SMM Hard X-Ray Observations of the Soft Gamma-Ray Repeater 1806-20

A new type of gamma-ray source has been recently discovered. Six bursts from this source were recorded with the Hard X-Ray Burst Spectrometer on the Solar Maximum Mission during a highly active phase in 1983. High time resolution measurements of one burst show rise and decay times of less than 5 ms, the fastest yet observed from this source. Time profiles of these events are simple, but indicate low-level emission before and after the main peaks. Two transients in the series show no spectral evolution over their durations of fractions of a second. Bursts from soft gamma-ray repeaters (SGRs) appear to form a separate class of events. The characteristic properites of the three known SGRs are briefly reviewed.