2021MNRAS.507.2602K

We present a detailed spectral and timing analysis of Cygnus X-1 with multi-epoch observations, during 2016-2019, by SXT and LAXPC onboard AstroSat. We model the spectra in broad energy range of 0.5-70.0 keV to study the evolution of spectral properties while Cygnus X-1 transited from hard state to an extreme soft state via intermediate states in 2017. Simultaneous timing features are also examined by modelling the power density spectra in 3.0-50.0 keV. We find that during high-soft state (HSS) observations, made by AstroSat on 2017 October 24 (MJD 58050), the energy spectrum of the source exhibits an inner disc temperature (kT_in) of 0.46 ±0.01 keV, a very steep photon index (Γ) of 3.15 ± 0.03 along with a fractional disc flux contribution of ∼45 per cent. The power density spectrum in the range of 0.006-50.0 Hz is also very steep with a power-law index of 1.12 ± 0.04 along with a high root mean square value of ∼25 per cent. Comparing the spectral softness of HSS with those of previously reported, we confirm that AstroSat observed Cygnus X-1 in the 'softest' state. The lowest MAXI spectral hardness ratio of ∼0.229 corroborates the softest nature of the source. Moreover, we estimate the spin of the black hole by continuum-fitting method, which indicates that Cygnus X-1 is a maximally rotating 'hole'. Further, Monte Carlo simulations are performed to estimate the uncertainty in spin parameter, which is constrained as a_* > 0.9981 with 3σ confidence interval. Finally, we discuss the implications of our findings.