GCN Circular 41511

M. H. Siegel (PSU), S. Dichiara (PSU), R. Gupta (NASA GSFC),
N. P. M. Kuin (UCL-MSSL), S. Lanava (PSU), K. L. Page (U Leicester),
D. M. Palmer (LANL) and B. Sbarufatti (INAF-OAB) report on behalf of
the Neil Gehrels Swift Observatory Team:

At 20:10:07 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 250823A (trigger=1344586).  Swift slewed immediately to the burst.
The BAT on-board calculated location is 
RA, Dec 297.521, -24.976 which is 
   RA(J2000) = 19h 50m 05s
   Dec(J2000) = -24d 58' 33"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty). The BAT light curve shows a complex structure 
with a duration of 200 sec.  The peak count rate
was ~1600 counts/sec (15-350 keV), at ~4 sec after the trigger.

The XRT began observing the field at 20:17:28.0 UT, 440.5 seconds after
the BAT trigger. Using promptly downlinked data we find a fading,
uncatalogued X-ray source with an enhanced position: RA, Dec 297.47788,
-24.99582 which is equivalent to:
   RA(J2000)  = 19h 49m 54.69s
   Dec(J2000) = -24d 59' 45.0"
with an uncertainty of 2.1 arcseconds (radius, 90% containment). This
location is 157 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. 

A power-law fit to a spectrum formed from promptly downlinked event
data gives a column density in excess of the Galactic value (1.12 x
10^21 cm^-2, Willingale et al. 2013), with an excess column of 3.9
(+3.01/-2.59) x 10^21 cm^-2 (90% confidence).

UVOT took a finding chart exposure of 250 seconds with the U filter starting
338 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 100% of the
XRT error circle. The typical 3-sigma upper limit has been about 19.2 mag. 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 18.0 mag. No
correction has been made for the expected extinction corresponding to E(B-V) of
0.111.

Some data, such as earlier XRT observations, are not available due to
a telemetry gap, and will be produced on the full ground download.

Burst Advocate for this burst is M. H. Siegel (siegel AT swift.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/)