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

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 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 observations, made by AstroSat on Oct 24, 2017 (MJD 58050), the energy spectrum of the source exhibits an inner disk temperature (kT in ) of 0.46±0.01 keV , a very steep photon index (Γ) of 3.15±0.03 along with a fractional disk flux contribution of ∼ 45%. 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 RMS value of ∼ 25%. Comparing the spectral softness of high-soft state 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 (MC) 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.

Study of mass outflow rates from magnetized advective accretion disk around rotating black holes

“…Here, we choose α B = 0.005. Subsequently, using ṁ and a k for a given black hole, we 6.31 [79], [79], [80], [81], [81] GRO J1655−40 [82], [83], [84], [81], [ [85], [86], [87], [88], [89] GX 339−4 3.81 [94], [95], [96,97], [81], [81] IGR J17091−3624 10.6 − 12. 3 29.00 [109], [110], [111], [112], [112] MAXI J0637−430 † 0.79 [113], [114], [115], [115] [113], [114], [115], [115] V404 Cyg 15.21 [116], [117], [118], [119], [119] H 1743−322 7.44 [120], [121], [121], [122], [122] ⊠ Aneesha et al (2024)(under review).…”