THE RARITY OF SOFT GAMMA-RAY REPEATERS DEDUCED FROM REACTIVATION OF SGR1806-20

ONLY two different types of gamma-ray transient sources are presently known: over one thousand gamma-ray bursters (GRBs) and only three soft gamma-ray repeaters (SGRs). The latter are distinguished by their pro pensity for recurrent burst behaviour(1-3), in contrast to the nonrepe ating GRB sources. Recurrent emission from one of the repeaters, SGR19 00+14, has been detected(4) earlier by the Burst and Transient Source Experiment (BATSE) aboard the Compton Gamma-Ray Observatory. Here we r eport renewed burst activity from SGR1806-20, the most prolific of the three known SGRs. This detection of reactivation of this source has b een rapidly followed by identification of an X-ray counterpart(5,6), w hich also coincides with a compact radio source(7) now identified as a plerionic (pulsar-powered) supernova remnants. In combination, these results are leading to a convergence of ideas about the nature of SGRs , which can now be firmly identified as neutron stars. That BATSE has detected no new sources in its two and a half years of operation indic ates that SGRs are rare in our Galaxy.