• RANJEEV MISRA

Articles written in Journal of Astrophysics and Astronomy

• Large Area X-Ray Proportional Counter (LAXPC) Instrument on AstroSat and Some Preliminary Results from its Performance in the Orbit

Large area X-ray propositional counter (LAXPC) instrument on AstroSat is aimed at providing high time resolution X-ray observations in 3–80 keV energy band with moderate energy resolution. To achieve large collecting area, a cluster of three co-aligned identical LAXPC detectors, is used to realize an effective area in access of ∼6000cm2 at 15 keV. The large detection volume of the LAXPC detectors, filled with xenon gas at ∼2 atmosphere pressure, results in detection efficiency greater than 50%, above 30 keV. In this article, we present salient features of the LAXPC detectors, their testing and characterization in the laboratory prior to launch and calibration in the orbit. Some preliminary results on timing and spectral characteristics of a few X-ray binaries and other type of sources, are briefly discussed to demonstrate that the LAXPC instrument is performing as planned in the orbit.

• Large Area X-ray Proportional Counter (LAXPC) in orbit performance: Calibration, background, analysis software

The Large Area X-ray Proportional Counter (LAXPC) instrument on-board AstroSat has three nominally identical detectors for timing and spectral studies in the energy range of 3–80 keV. The performance of these detectors during the five years after the launch of AstroSat is described. Currently, only oneof the detector is working nominally. The variation in pressure, energy resolution, gain and background with time are discussed. The capabilities and limitations of the instrument are described. A brief account of available analysis software is also provided.

• Multi-wavelength view of the galactic black-hole binary GRS 1716–249

The origins of X-ray and radio emissions during an X-ray binary outburst are comparatively better understood than those of ultraviolet, optical and infrared radiation. This is because multiple competing mechanisms – emission from intrinsic and irradiated disk, secondary star emission, synchrotron emissionfrom jet and/or non-thermal electron cloud, etc – peak in these mid-energy ranges. Ascertaining the true emission mechanism and segregating the contribution of different mechanisms, if present, is important for correct understanding of the energetics of the system and hence its geometry and other properties. We have studied the multi-wavelength spectral energy distribution of the galactic X-ray binary GRS 1716-249 ranging from near infrared ($5 \times 10^{-4}$ keV) to hard X-rays (120 keV) using observations from AstroSat, Swift, and Mount Abu Infrared Observatory. Broadband spectral fitting suggests that the irradiated accretion disk dominates emission in ultraviolet and optical regimes. The near infrared emission exhibits some excess thanthe prediction of the irradiated disk model, which is most likely due to Synchrotron emission from jets as suggested by radio emission. Irradiation of the inner disk by the hard X-ray emission from the Corona also plays a significant role in accounting for the soft X-ray emission.

• LAXPC instrument onboard AstroSat: Five exciting years of new scientific results specially on X-ray binaries

With its large effective area at hard X-rays, high time resolution and having co-aligned other instruments, AstroSat/LAXPC was designed to usher in a new era in rapid variability studies and wide spectral band measurements of the X-ray binaries. Over the last five years, the instrument has successfully achieved to a significant extent these Science goals. In the coming years, it is poised to make more important discoveries. This paper highlights the primary achievements of AstroSat/LAXPC in unraveling the behavior of black hole and neutron star systems and discusses the exciting possibility of the instrument’s contributionto future science.

• AstroSat view of the NLS1 galaxy Mrk 335

We present the results from the multi-wavelength monitoring observations of the Narrow-Line Seyfert 1 galaxy Mrk 335 with AstroSat. We analysed both the X-ray (SXT and LAXPC) and UV (UVIT) data of the source at two epochs, separated by $\sim$18 days. The source was in a low flux state during theobservations, and the X-ray spectra were found to be harder than usual. The presence of soft X-ray excess was identified in the observations, and the broadband X-ray continuum was modelled with power-law and black body (modified by intrinsic absorption) and a distant neutral reflection component. We did not find any variability in the X-ray spectral shape or the flux over this period. However, the UV flux is found to be variable between the observations. The obtained results from the X-ray analysis point to a scenario where theprimary emission is suppressed and the component due to distant reflection dominates the observed spectrum.

• An alternative scheme to estimate AstroSat/LAXPC background for faint sources

An alternative scheme is described to estimate the layer 1 LAXPC 20 background for faint sources where the source contribution to the 50–80 keV count rate is less than 0.25 counts/s (15 milli-crabs or $6\times 10^{-11}$ ergs/s/cm$^2$). We consider 12 blank sky observations and based on their 50–80 keV count rate in 100 second time-bins, generate four template spectra which are then used to estimate the background spectrum and lightcurve for a given faint source observation. The variance of the estimated background subtracted spectra for the 12 blank sky observations is taken as the energy dependent systematic uncertainty which will dominate over the statistical one for exposures longer than 5 ks. The estimated 100 second time bin background lightcurve in the 4–20 keV band with a 3% systematic error matches with the blank sky ones.The 4–20 keV spectrum can be constrained for a source with flux $\gtrapprox$1 milli-crab. Fractional rms variability of 10% can be determined for a $\sim$5 milli-crab source lightcurve binned at 100 seconds. To illustrate the scheme, the lightcurves and spectra of three different blank sky observations, three AGN sources (Mrk 0926, Mrk 110, NGC 4593) and LMC X-1 are shown.

• # Journal of Astrophysics and Astronomy

Volume 43, 2022
All articles
Continuous Article Publishing mode

• # 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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