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 elds 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 (identying 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 such transient lines has been brought into question by the failure of long-term monitoring to detect them. A seemingly critical observation of the GC by three diierent instruments simultaneously, in which transient emission was detected only by one, provides the strongest evidence that the earlier results were in error. Recent developments in accretion disk theory have weakened the theoretical case for the lines. I draw conclusions concerning the nature of scientiic inquiry and public outreach eeorts in science. Transient events pose considerable problems in a young eld such as-ray astronomy, in which the measurements are subject to poorly understood systematic errors. If two instruments disagree signiicantly when observing the same source, this may be due to the variability of the source in between measurements. Alternatively, the systematic errors in the two instruments may be responsible. Given the very poor spatial resolution of most-ray