External Compton emission from relativistic jets in Galactic black hole candidates and ultraluminous X-ray sources

Galactic binary systems that contain a black hole candidate show evidence of radio jets in their hard X-ray states. Unavoidably, photons from the companion star and/or the accretion disk are Compton-scattered by relativistic electrons in the jet, producing beamed X-rays and possibly gamma-rays. The importance of this process depends on the jet power and the Doppler boosting factor. For plausible values of these parameters, we show that the jet emission can contribute significantly to the hard state X-ray luminosity. Two sources – XTE J1118+480 and Cygnus X-1 – are modelled as representatives of black holes with low and high luminosity companion stars respectively. In XTE J1118+480, weak reflection features indicate that the jet emission is comparable to coronal emission. In Cygnus X-1, strong reflection features indicate coronal emission in the X-ray band, but the jet emission may emerge in the gamma-ray band. The absence of reflection features in the spectra of the ultraluminous compact X-ray sources in nearby galaxies suggests that they are dominated by jet emission. We show that a viable model for these sources is a stellar mass black hole with a high luminosity companion and a favourably oriented jet