TITLE:   GCN CIRCULAR
NUMBER:  39794
SUBJECT: GRB 250321A: Swift detection of a burst
DATE:    25/03/21 01:04:22 GMT
FROM:    David Palmer at LANL <palmer@lanl.gov>

R. Gupta (NASA GSFC), J. A. Kennea (PSU),
N. J. Klingler (GSFC/UMBC/CRESSTII), D. M. Palmer (LANL) and
M. H. Siegel (PSU) report on behalf of the Neil Gehrels Swift
Observatory Team:

At 00:42:38 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 250321A (trigger=1297508).  Swift slewed immediately to the burst.
The BAT on-board calculated location is 
RA, Dec 295.077, +21.041 which is 
   RA(J2000) = 19h 40m 19s
   Dec(J2000) = +21d 02' 27"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  Only the first 8 seconds of the BAT light
curve after the trigger is available due to a telemetry gap.
This data shows a complex structure with a duration of
at least 10 sec.  The peak count rate was ~2500 counts/sec (15-350 keV),
at ~0 sec after the trigger.

The XRT began observing the field at 00:44:43.60 UT, 125.6 seconds after
the BAT trigger. No source was detected in the 2.5-s promptly available
image. We are waiting for the full dataset to detect and localise the
XRT counterpart.

Due to a telemetry gap, UVOT data are not available at this time.

Although XRT did not report an X-ray counterpart in the limited immediately
transmitted data, the significance of the BAT image (>10 sigma), the
shape of the BAT light curve, and the simultaneous detection by Fermi/GBM
(trigger 764210564) give us confidence that this is an astrophysical
Gamma Ray Burst.

Burst Advocate for this burst is R. Gupta (rahulbhu.c157 AT gmail.com).
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/)