ANNIHILATION AND BACKSCATTERED LINES IN THE GALACTIC-CENTER REGION

We review the recent observations of the X-ray and gamma-ray sources i n the Galactic center region, emphasizing the features which have been detected in their spectra. Most are related to the annihilation of po sitrons, and therefore this discussion concentrates on pair plasma cre ation, positron transport, and annihilation. For a relatively complete picture of the Galactic center region and the 1E 1740.7-2942 source, we need to introduce compact object(s), some of them possibly black ho les. Clues for black hole detection are presented (dynamical arguments , spectral properties, QPO, and temporal behavior), as well as an upda ted list of candidates. We finally show how INTEGRAL would be a powerf ul tool for detecting black holes at gamma-ray energies.