TITLE:   GCN CIRCULAR
NUMBER:  14105
SUBJECT: GRB 121226A: Swift detection of a burst
DATE:    12/12/26 19:24:11 GMT
FROM:    Hans Krimm at NASA-GSFC  <krimm@milkyway.gsfc.nasa.gov>

H. A. Krimm (CRESST/GSFC/USRA), J. R. Cummings (NASA/UMBC),
V. D'Elia (ASDC), S. T. Holland (STScI), K. L. Page (U Leicester),
T. Sakamoto (AGU), M. H. Siegel (PSU) and
R. L. C. Starling (U Leicester) report on behalf of the Swift Team:

At 19:09:43 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 121226A (trigger=544027).  Swift slewed immediately to the burst. 
The BAT on-board calculated location is 
RA, Dec 168.658, -30.414 which is 
   RA(J2000) = 11h 14m 38s
   Dec(J2000) = -30d 24' 51"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  The BAT light curve showed a single peak
with a duration of about 2 sec.  The peak count rate was ~2700 counts/sec
(15-350 keV), at ~0 sec after the trigger.  Although the duration of this
burst is moderately short, it has a fairly soft spectrum.  Therefore we must
wait for further data to determine whether to classify GRB 121226A as
a short or long burst. 

The XRT began observing the field at 19:11:24.4 UT, 100.6 seconds after
the BAT trigger. Using promptly downlinked data we find an uncatalogued
X-ray source with an enhanced position: RA, Dec 168.6424, -30.4066
which is equivalent to:
   RA(J2000)  = 11h 14m 34.17s
   Dec(J2000) = -30d 24' 23.8"
with an uncertainty of 2.2 arcseconds (radius, 90% containment). This
location is 55 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 http://www.swift.ac.uk/sper.  We
cannot determine whether the source is fading at the present time. 

A power-law fit to a spectrum formed from promptly downlinked event
data gives a column density in excess of the Galactic value (6.11 x
10^20 cm^-2, Kalberla et al. 2005), with an excess column of 6.4
(+3.77/-3.06) x 10^21 cm^-2 (90% confidence). 

UVOT took a finding chart exposure of 150 seconds with the White filter
starting 104 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
0.05. 

Burst Advocate for this burst is H. A. Krimm (krimm AT milkyway.gsfc.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/too.html.)