The high-energy spectrum of Cygnus X-1 as measured by INTEGRAL

The INTEGRAL satellite observed the binary black hole system Cygnus X-1 on 2003 June 7-11. The source was detected up to 800 keV, its spectrum beeing compatible with a power law of photon index 2.3. In the framework of the accreting black hole phenomenology, the source was in an unusual state, probably the so-called “intermediate state”. Using the results of the analysis performed on these INTEGRAL data, we present here the spectral properties of our observations over the whole energy range covered by the high-energy instruments on board INTEGRAL. We then compare them with those obtained from earlier observations of Cygnus X-1 performed during the INTEGRAL performance and verification phase in the fall of 2002. At that time, the source was found in the low/hard state, indicating that Cygnus X-1 underwent a transition towards a softer state in 2003 June. We discuss the spectral parameters obtained from a Comptonization model.