AstroSat/LAXPC Observation of Cygnus X-1 in the Hard State
Abstract
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.
- Publication:
-
The Astrophysical Journal
- Pub Date:
- February 2017
- DOI:
- arXiv:
- arXiv:1612.08793
- Bibcode:
- 2017ApJ...835..195M
- Keywords:
-
- accretion;
- accretion disks;
- black hole physics;
- X-rays: binaries;
- X-rays: individual: Cyg X-1;
- Astrophysics - High Energy Astrophysical Phenomena
- E-Print:
- 6 pages, 8 figures