TITLE: GCN CIRCULAR NUMBER: 39396 SUBJECT: GRB 250221A: Swift detection of a burst with an optical counterpart DATE: 25/02/21 03:53:15 GMT FROM: David Palmer at LANL palmer@lanl.gov
R. Caputo (GSFC), C. Gronwall (PSU), R. Gupta (NASA GSFC), K. L. Page (U Leicester), D. M. Palmer (LANL), T. M. Parsotan (GSFC) and M. H. Siegel (PSU) report on behalf of the Neil Gehrels Swift Observatory Team:
At 03:34:37 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 250221A (trigger=1290305). Swift slewed immediately to the burst. The BAT on-board calculated location is RA, Dec 59.477, -15.139 which is RA(J2000) = 03h 57m 54s Dec(J2000) = -15d 08' 18" 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 5 sec. The peak count rate was ~2200 counts/sec (15-350 keV), at ~0 sec after the trigger.
The XRT began observing the field at 03:36:39.9 UT, 122.9 seconds after the BAT trigger. Using promptly downlinked data we find an uncatalogued X-ray source with an enhanced position: RA, Dec 59.46251, -15.13379 which is equivalent to: RA(J2000) = 03h 57m 51.00s Dec(J2000) = -15d 08' 01.6" with an uncertainty of 1.9 arcseconds (radius, 90% containment). This location is 53 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.
A power-law fit to a spectrum formed from promptly downlinked event data gives a column density in excess of the Galactic value (3.73 x 10^20 cm^-2, Willingale et al. 2013), with an excess column of 4.1 (+2.63/-2.31) x 10^21 cm^-2 (90% confidence).
UVOT took a finding chart exposure of 250 seconds with the U filter starting 340 seconds after the BAT trigger. There is a candidate afterglow in the rapidly available 2.7'x2.7' sub-image at RA(J2000) = 03:57:51.06 = 59.46274 DEC(J2000) = -15:07:59.2 = -15.13311 with a 90%-confidence error radius of about 0.76 arc sec. This position is 2.4 arc sec. from the center of the XRT error circle. The estimated magnitude is 17.82 with a 1-sigma error of about 0.15. No correction has been made for the expected extinction corresponding to E(B-V) of 0.046.
Burst Advocate for this burst is R. Caputo (regina.caputo 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/)
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