cosmic evolution. We also use statefinder and Om diagnostic tools to differentiate various dark energy models from \(\Lambda CDM\) model. Moreover, model parameter \(p\) and present value of deceleration parameter \(q_{0}\) is also estimated using Hubble data set and Pantheon Type 1a supernova data set. PubDate: 2021-01-13

Abstract: Abstract By expanding the perturbation function of tesseral harmonic \(J_{22}\) term of the earth’s non spherical gravitation, it is found that there is a term \(\cos 2\Omega _{e}\) related to the rotation of the earth. The period of the term is half a day, and the coefficient includes the factor \(\sin ^{2}i\) related to the orbital inclination angle. For the low polar orbit satellite with orbit inclination close to \(90^{\circ }\) , \(J_{22}\) term has a great impact on the orbit accuracy, which will reach the order of kilometers. Based on the measured orbit data (historical TLEs and GPS precise ephemeris) of the polar satellite GRACE-A/B, the influence of the tesseral harmonic \(J_{22}\) term on TLE orbit error is further verified. It is found that the system bias caused by ignoring \(J_{22}\) term exists in TLE orbit data for LEO, which shows a periodic change of 180 degrees with the geographic longitude in the in-track direction. In addition, the accuracy of TLE orbit in the northern hemisphere is significantly higher than that in the southern hemisphere, which is related to the distribution of SSN observation network. For GRACE-A satellite, the maximum system bias of TLE orbit can reach close to 1.5 km near the equator of the earth, and 8 km after a 7-day prediction period. PubDate: 2021-01-11

Abstract: Abstract We employ a perturbative analysis to study the evolution of large-scale peculiar velocity fields within the framework of Newtonian gravity and then compare our results to those of the corresponding relativistic treatment. In so doing, we use the same mathematical formalism and apply the same physical approach. This facilitates a direct and transparent comparison between the two treatments. Our study recovers and extends the familiar Newtonian results on the one hand, while on the other it shows that the Newtonian analysis leads to substantially weaker growth-rates for the peculiar velocity field, compared to the relativistic approach. This implies that, by using Newton’s rather than Einstein’s theory, one could seriously underestimate the overall kinematic evolution of cosmological peculiar motions. We are also in the position to identify the reason the two theories arrive at such considerably different results and conclusions. PubDate: 2021-01-06

Abstract: Abstract This paper presents a statistical analysis of soft X-ray flares during the period January 1986 to June 2017 covering the last three solar cycles (SCs) 22, 23, and 24. We have analyzed the distribution of flare number, the relationship between the occurrence rate of soft X-ray flares and solar activity, and the duration of different intensity class (B, C, M, and X) solar flares during the period of investigation. The total occurrence rates of M and X class flares are 10.33%, 6.77%, and 5.45% in SCs 22, 23, and 24, respectively, which indicates that SC 22 is an SC with frequent large flares. Meanwhile, the M and X class flares are mainly concentrated in solar maximum (71.13%) in SC 22. The correlation coefficients between the annual number of flares and solar F10.7 index, X-ray flux in 0.1–0.8 nm, and EUV flux in 0.1–50 nm in SC 24 (0.91, 0.88, and 0.96) are greater than that in SCs 23 (0.87, 0.85, and 0.86) and 22 (0.88 and 0.81). The median values of the duration of M and X class flares are 39.00 and 71.50 minutes in SC 22, 21.00 and 26.50 minutes in SC 23, and 19.00 and 24.00 minutes in SC 24. The duration of M and X class flares in SC 22 is obviously longer than that in SCs 23 and 24. In addition, the duration of flares increases with the increase of flare intensity. PubDate: 2021-01-04

