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      Volume 39, Issue 1

      February 2018

    • Editorial

      Samir Mandal Indranil Chattopadhyay Anuj Nandi Santabrata Das

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    • High energy transients: The millisecond domain

      A. R. RAO

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      The search for high energy transients in the millisecond domain has come to the focus in recent times due to the detection of gravitational wave events and the identification of fast radio bursts as cosmological sources. Here we highlight the sensitivity limitations in the currently operating hard X-ray telescopes and give some details of the search for millisecond events in the AstroSat CZT Imager data.

    • Standing shocks in magnetized dissipative accretion flow around black holes


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      We explore the global structure of the accretion flow around a Schwarzschild black hole where the accretion disc is threaded by toroidal magnetic fields. The accretion flow is optically thin and advection dominated. The synchrotron radiation is considered to be the active cooling mechanism in the flow. With this, we obtain the global transonic accretion solutions and show that centrifugal barrier in the rotating magnetized accretion flow causes a discontinuous transition of the flow variables in the form of shock waves. The shock properties and the dynamics of the post-shock corona are affected by the flow parameters such as viscosity,cooling rate and strength of the magnetic fields. The shock properties are investigated against these flow parameters.We further show that for a given set of boundary parameters at the outer edge of the disc, accretion flow around a black hole admits shock when the flow parameters are tuned for a considerable range.

    • $M_{\bullet}$ − $\sigma$ relation in spherical systems


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      To investigate the $M_{\bullet}$ − $\sigma$ relation, we consider realistic elliptical galaxy profiles that are taken to follow a single power-law density profile given by $\rho(r) = \rho_0(r/r_0)−\gamma$ or the Nuker intensity profile. We calculate the density using Abel’s formula in the latter case by employing the derived stellar potential; in both cases. We derive the distribution function $f(E)$ of the stars in the presence of the supermassive black hole(SMBH) at the center and hence compute the line-of-sight (LoS) velocity dispersion as a function of radius. For the typical range of values for masses of SMBH, we obtain $M_{\bullet} \propto \sigma^p$ for different profiles. An analytical relation $p = (2\gamma +6)/(2+ \gamma)$ is found which is in reasonable agreement with observations (for $\gamma = 0.75−1.4$,$p = 3.6−5.3$). Assuming that a proportionality relation holds between the black hole mass and bulge mass, $M_{\bullet} = f M_{\rm b}$, and applying this to several galaxies, we find the individual best fit values of $p$ as a function of $f$; also by minimizing $\chi^2$, we find the best fit global $p$ and $f$ . For Nuker profiles, we find that $p = 3.81 \pm 0.004$ and $f = (1.23 \pm 0.09) × 10^{−3}$ which are consistent with the observed ranges.

    • Observational aspects of outbursting black hole sources: Evolution of spectro-temporal features and X-ray variability


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      We report on our attempt to understand the outbursting profile of Galactic Black Hole sources, keeping in mind the evolution of temporal and spectral features during the outburst. We present results of evolution of quasi-periodic oscillations, spectral states and possible connection with jet ejections during the out burst phase. Further,we attempt to connect the observed X-ray variabilities (i.e., ‘class’/‘structured’ variabilities, similar to GRS 1915$+$105) with spectral states of black hole sources. Towards these studies, we consider three black hole sources that have undergone single (XTE J1859$+$226), a few (IGR J17091$-$3624) and many (GX 339-4) outbursts since the start of $RXTE$ era. Finally, we model the broadband energy spectra (3–150 keV) of different spectral states using $RXTE$ and $NuSTAR$ observations. Results are discussed in the context oftwo-component advective flow model, while constraining the mass of the three black hole sources.

