Thanks to observations of the black hole binary Cyg X-1 carried over 15 years the INTEGRAL satellite collected the largest data set in the hard X-ray band for this source, exceeding 18 Ms of exposure time. We analysed these data, complemented by the radio flux measured by AMI Large Array. To characterize the spectral and variability properties of the system we determined parameters such as the hard X-ray flux, photon index and fractional variability. We found that the distribution of the photon index determined for the 22-100 keV band can be decomposed into four Gaussian peaks. This result, interpreted within the Comptonization scenario as a dominant process responsible for the hard X-ray emission, leads to a conclusion that the hot plasma region in Cyg X-1 takes form of four specific geometries. A distinct character of each of these states is strengthened by different X-ray and radio variability patterns. In particular, the hardest state shows no flux - photon index correlation typical for the three other states, what can be interpreted as a lack of interaction between the plasma and accretion disk.
Memorie della Societa Astronomica Italiana
- Pub Date:
- Stars: black holes;
- X-rays: binaries;
- X-rays: individuals: Cygnus X-1