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Gamma-ray line transients

Early interpretations of the variability of the narrow 0.511 MeV annihilation line from the Galactic center (GC) were confused by the unrecognized presence of a very extended component (mid 70s–mid 80s). Transient behavior was suggested by comparisons of measurements made by instruments with comparable fields of view, and by detection of other lines which could be interpreted as broadened and Doppler-shifted versions of the line. A new generation of instruments (late 80s–early 90s) provided more compelling evidence, by improved imaging capability (identifying a known transient high-energy candidate source, 1E 1740.7–2942), and by improved sensitivity. Additional broadened, shifted, and Compton-scattered derivatives of the annihilation line were detected from this and other sources. At the same time, theoretical developments suggested exciting new physics which might arise in the accretion regimes around black holes, of which these features would be important diagnostics. In the GRO era the existence of su...