UNDERSTANDING SOFT GAMMA-RAY REPEATERS IN THE CONTEXT OF THE EXTRAGALACTIC RADIO PULSAR ORIGIN OF GAMMA-RAY BURSTS

Both the soft gamma-ray repeaters (SGRs) and the classical gamma-ray burst (GRB) sources may be neutron stars undergoing structural adjustments that produce the observed transient gamma-ray events. We propose a unified scenario, in which a cosmological population of young radio pulsars is responsible for both phenomena, and we argue that whether the radiative emission associated with a pulsar ''glitch'' is seen as a GRB or as an SGR event, is determined primarily by the direction of our line of sight relative to the stellar spin axis. We show that whereas the classical GRB spectrum arises from Compton upscattering by charges accelerated along the viewing direction, the SGR burst spectrum is due to the thermalization of Alfven wave energy away from this direction and is roughly that of a blackbody, whose characteristic temperature is correlated to the burst luminosity. Within the context of this model, the difference in time histories between the two classes results from selection effects, since their detection probabilities imply that we are far more likely to see nearby, weak (and presumably short) events as SGR bursts and the more distant, powerful, and longer events as GRBs. The crudely inferred source distance to SGR 0526-66 and SGR 1806-20 is almost-equal-to 170 kpc and almost-equal-to 18 kpc, respectively, consistent with their apparent positioning within the LMC (the former) and the Galactic plane (the latter). If the crustal adjustments occur during the first approximately 50,000 yr of a pulsar's lifetime, this model predicts that we should be able to see approximately two SGR sources within the galaxy, in agreement with current observations.