TITLE:   GCN CIRCULAR
NUMBER:  43166
SUBJECT: GRB 251221A: Swift detection of a burst
DATE:    25/12/21 05:38:42 GMT
FROM:    Elena Ambrosi at INAF-IASF <elena.ambrosi@inaf.it>

E. Ambrosi (INAF-IASFPA), C. Gronwall (PSU), R. Gupta (NASA GSFC),
A. Melandri (INAF-OAR), K. L. Page (U Leicester) and
D. M. Palmer (LANL) report on behalf of the Neil Gehrels Swift
Observatory Team:

At 05:24:31 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 251221A (trigger=1426088).  Swift slewed immediately to the burst.
The BAT on-board calculated location is 
RA, Dec 101.408, +0.729 which is 
   RA(J2000) = 06h 45m 38s
   Dec(J2000) = +00d 43' 45"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  The BAT light curve showed a single-peaked
structure with a duration of about 50 sec.  The peak count rate
was ~1300 counts/sec (15-350 keV), at ~1 sec after the trigger.

The XRT began observing the field at 05:26:50.3 UT, 138.4 seconds after
the BAT trigger. Using promptly downlinked data we find a fading,
uncatalogued X-ray source with an enhanced position: RA, Dec 101.42801,
0.73241 which is equivalent to:
   RA(J2000)  = 06h 45m 42.72s
   Dec(J2000) = +00d 43' 56.7"
with an uncertainty of 2.1 arcseconds (radius, 90% containment). This
location is 73 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 7.34
x 10^21 cm^-2 (Willingale et al. 2013).

UVOT took a finding chart exposure of 150 seconds with the White filter
starting 142 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 E. Ambrosi (elena.ambrosi AT inaf.it).
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/)