The high-energy spectrum of Cygnus X–1: corona and jet contributions

We have analyzed six years of INTEGRAL observations of the BH binary Cygnus X-1. We report on the evolution of the physical parameters of the accretion flow across spectral transitions. In particular, we have used for the first time the new model BELM which gives constraints on the intensity of the magnetic field in the X-ray corona of BH binaries. We have found that in the softer states, the magnetic field is at most of the order of 1E+06 G. In the harder states, if the non-thermal excess observed above a few hundred keV is produced in the same region as the bulk of the thermal Comptonization, the upper limit on the magnetic field is about 1E+05 G. On the other hand, as suggested by the recent polarization measurements, this high-energy excess may be produced in the jet: in this case the constraints on the magnetic field in the hard state are somewhat relaxed and the upper limit rises to 1E+07 G.