TITLE: GCN CIRCULAR NUMBER: 42146 SUBJECT: GRB 251007A: Swift detection of a burst with an optical counterpart DATE: 25/10/07 19:53:26 GMT FROM: David Palmer at LANL palmer@lanl.gov
E. Ambrosi (INAF-IASFPA), A. D'Ai (INAF-IASFPA), V. D'Elia (SSDC & INAF-OAR), S. Dichiara (PSU), R. Gupta (NASA GSFC), N. J. Klingler (GSFC/UMBC/CRESSTII), N. P. M. Kuin (UCL-MSSL), A. Melandri (INAF-OAR), M. J. Moss (GSFC), K. L. Page (U Leicester) and D. M. Palmer (LANL) report on behalf of the Neil Gehrels Swift Observatory Team:
At 19:36:58 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 251007A (trigger=1402466). Swift slewed immediately to the burst. The BAT on-board calculated location is RA, Dec 128.173, +21.823 which is RA(J2000) = 08h 32m 42s Dec(J2000) = +21d 49' 24" with an uncertainty of 3 arcmin (radius, 90% containment, including systematic uncertainty). The BAT light curve showed a complex structure with an initial sequence of peaks followed by a larger complex peak with a total duration of about 100 sec. The peak count rate was ~12000 counts/sec (15-350 keV), at ~56 sec after the trigger.
The XRT began observing the field at 19:38:07.4 UT, 69.4 seconds after the BAT trigger. XRT found a bright, uncatalogued X-ray source located at RA, Dec 128.1775, 21.8138 which is equivalent to: RA(J2000) = 08h 32m 42.60s Dec(J2000) = +21d 48' 49.7" with an uncertainty of 5.4 arcseconds (radius, 90% containment). This location is 36 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 78 seconds after the BAT trigger. There is a candidate afterglow in the rapidly available 2.7'x2.7' sub-image at RA(J2000) = 08:32:42.40 = 128.17665 DEC(J2000) = +21:48:46.1 = 21.81280 with a 90%-confidence error radius of about 0.63 arc sec. This position is 4.6 arc sec. from the center of the XRT error circle. The estimated magnitude is 18.38 with a 1-sigma error of about 0.15. No correction has been made for the expected extinction corresponding to E(B-V) of 0.039.
Burst Advocate for this burst is E. Ambrosi (elena.ambrosi AT inaf.it). 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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