Abstract
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 scenario's. The high pair creation rate during this state is indicated by the presence of a broad e −e + annihilation line-like feature and needs the presence of a high temperature radiation field. We put forward a scenario in which the observed spectrum originates from the inner region of an accretion disk around a rapidly rotating black hole. For a not too high inclination angle, the gravitational redshift lowers the high local radiation temperature to the observed value. This 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.
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