• Volume 40, Issue 5

      October 2019

    • Generalized Durgapal–Fuloria relativistic stellar models

      K. KOMATHIRAJ RANJAN SHARMA SHYAM DAS SUNIL D. MAHARAJ

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      We present a new class of closed-form solutions to the Einstein–Maxwell system of equations for a static spherically symmetric anisotropic star in the presence of an electric field by generalizing earlier approaches. The field equations are integrated by specifying the form of the electric field, anisotropic factor, and one of the gravitational potentials which are physically reasonable.We regain some of the earlier solutions of Durgapal and Fuloria (1985), Gupta and Maurya (2011) and Maurya et al. (2015) as special cases. A detailed physical analysis of the solution indicates the physical viability for modelling anisotropic charged superdense stars.

    • Irreversible thermodynamics of the universe in $f(R)$ gravity

      ATREYEE BISWAS

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      In this study, the irreversible thermodynamics of the universe in the framework of $f(R)$ gravity has been studied following standard Eckart theory of non-equilibrium thermodynamics. For a spatially flat universe, validity of GSLT and thermodynamic equilibrium have been examined both for apparent and eventhorizon as bounding horizons.The general result has been verified for exponential models of $f(R)$ gravity and it is shown that GSLT holds on both apparent and event horizon, but thermodynamic equilibrium does not holdon event horizon.

    • Gamma-ray emitting narrow-line Seyfert 1 galaxies: Past, present, and future

      VAIDEHI S. PALIYA

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      This article reviews our current understanding about $\gamma$-ray detected narrow-line Seyfert 1 ($\gamma$ -NLSy1) galaxies. The detection with the Large Area Telescope onboard Fermi Gamma-ray Space Telescope has provided the strongest evidence for the presence of closely aligned relativistic jet in these intriguing active galactic nuclei (AGN) and opened up a realm to explore the physical conditions needed to launch the jet in a different central engine and host galaxy environment than that is known for blazars. Promising results acquired from various multi-wavelength campaigns are converging to a scenario in which the $\gamma$-NLSy1 galaxies can be considered as ‘young’ blazars. These enigmatic sources hold the key to unravel the jet triggering mechanism and evolution of the AGN phase of a galaxy, in general. As such, $\gamma$-NLSy1s should be considered as one of the top priority targets for next generation observational facilities.

    • Modified Chaplygin gas with bulk viscous cosmology in FRW (2$+$1)-dimensional spacetime

      G. S. KHADEKAR PRAVEEN KUMAR SAFIQUL ISLAM

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      In this paper we study the bulk viscous cosmology by considering modified Chaplygin gas in the framework of ($2 + 1$)-dimensional spacetime. For this we consider various form of bulk viscosity coefficient $\zeta$ and then obtain the physical parameters energy density $\rho$, Hubble and deceleration parameters, $H$ and $\rho$, respectively. Finally we discuss the stability of the model by using the speed of sound.

    • Cold neutral hydrogen gas in galaxies

      RAJESHWARI DUTTA

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      This review summarizes recent studies of the cold neutral hydrogen gas associated with galaxies probed via the HI 21-cm absorption line. HI 21-cm absorption against background radio-loud quasars is a powerful tool to study the neutral gas distribution and kinematics in foreground galaxies from kilo-parsec toparsec scales. At low redshifts ($z$ < 0.4), it has been used to characterize the distribution of high column density neutral gas around galaxies and study the connection of this gas with the galaxy’s optical properties. The neutral gas around galaxies has been found to be patchy in distribution, with variations in optical depth observed at both kilo-parsec and parsec scales. At high redshifts ($z$ > 0.5), HI 21-cm absorption has been used to study the neutral gas in metal or Lyman − $\alpha$ absorption-selected galaxies. It has been found to be closely linked with the metal and dust content of the gas. Trends of various properties like incidence, spin temperature and velocity width of HI 21-cm absorption with redshift have been studied, which imply evolution of coldgas properties in galaxies with cosmic time. Upcoming large blind surveys of HI 21-cm absorption with next generation radio telescopes are expected to determine accurately the redshift evolution of the number densityof HI 21-cm absorbers per unit redshift and hence understand what drives the global star formation rate density evolution.

    • Radio pulsar sub-populations (I): The curious case of nulling pulsars

      SUSHAN KONAR UDDEEPTA DEKA

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      About $\sim$200 radio pulsars have been observed to exhibit nulling episodes – short and long. We find that the nulling fraction of a pulsar does not have any obvious correlation with any of the intrinsic pulsar parameters. It also appears that the phenomenon of nulling may be preferentially experienced by pulsars withemission coming predominantly from the polar cap region, and also having extremely curved magnetic fields.

    • Interacting quintessence and growth of structure

      AZAM HUSSAIN

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      In standard cosmologies, dark energy interacts only gravitationally with dark matter. An extension to this picture is interacting quintessence (IQ) model where scalar field coupled directly to cold darkmatter. The percentage deviation is studied in IQ model with respect to $\Lambda$CDM for varied values of the interacting parameter W.We investigated the effect of interaction on matter, kaiser and galaxy power spectrums.The deviation in power spectrum increases with interaction on both large and small scales. On small scales, variation is comparatively smaller than on large scale. On large scales, it is due to dark energy perturbation, while it is background evolution that causes a difference on small scale. These variations decreases with increase in redshift. Herein the thawing class of model with linear potential is studied.

  • Journal of Astrophysics and Astronomy | News

    • Continuous Article Publication

      Posted on January 27, 2016

      Since January 2016, the Journal of Astrophysics and Astronomy has moved to Continuous Article Publishing (CAP) mode. This means that each accepted article is being published immediately online with DOI and article citation ID with starting page number 1. Articles are also visible in Web of Science immediately. All these have helped shorten the publication time and have improved the visibility of the articles.

    • Editorial Note on Continuous Article Publication

      Posted on July 25, 2019

      Click here for Editorial Note on CAP Mode

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