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

TL;DR

This study analyzes 15 years of hard X-ray data from Cygnus X-1, revealing six distinct accretion modes with different geometries and variability patterns, advancing understanding of black hole accretion states.

Contribution

It identifies six unique plasma states in Cygnus X-1 based on spectral and variability analysis, highlighting distinct accretion geometries and their observational signatures.

Findings

Six clusters in photon index-flux distribution indicating different plasma states

Hardest and softest states lack short-term flux-photon index correlation

System transitions occur between these extreme states driven by plasma cooling

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.