A SEARCH FOR NONTRIGGERED GAMMA-RAY BURSTS IN THE BATSE DATABASE

We describe a search of archival data from the Burst and Transient Sou rce Experiment (BATSE). The purpose of the search is to find astronomi cally interesting transients that did not activate the burst-detection (or ''trigger'') system on board the spacecraft. Our search is sensit ive to events with peak fluxes (on the 1.024 s timescale) that are low er by a factor of similar to 2 than can be detected with the on-board burst trigger. In a search of 345 days of archival data, we detected 9 1 events in the 50-300 keV range that resemble classical gamma-ray bur sts but that did not activate the on-board burst trigger. We also dete cted 110 low-energy (25-50 keV) events of unknown origin that may incl ude activity from soft gamma repeater (SGR) 1805-20 and bursts and fla res from X-ray binaries. This paper gives the occurrence times, estima ted source directions, durations, peak fluxes, and fluences for the 91 gamma-ray burst candidates. The direction and intensity distributions of these bursts imply that the biases, inherent in the on-board trigg er mechanism have not significantly affected the completeness of the p ublished BATSE gamma-ray burst catalogs.