TITLE: GCN CIRCULAR NUMBER: 39958 SUBJECT: GRB 250331B: Swift detection of a burst DATE: 25/03/31 04:47:31 GMT FROM: David Palmer at LANL palmer@lanl.gov
S. B. Cenko (GSFC), C. Gronwall (PSU), R. Gupta (NASA GSFC), K. L. Page (U Leicester), D. M. Palmer (LANL) and M. H. Siegel (PSU) report on behalf of the Neil Gehrels Swift Observatory Team:
At 04:19:43 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 250331B (trigger=1299965). Swift slewed to the burst after a brief delay to clear an observing constraint. The BAT on-board calculated location is RA, Dec 71.381, +43.805 which is RA(J2000) = 04h 45m 31s Dec(J2000) = +43d 48' 17" with an uncertainty of 3 arcmin (radius, 90% containment, including systematic uncertainty). The BAT light curve showed a single-peak structure with a duration of about 8 sec. The peak count rate was ~1400 counts/sec (15-350 keV), at ~2 sec after the trigger.
The XRT began observing the field at 04:24:57.3 UT, 313.9 seconds after the BAT trigger. Using promptly downlinked data we find an uncatalogued X-ray source with an enhanced position: RA, Dec 71.37364, 43.81080 which is equivalent to: RA(J2000) = 04h 45m 29.67s Dec(J2000) = +43d 48' 38.9" with an uncertainty of 1.9 arcseconds (radius, 90% containment). This location is 28 arcseconds from the BAT onboard position, within the BAT error circle. This position may be improved as more data are received; the latest position is available at https://www.swift.ac.uk/sper. We cannot determine whether the source is fading at the present time.
A power-law fit to a spectrum formed from promptly downlinked event data does not constrain the column density.
UVOT took a finding chart exposure of 150 seconds with the White filter starting 317 seconds after the BAT trigger. No credible afterglow candidate has been found in the initial data products. The 2.7'x2.7' sub-image covers 100% of the XRT error circle. The typical 3-sigma upper limit has been about 19.6 mag. The 8'x8' region for the list of sources generated on-board covers 100% of the XRT error circle. The list of sources is typically complete to about 18 mag. No correction has been made for the large, but uncertain, extinction expected.
Burst Advocate for this burst is S. B. Cenko (brad.cenko AT nasa.gov). Please contact the BA by email if you require additional information regarding Swift followup of this burst. In extremely urgent cases, after trying the Burst Advocate, you can contact the Swift PI by phone (see Swift TOO web site for information: http://www.swift.psu.edu/)
View this GCN Circular online at https://gcn.nasa.gov/circulars/39958. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0LW...