The rarity of soft γ-ray repeaters deduced from reactivation of SGR1806 - 20

ONLY two different types of γ-ray transient sources are presently known: over one thousand γ-ray bursters (GRBs) and only three soft γ-ray repeaters (SGRs). The latter are distinguished by their propensity for recurrent burst behaviour^1-3, in contrast to the nonrepeating GRB sources. Recurrent emission from one of the repeaters, SGR1900 + 14, has been detected⁴ earlier by the Burst and Transient Source Experiment (BATSE) aboard the Compton Gamma-Ray Observatory. Here we report renewed burst activity from SGR1806 - 20, the most prolific of the three known SGRs. This detection of reactivation of this source has been rapidly followed by identification of an X-ray counterpart^5,6, which also coincides with a compact radio source⁷ now identified as a plerionic (pulsar-powered) supernova remnant⁸. 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 indicates that SGRs are rare in our Galaxy.