GCN Circular 13952

T. N. Ukwatta (MSU), V. D'Elia (ASDC), H. A. Krimm (CRESST/GSFC/USRA),
F. E. Marshall (NASA/GSFC), A. Melandri (INAF-OAB),
D. M. Palmer (LANL), B. Sbarufatti (INAF-OAB/PSU), M. H. Siegel (PSU)
and E. Sonbas (NASA/GSFC/Adiyaman Univ.) report on behalf of the Swift
Team:

At 17:47:39 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 121108A (trigger=537921).  Swift slewed immediately to the burst. 
The BAT on-board calculated location is 
RA, Dec 83.186, +54.491 which is 
   RA(J2000) = 05h 32m 45s
   Dec(J2000) = +54d 29' 27"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  The BAT light curve showed a complex
peak structure for about 60 seconds, followed by an additional
peak at T+120 s.  The peak count rate was ~1500 counts/sec 
(15-350 keV), at ~28 sec after the trigger. 

The XRT began observing the field at 17:48:33.2 UT, 54.2 seconds after
the BAT trigger. Using promptly downlinked data we find a bright,
fading, uncatalogued X-ray source located at RA, Dec 83.19368, 54.47191
which is equivalent to:
   RA(J2000)  = 05h 32m 46.48s
   Dec(J2000) = +54d 28' 18.9"
with an uncertainty of 4.3 arcseconds (radius, 90% containment). This
location is 70 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. 

A power-law fit to a spectrum formed from promptly downlinked event
data gives a column density consistent with the Galactic value of 1.87
x 10^21 cm^-2 (Kalberla et al. 2005). 

The initial flux in the 2.5 s image was 1.96e-09 erg cm^-2 s^-1 (0.2-10
keV). 

UVOT took a finding chart exposure of 150 seconds with the White filter
starting 63 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
0.37. 

Burst Advocate for this burst is T. N. Ukwatta (tilan.ukwatta 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/too.html.)