TITLE: GCN CIRCULAR NUMBER: 38847 SUBJECT: GRB 250108B: Swift detection of a burst DATE: 25/01/08 10:45:05 GMT FROM: K.L. Page at U Leicester klp5@leicester.ac.uk
N. J. Klingler (GSFC/UMBC/CRESSTII), R. Brivio (INAF-OAB), J. J. DeLaunay (PSU), R. Gupta (NASA GSFC), K. L. Page (U Leicester), C. Salvaggio (INAF-OAB) and T. Sbarrato (INAF-OAB) report on behalf of the Neil Gehrels Swift Observatory Team:
At 10:23:06 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 250108B (trigger=1280056). Swift slewed immediately to the burst. The BAT on-board calculated location is RA, Dec 201.297, +25.613 which is RA(J2000) = 13h 25m 11s Dec(J2000) = +25d 36' 46" with an uncertainty of 3 arcmin (radius, 90% containment, including systematic uncertainty). The BAT light curve showed a complex structure with a duration of about 30s sec. The peak count rate was ~1210 counts/sec (15-350 keV), at ~2 sec after the trigger.
The XRT began observing the field at 10:24:54.0 UT, 107.8 seconds after the BAT trigger. Using promptly downlinked data we find a bright, uncatalogued X-ray source with an enhanced position: RA, Dec 201.32686, 25.61541 which is equivalent to: RA(J2000) = 13h 25m 18.45s Dec(J2000) = +25d 36' 55.5" with an uncertainty of 2.0 arcseconds (radius, 90% containment). This location is 97 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 (1.16 x 10^20 cm^-2, Willingale et al. 2013), with an excess column of 4.6 (+2.37/-2.05) x 10^21 cm^-2 (90% confidence).
The initial flux in the 2.5 s image was 9.22e-10 erg cm^-2 s^-1 (0.2-10 keV).
UVOT took a finding chart exposure of 250 seconds with the U filter starting 274 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 19.2 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 18.0 mag. No correction has been made for the expected extinction corresponding to E(B-V) of 0.012.
Burst Advocate for this burst is N. J. Klingler (noelklin AT umbc.edu). 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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