TITLE: GCN CIRCULAR NUMBER: 41262 SUBJECT: GRB 250807A: Swift detection of a burst with an optical counterpart DATE: 25/08/07 11:03:36 GMT FROM: noelklin@umbc.edu

N. J. Klingler (GSFC/UMBC/CRESSTII), R. Brivio (INAF-OAB), J. J. DeLaunay (PSU), R. Gupta (NASA GSFC), J. A. Kennea (PSU), K. L. Page (U Leicester) and C. Salvaggio (INAF-OAB) report on behalf of the Neil Gehrels Swift Observatory Team:

At 10:46:08.44 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 250807A (trigger=1340514). Swift slewed immediately to the burst. The BAT on-board calculated location is RA, Dec 51.420, -47.877 which is RA(J2000) = 03h 25m 41s Dec(J2000) = -47d 52' 38" 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 80 sec. The peak count rate was ~19000 counts/sec (15-350 keV), at ~35 sec after the trigger.

The XRT began observing the field at 10:47:49.5 UT, 101.1 seconds after the BAT trigger. XRT found a bright, uncatalogued X-ray source located at RA, Dec 51.4478, -47.8880 which is equivalent to: RA(J2000) = 03h 25m 47.47s Dec(J2000) = -47d 53' 16.8" with an uncertainty of 4.8 arcseconds (radius, 90% containment). This location is 77 arcseconds from the BAT onboard position, within the BAT error circle. No event data are yet available to determine the column density using X-ray spectroscopy.

UVOT took a finding chart exposure of 150 seconds with the White filter starting 111 seconds after the BAT trigger. There is a candidate afterglow in the rapidly available 2.7'x2.7' sub-image at RA(J2000) = 03:25:47.59 = 51.44829 DEC(J2000) = -47:53:16.2 = -47.88782 with a 90%-confidence error radius of about 0.75 arc sec. This position is 1.3 arc sec. from the center of the XRT error circle. The estimated magnitude is 17.98 with a 1-sigma error of about 0.14. No correction has been made for the expected extinction corresponding to E(B-V) of 0.011.

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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