Abstract: Abstract In this paper, the prediction model for ionospheric total electron content (TEC) based on Long Short-Term Memory (LSTM) deep learning network and its performance are discussed. The input parameters of the model are previous values of daily TEC, solar radio flux at 10.7 cm parameter of 81 day moving average ( \(\overline{F107\_81}\) ), sunspot number (SSN), geomagnetic Kp index, and disturbance storm time (Dst) index, and the outputs are TEC values for the target day. TEC data from January 1, 2001 to December 31, 2016 were used in this study. The dataset almost covers most of the years of the last two solar cycles (23, 24), and it is separated as 81.3% for training, 6.2% for validation, and 12.5% for testing. At BJFS IGS station (39.61° N, 115.89° E), LSTM yielded good TEC estimates with an RMSE of 4.07 TECU in 2001, it was 33% and 48% lower than the RMSE observed in TEC prediction using BP and IRI-2016 models, respectively. In the year of low solar activity (2016), the RMSE predicted by LSTM was 1.78 TECU, it provided 30% and 54% lower RMSE for TEC prediction than for BP and IRI-2016 models. Under the condition of magnetic storm, the LSTM TEC predictions are more consistent with the corresponding IGS Global Ionospheric Maps (GIMs) TEC than TEC predictions by BP and IRI-2016 models. LSTM can better grasp the influence of different external conditions on TEC. Seventeen grid points along 120° E meridian in latitude range from 80° S to 80° N were selected to further study the performance of LSTM model in different latitude. Results show that the prediction accuracy of LSTM is better than that of BP at different latitudes, especially at low latitudes. The performances of the two models are highly correlated with latitude and solar activity, and are both better than that of IRI-2016. PubDate: 2021-01-04

Abstract: Abstract We present new observations of the reconnection outflow jet obtained from the crossing of a solar wind reconnection exhaust by the Wind spacecraft on 5 March 2001. The outflow jet is characterized by a bifurcated configuration, which is different from typical reconnection exhausts. The two outflow jets are mainly distributed in the region close to the exhaust boundaries. Between the two jets, the bulging of the magnetic field and the slight depression of the plasma density are observed. The possible mechanism for the bifurcation of the outflow jet has also been investigated by using a magnetohydrodynamics (MHD) simulation of Petschek-type reconnection. The dynamic patterns of the reconnection exhaust are analyzed in detail. We find that the bifurcated jet is one of the transient configurations produced in the magnetic reconnection. Other observed features of the exhaust show similar agreement with the predictions from the simulation. The study of the reconnection exhaust, over multiple spatial scales, provides an insight into the dynamic evolution of solar wind reconnection exhausts. PubDate: 2021-01-04

Abstract: Abstract The Mücket-Treder-type potential was initially introduced to explain the discrepancy between Mercury’s observed perihelion advance and the computed value based on Newton’s law, but it can also be used for many astronomical situations as, for instance, the study of the very eccentric cometary orbits in the neighborhood of the Sun. In this paper we tackle the two-body problem in the Mücket-Treder post-Newtonian field from the particular standpoint of orbits stability. Starting from the equations of motion and first integrals written in standard polar coordinates, we apply McGehee-type transformations of the second kind. Then we depict the phase-space structure considering the foliations by the energy constant and the angular momentum constant. Various stability regions are found for each case. PubDate: 2020-12-21

Abstract: Abstract Metallicities of the Milky Way globular cluster (GC) system are interpreted within the framework of a closed box model of chemical evolution. Model age-metallicity relations normalised to the Sun are compared with an observed age-metallicity relation constructed from literature data for the Galactic globular clusters and solar neighbourhood dwarf stars. One branch of the GC age-metallicity relation is seen to form a natural extension of the solar neighbourhood relation to low metallicities. Such clusters may fit in situ into the same Galactic closed-box model as many disk stars. Timescales for halo and disk GC formation are computed based on this assumption. However, a single closed box cannot account for a second branch in the GC age-metallicity data. This problem is addressed by assuming that some GCs did not form in situ within the closed box of the Galaxy, but rather were formed within separate parent objects whose chemical evolution was characterised by longer star formation timescales than the Milky Way box. These parent objects were eventually acquired by the Milky Way. Estimates are made of the initial mass and star formation timescale of those objects that contributed GCs to the accreted component of the Milky Way. PubDate: 2020-12-21

Abstract: Abstract This paper presents different families of transfers from Low Lunar Orbit to Distant Retrograde Orbits in the Earth-Moon system and analyzes their characteristics from the perspective of velocity increment requirement, transfer time, and sensitivity to small perturbations. Initially, families of transfer trajectories in the Circular Restricted Three-Body Problem are found by the predictor-corrector method and numerical continuation. Each transfer uses two impulsive maneuvers. These families are computed for different parking orbit altitudes, constants of motion, and insertion points. Then, to investigate the sensitivity characteristics of each family, a metric based on Local Lyapunov Exponent is introduced and computed along each trajectory. This work provides various choices for future cargo and human-crewed missions in the cislunar space. PubDate: 2020-12-21

