AstroSat/LAXPC observation of Cygnus X-1 in the hard state
Ranjeev Misra, J S Yadav, Jai Verdhan Chauhan, P C Agrawal, H M Antia,, Mayukh Pahari, V R Chitnis, Dhiraj Dedhia, Tilak Katoch, P. Madhwani, R K, Manchanda, B Paul, Parag Shah

TL;DR
This paper presents the first analysis of AstroSat/LAXPC data of Cygnus X-1 in the hard state, revealing spectral and timing properties consistent with thermal Comptonization and fluctuation propagation models.
Contribution
It provides new insights into the spectral and timing behavior of Cygnus X-1 in the hard state using AstroSat/LAXPC observations, including flux-resolved spectral analysis.
Findings
Spectral analysis shows a thermal Comptonization component with photon index 1.8 and electron temperature > 60 keV.
Timing analysis reveals two broad Lorentzian components at 0.4 and 3 Hz with energy-dependent rms.
Time lags between hard and soft bands vary with frequency and energy, supporting fluctuation propagation models.
Abstract
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…
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