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

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 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.