Abstract: Abstract Two parameters such as magnetic fields near the light cylinder \(B_{lc} \) and rates of rotational energy losses \(\frac{dE}{dt} \) for 1795 radio pulsars without high energy emission (R), 52 radio loud gamma-ray pulsars ( \(\gamma\) +R), 33 radio quiet gamma-ray pulsars ( \(\gamma\) ) and 61 radio loud X-ray pulsars (X+R) are compared with each other in this work. Pulsars with registered X-ray and gamma-ray emission have the higher median values of \(B_{lc} \) and \(\frac{dE}{dt} \) as compared to the population of ordinary radio pulsars. There is a trend of increasing of luminosities in X-ray and gamma-ray ranges for higher values of magnetic field near the light cylinder. We hypothesize that non-thermal X-ray and gamma-ray emissions are generated near the light cylinder by the synchrotron mechanism. PubDate: 2020-12-21

Abstract: Abstract In this paper, we present the multi-wavelength study of a high level of solar activity during which a single active region produced multiple flares/CMEs. According to the sunspot observations, the current solar cycle 24 manifest to be less intense in comparison with the previous recent sunspot cycles. In the course of the current sunspot cycle 24, several small and large sunspot groups have produced various moderate and intense flare/CME events. There are a few active regions with a large number of flaring activities passed across the visible disk of the Sun during 2012-2015. In this study, we consider the three periods 22-29 Oct 2013, 01-08 Nov 2013, and 25 Oct-08-Nov 2014, during which 228 flares have been observed. Considering only active regions near the central part of the disk, 59 CMEs (halo or partial) have been reported among which only 39 events are associated with flares. We conclude that an active region with a larger area, more complex morphology and stronger magnetic field has a comparatively higher possibility of producing extremely fast CMEs (speed \(>1500\) km/sec). So that among the 5 X class flares of the reported periods, 3 of them (60%) are associated with a CME. The lift-off time for CME-flare associated events has a +15 to +30 minute time interval range after the occurrence time of associated flares suggesting that the flares produce the CMEs. Additionally, we compiled the geomagnetic storms occurring within 1-5 days after the CME onset. 10% of the 59 CMEs are related to a magnetic storm but all are moderate storms. PubDate: 2020-12-18

Abstract: Abstract This work presents an analytical approach for studying the cosmological 21cm background signal from the Dark Ages (DA) and subsequent Epoch of Reionization (EoR). We simulate differential observations of a galaxy cluster to demonstrate how these epochs can be studied with a specific form of the Sunyaev-Zel’dovich Effect called the SZE-21cm. This work produces simulated maps of the SZE-21cm and shows that the SZE-21cm can be extracted from future observations with low-frequency radio interferometers such as the Hydrogen Epoch of Reionization Array (HERA) and the Square Kilometre Array (SKA). In order to simulate near realistic scenarios, we look into cosmic variance noise, incorporate and take into account the effects of foregrounds, thermal noise, and angular resolution for our simulated observations. We further extend this exploration by averaging over a sample of galaxy clusters to mitigate the effects of cosmic variance and instrumental noise. The impact of point source contamination is also studied. Lastly, we apply this technique to the results of the EDGES collaboration, which in 2018 reported an absorption feature of the global 21cm background signal centred at 78 MHz. The challenges to be addressed in order to achieve the objectives of this work include errors that arise due to cosmic variation, instrumental noise and point source contamination. Our approach demonstrates the potential of the SZE-21cm as an indirect probe for the DA and EoR, and we conclude that the spectral features of the SZE-21cm from our simulated observations yield results that are close to prior theoretical predictions and that the SZE-21cm can be used to test the validity of the EDGES detection. PubDate: 2020-12-16

Abstract: Abstract The coincidental detection of the gravitational wave event GW 170817 and the gamma-ray burst GRB 170817A marked the advent of multi-messenger astronomy and represented a milestone in the study of GRBs. Significant progress in this field is expected in the coming years with the increased sensitivity of gravitational waves detectors and the launch of new facilities for the high-energy survey of the sky. In this context, the launch of SVOM in mid-2022, with its two wide-field high-energy instruments ECLAIRs and GRM, will foster the possibilities of coincidental transient detection with gravitational waves and gamma-rays events. The purpose of this paper is to assess the ability of SVOM/ECLAIRs to detect and quickly characterize high-energy transients in the local Universe (z ≤ 0.3), and to discuss the contribution of this instrument to multi-messenger astronomy and to gamma-ray burst (GRB) astrophysics in the 2020’s. A list of local HE transients, along with their main characteristics, is constructed through an extensive literature survey. This list includes 41 transients: 24 long GRBs, 10 short GRBs and 7 SGR Giant Flares. The detectability of these transients with ECLAIRs is assessed with detailed simulations using tools developed for the SVOM mission, including a GEANT4 simulation of the energy response and a simulated trigger algorithm representative of the onboard trigger algorithm. SVOM/ECLAIRs would have been able to detect 88% of the short high-energy transients in our list: 22 out of 24 long GRBs, 8 out of 10 short GRBs and 6 out of 7 SGR Giant Flares. The SNR for almost all detections will be sufficiently high to allow the on-board ECLAIRs trigger algorithm to derive the localisation of the transient, transmitting it to the SVOM satellite and ground-based instruments. Coupled with the anti-solar pointing strategy of SVOM, this will enable an optimal follow-up of the events, allowing the observation of their afterglows, supernovae/kilonovae counterparts, and host galaxies. We conclude the paper with a discussion of the unique contribution expected from SVOM and of the possibility of simultaneous GW detection for each type of transient in our sample. PubDate: 2020-12-14

