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 2016 January. 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 of kTe > 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 rms 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 (20–40 keV) and soft (5–10 keV) bands varies in a step-like manner being nearly constant at ~50 milliseconds from 0.3 to 0.9 Hz, decreasing to ~8 milliseconds from 2 to 5 Hz and finally dropping to ~2 milliseconds 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 timescale of 1 s, 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.