1995A&A...302..154V

The non-relativistic temperature of the hard X-ray continuum in the spectrum of the black hole candidate 1E 1740.7-2942 during its hard state cannot be understood in terms of the conventional photon-photon pair production scenarios. The high pair-creation rate during this state is indicated by the presence of a broad pair annihilation line-like feature and demands the presence of a high-temperature radiation field. We propose a scenario in which the observed spectrum originates from the inner region of an accretion disk around a rapidly rotating black hole. The gravitational red-shift lowers the high local radiation temperature to the observed value, provided that the inclination of the system is less than approximately 40deg. For higher inclination angles the Doppler blue-shift caused by the high speed of the accretion flow opposes the gravitational red-shift. The created pairs are driven from the inner disk region by radiation pressure. In the hard state the pair wind is intercepted by the disk at a somewhat larger radial distance. This radial distance is inferred from the observed (weak) gravitational red-shift of the line. The mechanism necessitates a region of extremely strong gravity and is therefore unique for black holes. The presence of an annihilation line together with a low-temperature X-ray continuum could therefore be considered as a black hole signature.