TITLE:   GCN CIRCULAR
NUMBER:  31316
SUBJECT: GRB 211227A: Swift detection of a burst
DATE:    21/12/27 23:48:49 GMT
FROM:    David Palmer at LANL  <palmer@lanl.gov>

A. P. Beardmore (U Leicester), S. D. Barthelmy (GSFC),
F. E. Marshall (NASA/GSFC), K. L. Page (U Leicester),
D. M. Palmer (LANL), T. M. Parsotan (GSFC/UMBC/CRESSTII),
B. Sbarufatti (PSU), M. H. Siegel (PSU) and A. Tohuvavohu (U Toronto)
report on behalf of the Neil Gehrels Swift Observatory Team:

At 23:32:06 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 211227A (trigger=1091101).  Swift slewed immediately to the burst. 
The BAT on-board calculated location is 
RA, Dec 132.126, -2.702 which is 
   RA(J2000) = 08h 48m 30s
   Dec(J2000) = -02d 42' 05"
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 100 sec.  The peak count rate
was ~5200 counts/sec (15-350 keV), at ~1 sec after the trigger. 

The XRT began observing the field at 23:33:20.3 UT, 73.5 seconds after
the BAT trigger. Using promptly downlinked data we find a bright,
fading, uncatalogued X-ray source located at RA, Dec 132.14898,
-2.73556 which is equivalent to:
   RA(J2000)  = 08h 48m 35.76s
   Dec(J2000) = -02d 44' 08.0"
with an uncertainty of 3.8 arcseconds (radius, 90% containment). This
location is 146 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 in excess of the Galactic value (2.37 x
10^20 cm^-2, Willingale et al. 2013), with an excess column of 4.3
(+3.24/-2.79) x 10^21 cm^-2 (90% confidence). 

The initial flux in the 2.5 s image was 6.84e-09 erg cm^-2 s^-1 (0.2-10
keV). 

UVOT took a finding chart exposure of 150 seconds with the White filter
starting 81 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 expected extinction corresponding to E(B-V) of
0.020. 

Burst Advocate for this burst is A. P. Beardmore (apb AT star.le.ac.uk). 
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/)