arXiv subjects

Tilak Katoch

Publications and source records attributed to Tilak Katoch.

22 records · Page 2Linked to original sources

arXiv2017

Calibration of the Large Area X-ray Proportional Counter (LAXPC) instrument on-board AstroSat

We present the calibration and background model for the Large Area X-ray Proportional Counter (LAXPC) detectors on-board AstroSat. LAXPC instrument has three nominally identical detectors to achieve large collecting area. These detectors are independent of each other and in the event analysis mode, they record the arrival time and energy of each photon that is detected. The detectors have a time-resolution of 10 $μ$s and a dead-time of about 42 $μ$s. This makes LAXPC ideal for timing studies. The energy resolution and peak channel to energy mapping were obtained from calibration on ground using radioactive sources coupled with GEANT4 simulations of the detectors. The response matrix was further refined from observations of the Crab X-ray source after launch. At around 20 keV the energy resolution of detector is about 10--15\%, while the combined effective area of the 3 detectors is about 6000 cm$^2$.

astro-ph.IM↗

arXiv2017

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 to 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 about 6000 cm2 at 15 keV. The large detection volume of the LAXPC detectors, filled with xenon gas at about 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.

astro-ph.IM↗

arXiv2016

AstroSat/LAXPC observation of Cygnus X-1 in the hard state

We report the first analysis of data from AstroSat/LAXPC observations of Cygnus X-1 in January 2016. LAXPC spectra reveals that the source was in the canonical hard state, represented by a prominent thermal Comptonization component having a photon index of 1.8 and high temperature kT of electron > 60 keV along with weak reflection and possible disk emission. The power spectrum can be characterized by two broad lorentzian functions centered at 0.4 and 3 Hz. The r.m.s of the low frequency component decreases from 15% at around 4 keV to 10% at around 50 keV, while that of the high frequency one varies less rapidly from 13.5% to 11.5% in the same energy range. The time lag between the hard (20 to 40 keV) and soft (5 to 10 keV) bands varies in a step-like manner being nearly constant at 50 Milli-seconds from 0.3 to 0.9 Hz, decreasing to 8 Milli-seconds from 2 to 5 Hz and finally dropping to 2 Milli-seconds for higher frequencies. The time lags increase with energy for both the low and high frequency components. The event mode LAXPC data allows for flux resolved spectral analysis on a time-scale of 1 second, which clearly shows that the photon index increased from 1.72 to 1.80 as the flux increased by nearly a factor of two. We discuss the results in the framework of the fluctuation propagation model.

astro-ph.HE↗

arXiv2016

AstroSat/LAXPC reveals the high energy variability of GRS 1915+105 in the chi class

We present the first quick look analysis of data from nine {\it AstroSat}'s LAXPC observations of GRS 1915+105 during March 2016 when the source had the characteristics of being in Radio-quiet $χ$ class. We find that a simple empirical model of a disk blackbody emission, with Comptonization and a broad Gaussian Iron line can fit the time averaged 3--80 keV spectrum with a systematic uncertainty of 1.5\% and a background flux uncertainty of 4\%. A simple deadtime-corrected Poisson noise level spectrum matches well with the observed high frequency power spectra till 50 kHz and as expected the data show no significant high frequency ($> 20$ Hz) features. Energy dependent power spectra reveal a strong low frequency (2 - 8 Hz) Quasi-periodic oscillation (LFQPO) and its harmonic along with broad band noise. The QPO frequency changes rapidly with flux (nearly 4 Hz in ~ 5 hours). With increasing QPO frequency, an excess noise component appears significantly in the high energy regime (> 8 keV). At the QPO frequencies, the time-lag as a function of energy has a non-monotonic behavior such that the lags decrease with energy till about 15 -20 keV and then increase for higher energies. These first look results benchmark the performance of LAXPC at high energies and confirms that its data can be used for more sophisticated analysis such as flux or frequency-resolved spectro-timing studies.

astro-ph.HE↗