TITLE: GCN GRB OBSERVATION REPORT NUMBER: 1084 SUBJECT: Dramatic Increase in the Rate of X-ray Bursts Reported by HETE DATE: 01/07/30 21:06:05 GMT FROM: George Ricker at MIT Dramatic Increase in the Rate of X-ray Bursts Reported by HETE G. Ricker (MIT), on behalf of the HETE Team, reports: Over the past few weeks, the number of X-ray bursts (XRBs) reported by HETE has been gradually increasing, reaching ~10 per day during the past 2-3 days. This seeming storm of XRBs is being produced by an ensemble of ~10 distinct XRB sources, which are optimally placed in the HETE field-of-view (FOV) for detectability. This dramatic overall increase is due to two factors: 1) Improvements in the on-board detection algorithm, resulting from lowered trigger thresholds and increased numbers of trigger time scales being activated; 2) Sco X-1 has moved out of the HETE instruments' FOV, greatly reducing the overall background, thus improving the instruments' burst detection efficiency. (NB: Despite the flurry of XRBs, soft gamma repeaters [SGR] and gamma-ray bursts [GRB] positions are now being, and will continue to be, reported for events which trigger the high energy bands of the FREGATE instrument directly.) Many of the faint XRB detections are near threshold, and thus their onboard localization is unreliable. In order not to saturate rapid response telescopes and observing programs which primarily target SGRs and GRBs, these XRB events are being reported to the GCN with no positions. (In fact, reliable positions are subsequently established by the WXM ground software for a large fraction of these XRB events.) A refined reporting algorithm, which will more clearly distinguish fainter, poorly-localized XRBs from brighter, well-localized XRBs will be implemented in the near future. In the meantime, the XRB "alert only" reports will continue. Beginning in late August, when the Galactic Bulge Region moves out of HETE's FOV, we anticipate a precipitous drop in the reported XRB rate.