AstroSat and MAXI view of Cygnus X-1: Signature of an `extreme' soft nature

TL;DR

This paper presents a comprehensive spectral and timing analysis of Cygnus X-1 during its transition to an extreme soft state, revealing it as the softest observed state and confirming the black hole's near-maximal spin.

Contribution

The study provides detailed multi-epoch spectral and timing data during state transition, and estimates the black hole spin as nearly maximal using continuum-fitting and Monte Carlo simulations.

Findings

Cygnus X-1 reached its softest state with a photon index of 3.15.

Black hole spin is constrained to be greater than 0.9981.

Spectral and timing properties confirm the extreme soft state.

Abstract

We present a detailed spectral and timing analysis of Cygnus X-1 with multi-epoch observations, during $2016$ to $2019$, by SXT and LAXPC on-board AstroSat. We model the spectra in broad energy range of $0.5\!-\!70.0\,\rm{keV}$ 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 $3.0\!-\!50.0\,\rm{keV}$ . We find that during high-soft state observations, made by AstroSat on Oct $24,\,2017$ (MJD $58050$), the energy spectrum of the source exhibits an inner disk temperature (kT$\rm_{in}$) of $0.46\!\pm\!0.01\,\rm{keV}$ , a very steep photon index ($\Gamma$) of $3.15\!\pm\!0.03$ along with a fractional disk flux contribution of $\sim\!45\%$. The power density spectrum in the range of $0.006\!-\!50.0\,\rm{Hz}$ is also very steep with a power-law index of $1.12\!\pm\!0.04$ along with a high RMS value of $\sim\!25\%$. Comparing the spectral softness of high-soft state with those of previously reported, we confirm that {\it AstroSat} observed Cygnus X-1 in the `softest' state. The lowest MAXI spectral hardness ratio of $\sim\!0.229$ 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 (MC) simulations are performed to estimate the uncertainty in spin parameter, which is constrained as a$_{\ast}>0.9981$ with $3\sigma$ confidence interval. Finally, we discuss the implications of our findings.