GCN Circular 41403

S. B. Cenko (GSFC), C. Gronwall (PSU), R. Gupta (NASA GSFC),
H. A. Krimm (NSF), A. Y. Lien (U Tampa), K. L. Page (U Leicester),
D. M. Palmer (LANL) and M. H. Siegel (PSU) report on behalf of the
Neil Gehrels Swift Observatory Team:

At 01:31:22 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 250818A (trigger=1343270).  Swift slewed immediately to the burst.
The BAT on-board calculated location is 
RA, Dec 213.833, -58.007 which is 
   RA(J2000) = 14h 15m 20s
   Dec(J2000) = -58d 00' 26"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  Due to a communications gap, only the
BAT light curve after T+8s is available.  This later lightcurve
shows a complex structure out to at least T+150s.  The peak count
rate in the available lightcurve was ~5500 counts/sec (15-350 keV), 
at ~80 sec after the trigger.

The XRT began observing the field at 01:33:00.6 UT, 97.9 seconds after
the BAT trigger. XRT found a bright, uncatalogued X-ray source located
at RA, Dec 213.8002, -58.0134 which is equivalent to:
   RA(J2000)  = 14h 15m 12.05s
   Dec(J2000) = -58d 00' 48.2"
with an uncertainty of 4.9 arcseconds (radius, 90% containment). This
location is 66 arcseconds from the BAT onboard position, within the BAT
error circle. 

The initial flux in the 0.1 s image was 1.09e-07 erg cm^-2 s^-1 (0.2-10
keV).

UVOT took a finding chart exposure of 250 seconds with the U filter starting
162 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. Because of the density of catalogued
stars, further analysis is required to report an upper limit for any afterglow
in the region. No correction has been made for the large, but uncertain,
extinction expected.

Burst Advocate for this burst is S. B. Cenko (brad.cenko AT nasa.gov).
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/)