Long-term spectral study of the black hole Cygnus X-1 using INTEGRAL

TL;DR

This study analyzes the spectral and polarization properties of Cygnus X-1 across different states using INTEGRAL, revealing a high-energy tail and variable polarization in the low-hard state.

Contribution

It provides the first detailed spectral and polarization analysis of Cygnus X-1 over a broad energy range with state resolution, highlighting the high-energy tail and polarization variability.

Findings

Detection of a high-energy tail above 400 keV in both states

Polarization detected in the low-hard state, with potential temporal variations

No polarization detected in the high-soft state

Abstract

We utilize the joint capabilities of IBIS and SPI to perform a state-resolved 20--2000\,keV analysis of the microquasar Cygnus X-1. In both LHS and HSS, the spectral analysis reveals the presence of a high-energy tail above 400\,keV in addition to a standard, Compton-like continuum in the 20--400\,keV range. We study the polarisation properties of the hard X-ray radiation, as well as of the polarisation of this high-energy tail. Polarisation is detected in the LHS in agreement with previous work. We find potential variations of the LHS polarisation with time and no polarisation detection in the HSS.