TITLE:   GCN CIRCULAR
NUMBER:  37274
SUBJECT: GRB 240825A: Swift detection of a burst with a bright optical counterpart
DATE:    24/08/25 16:04:17 GMT
FROM:    David Palmer at LANL <palmer@lanl.gov>


R. Gupta (NASA GSFC), R. Brivio (INAF-OAB), S. Dichiara (PSU),
M. Ferro (INAF-OAB), J. A. Kennea (PSU), K. L. Page (U Leicester),
D. M. Palmer (LANL) and T. Sbarrato (INAF-OAB) report on behalf of the
Neil Gehrels Swift Observatory Team:

At 15:52:59 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 240825A (trigger=1250617).  Swift slewed immediately to the burst. 
The BAT on-board calculated location is 
RA, Dec 344.551, +1.036 which is 
   RA(J2000) = 22h 58m 12s
   Dec(J2000) = +01d 02' 10"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  The BAT light curve showed a bright complex peak
structure with a duration of at least 10 sec.  The peak count rate
was ~160k counts/sec (15-350 keV), at ~1 sec after the trigger. 

The XRT began observing the field at 15:54:22.9 UT, 83.1 seconds after
the BAT trigger. XRT found a bright, uncatalogued X-ray source located
at RA, Dec 344.5691, 1.0253 which is equivalent to:
   RA(J2000)  = 22h 58m 16.58s
   Dec(J2000) = +01d 01' 31.1"
with an uncertainty of 12.3 arcseconds (radius, 90% containment). This
location is 75 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 92 seconds after the BAT trigger. There is a candidate afterglow in
the rapidly available 2.7'x2.7' sub-image at
  RA(J2000)  =	22:58:17.26 = 344.57192
  DEC(J2000) = +01:01:36.7  =	1.02686
with a 90%-confidence error radius of about 0.74 arc sec. This position is 11.6
arc sec. from the center of the XRT error circle. The estimated magnitude is
15.57 with a 1-sigma error of about  0.14. No correction has been made for the
expected extinction corresponding to E(B-V) of 0.063. 

Burst Advocate for this burst is R. Gupta (rahulbhu.c157 AT gmail.com). 
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/)