A stochastic propagation model to the energy dependent rapid temporal behaviour of Cygnus X-1 as observed by AstroSat in the hard state
Abstract
We report the results from analysis of six observations of Cygnus X-1 by Large Area X-ray Proportional Counter (LAXPC) and Soft X-ray Telescope (SXT) onboard AstroSat, when the source was in the hard spectral state as revealed by the broad-band spectra. The spectra obtained from all the observations can be described by a single-temperature Comptonizing region with disc and reflection components. The event mode data from LAXPC provides unprecedented energy dependent fractional root mean square (rms) and time-lag at different frequencies which we fit with empirical functions. We invoke a fluctuation propagation model for a simple geometry of a truncated disc with a hot inner region. Unlike other propagation models, the hard X-ray emission (>4 keV) is assumed to be from the hot inner disc by a single-temperature thermal Comptonization process. The fluctuations first cause a variation in the temperature of the truncated disc and then the temperature of the inner disc after a frequency dependent time delay. We find that the model can explain the energy dependent rms and time-lag at different frequencies.
- Publication:
-
Monthly Notices of the Royal Astronomical Society
- Pub Date:
- June 2019
- DOI:
- arXiv:
- arXiv:1904.01598
- Bibcode:
- 2019MNRAS.486.2964M
- Keywords:
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- accretion;
- accretion discs;
- black hole physics;
- X-rays: binaries;
- X-rays: individual: Cygnus X-1;
- Astrophysics - High Energy Astrophysical Phenomena
- E-Print:
- Accepted for Publication in MNRAS