TITLE: GCN CIRCULAR NUMBER: 41268 SUBJECT: GRB 250807B: Swift detection of a burst with an optical counterpart. DATE: 25/08/07 15:59:34 GMT FROM: noelklin@umbc.edu

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

At 15:43:07 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 250807B (trigger=1340561). Swift slewed immediately to the burst. The BAT on-board calculated location is RA, Dec 15.585, -59.094 which is RA(J2000) = 01h 02m 21s Dec(J2000) = -59d 05' 37" with an uncertainty of 3 arcmin (radius, 90% containment, including systematic uncertainty). The BAT light curve shows complex activity during the slew that immediately preceded this trigger, with a total observed duration of about 100 sec. The peak count rate was ~1200 counts/sec (15-350 keV), at ~15 sec after the trigger.

The XRT began observing the field at 15:45:32.3 UT, 144.5 seconds after the BAT trigger. XRT found a bright, uncatalogued X-ray source located at RA, Dec 15.6378, -59.1186 which is equivalent to: RA(J2000) = 01h 02m 33.07s Dec(J2000) = -59d 07' 07.0" with an uncertainty of 5.6 arcseconds (radius, 90% containment). This location is 131 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 153 seconds after the BAT trigger. There is a candidate afterglow in the rapidly available 2.7'x2.7' sub-image at RA(J2000) = 01:02:32.98 = 15.63741 DEC(J2000) = -59:07:07.9 = -59.11887 with a 90%-confidence error radius of about 0.75 arc sec. This position is 1.2 arc sec. from the center of the XRT error circle. The estimated magnitude is 16.31 with a 1-sigma error of about 0.14. No correction has been made for the expected extinction corresponding to E(B-V) of 0.016.

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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