GCN Circular 41268

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/)