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/)