ON THE NATURE OF SOFT GAMMA-REPEATERS

Bursts from the soft gamma repeaters (SGRs) form a unique class in several respects. Temporal characteristics include short, 0.1 s, simple event profiles with very fast rise times. Event recurrence patterns are stochastic. Spectral shape is independent of burst intensity; neither spectral variability nor soft X-ray tails are observed. The ratios of burst peak fluxes for the three known SGRs is consistent with a Large Magellanic Cloud distance for 0526 -66 and approximately galactocentric distances for 1806-20 and 1900+14. A/Amax and V/Vmax tests imply that present instrumentation has sampled to the edge of the SGR population. The SGR source distribution and sparse population statistics are consistent with radio pulsars being the progenitors of SGRs, and the latter having short burst-active lifetimes. A Large Magellanic Cloud distance for the 1979 March 5 superburst is allowed and the synchrotron selfabsorption problem obviated if expansion and the observed spectral evolution are taken into account. The narrow, luminosity-independent spectra in 1806-20 bursts can be explained if the high photon flux is scattered by a dense pair plasma, destroying high-energy y-rays, and converting soft X-rays by cyclotron resonant Compton scattering. We present limits on the lack of persistent X-ray flux from all three SGR sources, and discuss other reported nondetections in the infrared, optical, and X-ray bands. In this connection we review previous arguments against proposed energy release mechanisms, including thermonuclear events, magnetic gating phenomena, and accretion events. Some models deriving the burst energy from neutron star quakes for source distances of tens of kiloparsecs are not excluded and appear plausible.