TITLE:   GCN CIRCULAR
NUMBER:  33108
SUBJECT: GRB 221226B: Swift detection of a burst
DATE:    22/12/26 23:00:32 GMT
FROM:    David Palmer at LANL  <palmer@lanl.gov>

S. B. Cenko (GSFC), J.D. Gropp (PSU), J. A. Kennea (PSU),
H. A. Krimm (NSF), S. Laha (GSFC/UMBC), A. Y. Lien (U Tampa),
F. E. Marshall (NASA/GSFC), K. L. Page (U Leicester),
D. M. Palmer (LANL) and T. M. Parsotan (GSFC/UMBC/CRESSTII) report on
behalf of the Neil Gehrels Swift Observatory Team:

At 22:41:20 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 221226B (trigger=1145959).  Swift slewed immediately to the burst. 
The BAT on-board calculated location is 
RA, Dec 22.917, -41.554 which is 
   RA(J2000) = 01h 31m 40s
   Dec(J2000) = -41d 33' 13"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  The BAT light curve showed a multi-peaked
structure with a duration of about 10 sec.  The peak count rate
was ~2200 counts/sec (15-350 keV), at ~1 sec after the trigger. 

The XRT began observing the field at 22:42:32.4 UT, 72.4 seconds after
the BAT trigger. Using promptly downlinked data we find a fading,
uncatalogued X-ray source located at RA, Dec 22.90890, -41.52648 which
is equivalent to:
   RA(J2000)  = 01h 31m 38.14s
   Dec(J2000) = -41d 31' 35.3"
with an uncertainty of 3.5 arcseconds (radius, 90% containment). This
location is 101 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 1.69
x 10^20 cm^-2 (Willingale et al. 2013). 

UVOT took a finding chart exposure of 150 seconds with the White filter
starting 75 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 expected
extinction corresponding to E(B-V) of 0.017. 

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/)