GCN Circular 39958

S. B. Cenko (GSFC), C. Gronwall (PSU), R. Gupta (NASA GSFC),
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 04:19:43 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 250331B (trigger=1299965).  Swift slewed to the burst
after a brief delay to clear an observing constraint.
The BAT on-board calculated location is 
RA, Dec 71.381, +43.805 which is 
   RA(J2000) = 04h 45m 31s
   Dec(J2000) = +43d 48' 17"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  The BAT light curve showed a single-peak
structure with a duration of about 8 sec.  The peak count rate
was ~1400 counts/sec (15-350 keV), at ~2 sec after the trigger.

The XRT began observing the field at 04:24:57.3 UT, 313.9 seconds after
the BAT trigger. Using promptly downlinked data we find an uncatalogued
X-ray source with an enhanced position: RA, Dec 71.37364, 43.81080
which is equivalent to:
   RA(J2000)  = 04h 45m 29.67s
   Dec(J2000) = +43d 48' 38.9"
with an uncertainty of 1.9 arcseconds (radius, 90% containment). This
location is 28 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 does not constrain the column density.

UVOT took a finding chart exposure of 150 seconds with the White filter
starting 317 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 19.6 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 mag. 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/)