    • A comparative study of single-temperature and two-temperature accretion flows around black holes


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      We study the properties of sub-Keplerian accretion disk around a stationary black hole, considering bremsstrahlung, synchrotron and Comptonization of synchrotron photons as radiative cooling mechanisms active in the disk. We obtain the solutions of two-temperature global accretion flow (TTAF) and compare it with the results obtained from single-temperature (STAF) model.We observe that flow properties, in particular, the radial profile of electron and ion temperatures differ noticeably in the adopted models for flows with identical boundary conditions fixed at the outer edge of the disk. Since the electron temperature is one of the key factors to regulate the radiative processes, we argue that physically motivated description of electron temperature needsto be considered in studying the astrophysical phenomena around black holes.

    • $Suzaku$ observation of the eclipsing high mass X-ray binary pulsar XTE J1855-026


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      We report results from analysis performed on an eclipsing supergiant high mass X-ray binary pulsar XTE J1855-026 observed with the X-ray Imaging Spectrometer (XIS) on-board Suzaku Observatory in April 2015. Suzaku observed this source for a total effective exposure of ∼87 ks just before an eclipse. Pulsations are clearly observed and the pulse profiles of XTE J1855-026 did not show significant energy dependence duringthis observation consistent with previous reports. The time averaged energy spectrum of XTE J1855-026 in the1.0–10.5 keV energy range can be well fitted with a partial covering power law model modified with interstellarabsorption along with a black-body component for soft excess and a gaussian for iron fluorescence line emision. The hardness ratio evolution during this observation indicated significant absorption of soft X-rays in some segments of the observation. For better understanding of the reason behind this, we performed time-resolved spectroscopy in the 2.5–10.5 keV energy band which revealed significant variations in the spectral parameters,especially the hydrogen column density and iron line equivalent width with flux. The correlated variations in the spectral parameters indicate towards the presence of clumps in the stellar wind of the companion star accounting for the absorption of low energy X-rays in some time segments.

    • Searching for dual active galactic nuclei


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      Binary or dual active galactic nuclei (DAGN) are expected from galaxy formation theories. However, confirmed DAGN are rare and finding these systems has proved to be challenging. Recent systematic searches for DAGN using double-peaked emission lines have yielded several new detections, as have the studiesof samples of merging galaxies. In this paper, we present an updated list of DAGN compiled from published data. We also present preliminary results from our ongoing Expanded Very Large Array (EVLA) radio study of eight double-peaked emission-line AGN (DPAGN). One of the sample galaxy shows an S-shaped radio jet. Using new and archival data, we have successfully fitted a precessing jet model to this radio source. We find that the jet precession could be due to a binary AGN with a super-massive black-hole (SMBH) separation of $\sim$0.02 pc or a single AGN with a tilted accretion disk. We have found that another sample galaxy, which is undergoing a merger, has two radio cores with a projected separation of 5.6 kpc. We discuss the preliminaryresults from our radio study.

    • Role of primordial black holes in the direct collapse scenario of supermassive black hole formation at high redshifts


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      In this paper, we explore the possibility of accreting primordial black holes as the source of heating for the collapsing gas in the context of the direct collapse black hole scenario for the formation of super-massive black holes (SMBHs) at high redshifts, $z \sim 6–7$. One of the essential requirements for the direct collapse model to work is to maintain the temperature of the in-falling gas at $\approx$104 K. We show that even under the existingabundance limits, the primordial black holes of masses $\gtrsim$10−2M$_{\odot}$, can heat the collapsing gas to an extent that the H$_2$ formation is inhibited. The collapsing gas can maintain its temperature at 104 K till the gas reachesa critical density $n_c \approx 10^3$ cm$^{−3}$, at which the roto-vibrational states of H$_2$ approaches local thermodynamicequilibrium and H$_2$ cooling becomes inefficient. In the absence of H$_2$ cooling, the temperature of the collapsing gas stays at $\approx$10$^4$ Keven as it collapses further. We discuss scenarios of subsequent angular momentum removaland the route to find collapse through either a supermassive star or a supermassive disk.