Abstract: Abstract The regional ionospheric models are successful in capturing the variability of ionosphere with the inclusion of local ground-based GPS observations and location dependent ionospheric dynamics. In this context, there is a need to develop regional ionospheric maps that aids in improving the consistency of global models. In this paper, an attempt is made to understand the potentiality of multi-instrument observations over Indian region. Four different Total Electron Content (TEC) data sources namely from network of GPS receivers, Ionosonde stations, space based COSMIC radio occultation profiles and SWARM mission data is utilized. The multi-source data is chosen for the geo-magnetic storm conditions prevailed during March 2015. Data from multiple-sources is observed over the period from 15th March 2015 to 20th March 2015. Validation of ground and space-based TEC data with International GNSS service (IGS) station data is significantly observed. PubDate: 2020-12-14

Abstract: Abstract We investigate whether certain extra-solar multi-planet systems simultaneously follow the scaling and spacing rules of the angular-momentum-deficit model. The masses and semi-major axes of exoplanets in ten multi-planet systems are considered. It is found that GJ 667C, HD 215152, HD 40307, and Kepler-79 systems are currently close to configurations of the angular-momentum-deficit model. In a gas-poor scenario, GJ 3293, HD 141399, and HD 34445 systems are those which had a configuration of the angular-momentum-deficit model in the past and get scattered away due to post gaseous effects. In addition, no matter in gas-free or gas-poor scenario, 55 Cnc, GJ 876, and WASP-47 systems do not follow the angular-momentum-deficit model. Therefore, our results reveal important formation histories of these multi-planet systems. PubDate: 2020-12-14

Abstract: Abstract Impacts of solar flare vary at different parts of the lower ionosphere depending on it’s proximity to the direct exposure of incoming solar radiation. The quantitative analysis of this phenomena can be attributed to ‘solar zenith angle ( \(\chi (t)\) )’ profile over ionosphere. We numerically solve the ‘electron continuity equation’ to obtain the lower ionospheric electron density profile ( \(N_{e}(t)\) ). The electron production rate ( \(q(t)\) ) is governed by the (i) X-ray profile ( \(\phi (t)\) ) of the flare, (ii) \(\chi (t)\) -values during the flare occurrence etc. For analyzing the X-ray profile during flares, we use the GOES-15 satellite observations. Since we’re working on electron continuity equation based simplified ionospheric model, we confined our analysis for comparatively stable mid-latitude ionosphere only. We choose three flares each from C, M and X-classes for \(N_{e}(t)\) -profile computation. We observe that temporal \(N_{e}(t)\) -profiles differ when computed for lower ionosphere over different discrete latitudes. Further, we compute the spatial \(N_{e}(t)\) -profile across mid-latitude at the time when \(\phi (t)=\phi _{max}\) . Now we assume that, these flares repeat themselves every day of a year ( \(DoY\) ) at the same time of a day and we compute \(N_{e}(t)\) -profiles for each day. We found a seasonal effect on \(N_{e}(t)\) -profile due to solar flare. Further, we investigate the response time delay ( \(\Delta t\) ) of the lower ionosphere, which is the time difference between incidence of X-ray and the respective change in \(N_{e}(t)\) -profiles during solar flares. Strong seasonal effects on \(N_{e}(t)\) -profile and \(\Delta t\) are the unique results of this work. PubDate: 2020-12-11

