Distinct accretion modes of Cygnus X-1 revealed from hard X-rays

Abstract

Thanks to recurrent observations of the black hole binary Cyg X-1 carried out over 15 years the INTEGRAL satellite has collected the largest data set in the hard X-ray band for this source. We have analyzed these data, complemented by data collected by other X-ray satellites and radio flux at 15 GHz. To characterize the spectral and variability properties of the system we have examined parameters such as the hard X-ray flux, photon index and fractional variability. Our main result is that the 2D distribution of the photon index and flux determined for the 22-100 keV band forms six clusters. This result, interpreted within the Comptonization scenario as the dominant process responsible for the hard X-ray emission, leads to a conclusion that the hot plasma in Cyg X-1 takes the form of six specific geometries. The distinct character of each of these plasma states is reinforced by their different X-ray and radio variability patterns. In particular, the hardest and softest plasma states show no short-term flux - photon index correlation typical for the four other states, implying a lack of interaction between the plasma and accretion disk. The system evolves between these two extreme states, with the spectral slope regulated by a variable cooling of the plasma by the disk photons.

arXiv comment21 pages, 11 figures, accepted for publication in ApJ

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• Detection of a low frequency quasi-periodic oscillation in the soft state of Cygnus X-1 with Insight-HXMT
• A Hidden Pulse: Uncovering a New Timing Signal in Cygnus X-1 with AstroSat
• Using collimated CZTI as all sky X-ray detector based on Earth Occultation Technique
• Unprecedentedly bright X-ray flaring in Cygnus X-1 observed by INTEGRAL