Hard Burst Emission from the Soft Gamma Repeater SGR 1900+14

Abstract

We present evidence for burst emission from SGR 1900+14 with a power-law high energy spectrum extending beyond 500 keV. Unlike previous detections of high energy photons during bursts from SGRs, these emissions are not associated with high-luminosity burst intervals. Not only is the emission hard, but the spectra are better fit by Band's GRB function rather than by the traditional optically-thin thermal bremsstrahlung model. We find that the spectral evolution within these hard events obeys a hardness/intensity anti-correlation. Temporally, these events are distinct from typical SGR burst emissions in that they are longer (~ 1 s) and have relatively smooth profiles. Despite a difference in peak luminosity of > 1E+11 between these bursts from SGR 1900+14 and cosmological GRBs, there are striking temporal and spectral similarities between the two kinds of bursts, aside from spectral evolution. We outline an interpretation of these events in the context of the magnetar model.