TITLE:   GCN CIRCULAR
NUMBER:  13934
SUBJECT: GRB 121031A: Swift detection of a burst
DATE:    12/10/31 23:17:04 GMT
FROM:    David Palmer at LANL  <palmer@lanl.gov>

V. D'Elia (ASDC), S. D. Barthelmy (GSFC),
W. H. Baumgartner (GSFC/UMBC), D. N. Burrows (PSU),
M. M. Chester (PSU), J. R. Cummings (NASA/UMBC),
N. Gehrels (NASA/GSFC), S. T. Holland (STScI), J. A. Kennea (PSU),
F. E. Marshall (NASA/GSFC) and D. M. Palmer (LANL) report on behalf of
the Swift Team:

At 22:50:30 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 121031A (trigger=537195).  Swift slewed immediately to the burst. 
The BAT on-board calculated location is 
RA, Dec 170.771, -3.511 which is 
   RA(J2000)  =  11h 23m 05s
   Dec(J2000) = -03d 30' 37"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  The BAT light curve shows a large peak
with several smaller peaks riding on top with a total duration
of about 50 sec.  The peak count rate was ~4000 counts/sec (15-350 keV),
at ~4 sec after the trigger. 

The XRT began observing the field at 22:51:32.6 UT, 61.9 seconds after
the BAT trigger. XRT found a bright, uncatalogued X-ray source located
at RA, Dec 170.7694, -3.5153 which is equivalent to:
   RA(J2000)  = +11h 23m 4.66s
   Dec(J2000) = -03d 30' 55.1"
with an uncertainty of 4.8 arcseconds (radius, 90% containment). This
location is 17 arcseconds from the BAT onboard position, within the BAT
error circle. Despite the onboard localisation, no X-ray source was
detected in 732 s of promptly-downlinked data, suggesting that the
initial centroid may equally have been a cosmic ray. This position
should	therefore be treated with caution. 

The initial flux in the 2.5 s image was 5.90e-09 erg cm^-2 s^-1 (0.2-10
keV). 

UVOT took a finding chart exposure of 150 seconds with the White 
filter starting 70 seconds after the BAT trigger. No aspect solution
is available for the 2.7'x2.7' sub-image; results from the list of 
sources  generated on-board are not available at this time. The 
expected extinction corresponding to E(B-V) of 0.05. 

We note that the XRT position is 1.4 arcminutes from the galaxy 
2MASX J11225935-0331287, at redshift 0.1126, which has a nominal 
radius of 0.3 arcminutes according to NED. 

Burst Advocate for this burst is V. D'Elia (delia AT asdc.asi.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/too.html.)