SEARCH FOR SOFT GAMMA-REPEATERS IN THE SMM HXRBS DATA

Bursts from the soft gamma-ray repeaters (SGRs) appear to constitute a class of high-energy transients separate from typical gamma-ray bursts. Their distinct characteristics include stochastic burst repetition, short (approximately 0.1 s) durations, simple time histories, rise times as fast as 5 ms, spectra with characteristic energies of approximately 30 keV, and no indication of spectral evolution on time scales as short as approximately 0.1 s. Presently, only three repeaters are known. Thus, fundamental questions regarding their typical active lifetimes, celestial distribution, and luminosity function remain unresolved. We describe the results of a search for short transients resembling SGR bursts using the triggered fast memory of the hard X-ray burst spectrometer (HXRBS) on board the Solar Maximum Mission (SMM). This instrument operated for almost 10 years, from its launch in 1980 February until its demise in 1989 December. Each year its 40-degrees FWHM field of view scanned approximately one-quarter of the sky in the form of a band about 80-degrees wide centered on the ecliptic. HXRBS had a unique triggering capability with very short integration times of 4, 32, and 256 ms that made it ideal for detecting short, intense events. The effective HXRBS sensitivity for detecting SGR-like bursts longer than approximately 20 ms was routinely approximately 4 x 10(-7) ergs cm-2, and occasionally 8 x 10(-8) ergs cm-2. This search adds to the coverage by the International Cometary Explorer (ICE) of the most active repeater, SGR 1806-20, and indicates that the highly active phase of this source has not continued beyond 1984. We have found no other SGR-like source, but we identify some candidate events. Our results, combined with results from ICE and Konus observations and the assumption of a neutron star origin, enable us to make estimates of the burst-active time of an SGR source. In addition, we calculate the noise environment of the HXRBS CsI(Na) crystal on board SMM, a spacecraft in a low-inclination/low-altitude orbit, to be 1.5 x 10(-4) events s-1.