GCN Circular 41462

S. Dichiara (PSU), R. A. J. Eyles-Ferris (U Leicester),
R. Gupta (NASA GSFC), K. L. Page (U Leicester),
B. Sbarufatti (INAF-OAB) and M. H. Siegel (PSU) report on behalf of
the Neil Gehrels Swift Observatory Team:

At 10:46:17 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 250821A (trigger=1344131).  Swift slewed immediately to the burst.
The BAT on-board calculated location is 
RA, Dec 358.412, -28.618 which is 
   RA(J2000) = 23h 53m 39s
   Dec(J2000) = -28d 37' 05"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  The BAT light curve showed a complex
structure with a duration of about 20 sec.  The peak count rate
was ~2476 counts/sec (15-350 keV), at ~0 sec after the trigger.

The XRT began observing the field at 10:48:40.1 UT, 142.8 seconds after
the BAT trigger. Using promptly downlinked data we find an uncatalogued
X-ray source located at RA, Dec 358.43969, -28.65380 which is
equivalent to:
   RA(J2000)  = 23h 53m 45.53s
   Dec(J2000) = -28d 39' 13.7"
with an uncertainty of 4.8 arcseconds (radius, 90% containment). This
location is 155 arcseconds from the BAT onboard position, within the
BAT error circle. This position may be improved as more data are
received; the latest position is available at
https://www.swift.ac.uk/sper.  We cannot determine whether the source
is fading at the present time. 

A power-law fit to a spectrum formed from promptly downlinked event
data gives a column density consistent with the Galactic value of 1.65
x 10^20 cm^-2 (Willingale et al. 2013).

UVOT took a finding chart exposure of 150 seconds with the White filter
starting 147 seconds after the BAT trigger. No credible afterglow candidate has
been found in the initial data products. The 2.7'x2.7' sub-image covers none of
the XRT error circle. The 8'x8' region for the list of sources generated
on-board covers 100% of the XRT error circle. The list of sources is typically
complete to about 18 mag. No correction has been made for the expected
extinction corresponding to E(B-V) of 0.015.

Burst Advocate for this burst is S. Dichiara (sbd5667 AT psu.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/)