    • Study of magnetized accretion flow with cooling processes


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      We have studied shock in magnetized accretion flow/funnel flow in case of neutron star with bremsstrahlung cooling and cyclotron cooling. All accretion solutions terminate with a shock close to the neutron star surface, but at some regions of the parameter space, it also harbours a second shock away from thestar surface.We have found that cyclotron cooling is necessary for correct accretion solutions which match the surface boundary conditions.

    • Study of X-ray transients with Scanning Sky Monitor (SSM) onboard AstroSat


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      Scanning Sky Monitor (SSM) onboard AstroSat is an X-ray sky monitor in the energy range 2.5–10 keV. SSM scans the sky for X-ray transient sources in this energy range of interest. If an X-ray transient source is detected in outburst by SSM, the information will be provided to the astronomical community for follow-up observations to do a detailed study of the source in various other bands. SSM instrument, since its power-ON in orbit, has observed a number of X-ray sources. This paper discusses observations of few X-ray transients by SSM. The flux reported by SSM for few sources during its Performance Verification phase (PV phase) is studied and the results are discussed.

    • Radiatively-driven general relativistic jets


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      We use moment formalism of relativistic radiation hydrodynamics to obtain equations of motion of radial jets and solve them using polytropic equation of state of the relativistic gas.We consider curved spacetime around black holes and obtain jets with moderately relativistic terminal speeds. In addition, the radiation field from the accretion disc, is able to induce internal shocks in the jet close to the horizon. Under combinedeffect of thermal as well as radiative driving, terminal speeds up to 0.75 (units of light speed) are obtained.

    • High energy power-law tail in X-ray binaries and bulk Comptonization due to an outflow from a disk


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      We study the high energy power-law tail emission of X-ray binaries (XRBs) by a bulk Comptonization process which is usually observed in the very high soft (VHS) state of black hole (BH) XRBs and the high soft (HS) state of the neutron star (NS) and BH XRBs. Earlier, to generate the power-law tail in bulk Comptonization framework, a free-fall converging flow into BH or NS had been considered as a bulk region. In this work, for a bulk region we consider mainly an outflow geometry from the accretion disk which is bounded by a torus surrounding the compact object. We have two choices for an outflow geometry: (i) collimated flowand (ii) conical flow of opening angle $\theta_b$ and the axis is perpendicular to the disk. We also consider an azimuthalvelocity of the torus fluids as a bulk motion where the fluids are rotating around the compact object (a torus flow). We find that the power-law tail can be generated in a torus flow having large optical depth and bulk speed (>0.75$c$), and in conical flow with $\theta_b$ > $\sim 30^{\circ}$ for a low value of Comptonizing medium temperature. Particularly, in conical flow the low opening angle is more favourable to generate the power-law tail in both theHS state and the VHS state. We notice that when the outflow is collimated, then the emergent spectrum does not have power-law component for a low Comptonizing medium temperature.

    • Collapsing supra-massive magnetars: FRBs, the repeating FRB121102 and GRBs


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      Fast Radio Bursts (FRBs) last for $\sim$ few milli-seconds and, hence, are likely to arise from the gravitational collapse of supra-massive, spinning neutron stars after they lose the centrifugal support (Falcke & Rezzolla 2014). In this paper, we provide arguments to show that the repeating burst, FRB 121102, can also be modeled in the collapse framework provided the supra-massive object implodes either into a Kerr black holesurrounded by highly magnetized plasma or into a strange quark star. Since the estimated rates of FRBs and SN Ib/c are comparable, we put forward a common progenitor scenario for FRBs and long GRBs in which only those compact remnants entail prompt $\gamma$-emission whose kick velocities are almost aligned or anti-aligned with the stellar spin axes. In such a scenario, emission of detectable gravitational radiation and, possibly, of neutrinos are expected to occur during the SN Ib/c explosion as well as, later, at the time of magnetar implosion.

