TITLE:   GCN CIRCULAR
NUMBER:  39959
SUBJECT: GRB 250331C: Swift detection of a burst
DATE:    25/03/31 04:55:40 GMT
FROM:    David Palmer at LANL <palmer@lanl.gov>

S. B. Cenko (GSFC), C. Gronwall (PSU), 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 04:37:40 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 250331C (trigger=1299967).  Swift slewed immediately to the burst.
The BAT on-board calculated location is 
RA, Dec 80.993, +33.029 which is 
   RA(J2000) = 05h 23m 58s
   Dec(J2000) = +33d 01' 43"
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 25 sec.  The peak count rate
was ~800 counts/sec (15-350 keV), at ~1 sec after the trigger.

The XRT began observing the field at 04:39:51.5 UT, 130.9 seconds after
the BAT trigger. Using promptly downlinked data we find a bright,
uncatalogued X-ray source located at RA, Dec 80.95740, 33.06310 which
is equivalent to:
   RA(J2000)  = 05h 23m 49.78s
   Dec(J2000) = +33d 03' 47.2"
with an uncertainty of 3.7 arcseconds (radius, 90% containment). This
location is 163 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 consistent with the Galactic value of 6.49
x 10^21 cm^-2 (Willingale et al. 2013). 

The initial flux in the 2.5 s image was 1.04e-09 erg cm^-2 s^-1 (0.2-10
keV).

UVOT took a finding chart exposure of 150 seconds with the White filter
starting 139 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 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/)