Abstract: Abstract Despite that the tricarbon monosulfide (C3S) is among the first sulfur-containing carbon-chain molecules to be detected in the interstellar medium, no studies focused on the determination of its collisional rates. These rate coefficients are essential to estimate the abundance of C3S in the interstellar medium. Computations of the C3S( \(^{1}\Sigma^{+}\) ) downward rate coefficients, induced by collision with He, are performed by averaging the integral cross sections at low temperature (below \(25~\text{K}\) ). Calculations of the cross sections in the close-coupling quantum time independent formalism for \(E_{c}\leq110~\text{cm}^{-1}\) and \(J\leq10\) are based on a new 2-D potential energy surface. This PES is obtained from the explicit correlated coupled cluster with a single, double and perturbative triple excitation [ccsd(t)-f12] ab initio approach and the aug-cc-pVTZ basis sets. The PES have a global minimum of \(-55.69~\text{cm}^{-1}\) located at \(R=6.25\) bohr and \(\theta=94^{\circ}\) , and a second minimum of \(-36.95~\text{cm}^{-1}\) at \(R=9.35\) bohr and \(\theta=0^{\circ}\) . A comparison of C3S rates with those of the isoelectronic molecule C3O was made. The results indicate a great temperature dependence of the rates for transitions of \(\Delta J>2\) . We expect that the new collisional data will allow for accurate determination of the C3S abundance in several interstellar regions. PubDate: 2020-12-07

Abstract: Abstract The global features of the modulation of galactic cosmic ray protons and helium nuclei in a very quiet heliosphere are studied with a comprehensive, three-dimensional, drift model and compared to proton and helium observations measured by PAMELA from 2006 to 2009. Combined with accurate very local interstellar spectra (VLIS) for protons and helium nuclei, this provides the opportunity to study in detail how differently cosmic ray species with dissimilar mass-to-charge ratio ( \(A/Z\) ) are modulated down to a few GV. The effects at Earth of the difference in their VLIS’s and those caused by the main modulation mechanisms are illustrated. We find that both the PAMELA proton and helium spectra are reproduced well with the numerical model, assuming the same set of modulation parameters and diffusion coefficients. A comparative study of 3He2 (He-3) and 4He2 (He-4) modulated spectra reveals that they do not undergo identical spectral changes below 3 GV mainly due to differences in their VLIS’s. This result is important to uncover and investigate the effects on the proton to total helium ratio ( \(p\) /He) caused by the difference in their VLIS’s and those by \(A/Z\) . The computed \(p\) /He displays three modulation regimes, reflecting the complex interplay of modulation processes in the heliosphere. At rigidities above ∼3 GV, the \(p\) /He ratio at the Earth is found to deviate modestly from a value of ∼5.5, largely independent of the assumed modulation conditions. This result indicates that the PAMELA measurement of \(p\) /He reveals at these rigidities the shapes of their VLIS’s. Below ∼0.6 GV, \(p\) /He increases with decreasing rigidity from 2006 to 2009 and significant variations are predicted depending on the assumed solar modulation conditions. This result indicates that as modulation levels decreased from 2006 to 2009, the contribution of adiabatic energy changes dissipated faster for protons than for helium nuclei at the same rigidity mainly due to different slopes of their VLIS’s. The differences between modulation effects for protons and helium are found to be the consequence of how the combined interplaying modulation mechanisms in the heliosphere affect the modulated spectra based on their \(A/Z\) and particularly on their VLIS’s. PubDate: 2020-11-27

Abstract: Abstract We demonstrate a technique to generate new class of exact solutions to the Einstein-Maxwell system describing a static spherically symmetric relativistic star with anisotropic matter distribution. An interesting feature of the new class of solutions is that one can easily switch off the electric and/or anisotropic effects in this formulation. Consequently, we show that a plethora of well known stellar solutions can be identified as sub-class of our class of solutions. We demonstrate that it is possible to express our class of solutions in a simple closed form so as to examine its physical viability for the studies of relativistic compact stars. PubDate: 2020-11-23

Abstract: Abstract This paper explores and analyzes a set of solutions describing the interior structure of relativistic compact stellar structures with variable cosmological constant \(\Lambda (r)\) . We consider the solution of Krori–Barua space-time to a static spherical symmetric metric. Furthermore, we match our interior stellar structure with the exterior Schwarzschild geometry to determine the values of the constants used in the solution of the Krori–Barua space-time. The numerical values of these constants were determined for a set of different compact stars, and using these constants in our solutions, we have studied the viability of matter content, stability, TOV equations, and surface red-shift; and we predicted some physical aspects like central and surface densities, stresses, masses, and radii. PubDate: 2020-11-20