    • Measurement of coronal properties of Seyfert galaxies from NuSTAR’s hard X-ray spectrum


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      Precise measurement of the coronal properties of Active Galactic Nuclei (AGN) requires the availability of high signal-to-noise ratio data covering awide range of X-ray energies.The Nuclear Spectroscopic Telescope Array ($NuSTAR$) which is highly sensitive to earlier missions in its operational energy range of 3–79 keV, allows us to arrive at precise estimates of the coronal parameters such as cut-off energy ($E_{\rm cut}$), coronaltemperature ($kT_e$) and geometry of the corona at least for sources that have $E_{\rm cut}$ within the energy range of$NuSTAR$. In this paper,we present our preliminary results on the spectral analysis of two Seyfert galaxies namely 3C 120 and NGC 4151 using $NuSTAR$ observations in the 3–79 keV band. We investigated the continuum and coronal parameters, the photon index $\Gamma$, $E_{\rm cut}$ and $kT_e$. By fitting the X-ray spectrum of 3C 120 and NGC 4151 with a simple phenomenological model, we found that both the sources showed a clear cut-off in their spectrum.

    • Thermonuclear X-ray burst of MXB 1658-298 with $NuSTAR$


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      MXB 1658-298 is a transient Low-Mass X-ray Binary (LMXB), which shows eclipses, dips and bursts in its light curve. This source has undergone three active periods separated by long quiescent phases. The latest phase of enhanced X-ray emission was observed during 2015–2016. We have analysed broadband data from Swift/XRT and NuSTAR observations carried out in 2015. During NuSTAR observation, one thermonuclear X-ray burst took place. The X-ray emission during the burst was brighter by a factor of $\sim$200, compared to the pre-burst emission. This work focuses on the spectral analysis of MXB 1658-298 during the persistentand the burst phases using NuSTAR observation of 2015. We have also determined the temperature and radiusevolution during the burst using the time-resolved spectroscopy. The burst phase shows mild Photospheric Radius Expansion (PRE).

    • Advective accretion flow properties around rotating black holes – application to GRO J1655-40


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      We examine the properties of the viscous dissipative accretion flow around rotating black holes in the presence of mass loss. Considering the thin disc approximation, we self-consistently calculate the inflow-outflow solutions and observe that the mass outflow rates decrease with the increase in viscosity parameter ($\alpha$). Further, we carry out the model calculation of quasi-periodic oscillation frequency ($\nu_{\rm QPO}$) that is frequentlyobserved in black hole sources and observe that $\nu^{\max}_{\rm QPO}$ increases with the increase of black hole spin ($a_{\rm k}$). Then, we employ our model in order to explain the High Frequency Quasi-Periodic Oscillations (HFQPOs) observedin black hole source GROJ1655-40. While doing this, we attempt to constrain the range of $a_k$ based on observed HFQPOs ($\sim$300 Hz and $\sim$450 Hz) for the black hole source GRO J1655-40.

    • Dense matter in strong gravitational field of neutron star


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      Mass, radius and moment of inertia are direct probes of compositions and Equation of State (EoS) of dense matter in neutron star interior. These are computed for novel phases of dense matter involving hyperons and antikaon condensate and their observable consequences are discussed in this article. Furthermore, the relationship between moment of inertia and quadrupole moment is also explored.

    • Spectral properties of the accretion discs around rotating black holes


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      We study the radiation properties of an accretion disc around a rotating black hole. We solve the hydrodynamic equations and calculate the transonic solutions of accretion disc in the presence of shocks. Then we use these solutions to generate the radiation spectrum in the presence of radiative heating and coolingprocesses.We present the effect of spin parameter of the black hole on the emitted radiation spectrum. In addition,attention has also been paid to the variation in energy spectral index with Kerr parameter and accretion rate. We find that spectral index becomes harder as the spin parameter changes from negative (accretion disc is counter-rotating with respect to the black hole spin) to a positive value. Finally, we compute and compare the spectral characteristics due to a free-fall flow and a transonic flow. We notice significant differences in highenergy contributions from these two solutions.

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