Swift-BAT detection of a possible burst

TITLE: GCN GRB OBSERVATION REPORT
NUMBER: 4376
SUBJECT: Swift-BAT detection of a possible burst
DATE: 05/12/21 20:51:54 GMT
FROM: Scott Barthelmy at NASA/GSFC Scott@lheamail.gsfc.nasa.gov

P. Boyd (GSFC), S. Barthelmy (GSFC), D. Burrows (PSU), J. Cummings (GSFC/NRC), N. Gehrels (GSFC), H. Krimm (GSFC/USRA), C. Markwardt (GSFC/UMD), F. Marshall (GSFC), K. Page (U Leicester), D. Palmer (LANL), A. Parsons (GSFC), J. Racusin (PSU), P. Roming (PSU), T. Sakamoto (GSFC) on behalf of the Swift team:

At 20:03:20 UT, Swift-BAT triggered and located a source (trigger=173904). The spacecraft slewed promptly after the end of the image trigger. The BAT on-board calculated location is RA,Dec 312.395d,+53.054d {20h 49m 35s,+53d 03′ 14″} (J2000), with an uncertainty of 3 arcmin (radius, 90% containment, stat+sys). All we have at this time is the TDRSS lightcurve. This plus the fact that this is a 192-sec image trigger means that we can not say if this trigger is due to a real GRB, a hard x-ray transient, or a noise event. We note that the galactic latitude is 6 deg.

The XRT began observing the location at 20:07:56 UT, 276 sec after the BAT trigger. The on-board detection algorithm did not centroid on a source due to insufficient counts, so no prompt X-ray position is available. However, both the prompt XRT light-curve and raw spectrum indicate there is a faint X-ray source in the field. More information will be available after the next Malindi pass in a few hours.

UVOT took a finding chart exposure of 200 seconds with the V filter starting 275 seconds after the BAT trigger. No afterglow candidate has been found in the initial data products. The 2.7’x2.7′ sub-image covers 25% of the BAT error circle. The typical 3-sigma upper limit has been about 18th mag. The 8’x8′ region for the list of sources generated on-board covers 100% of the BAT error circle. The list of sources is typically complete to about 18.0 mag. No correction has been made for the expected visual extinction of about 4.6 magnitudes.