GCN Circular 1084

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.