GCN Circular 13938

V. D'Elia (ASDC), S. D. Barthelmy (GSFC),
W. H. Baumgartner (GSFC/UMBC), D. N. Burrows (PSU),
J. R. Cummings (NASA/UMBC), N. Gehrels (NASA/GSFC),
J. A. Kennea (PSU), F. E. Marshall (NASA/GSFC), D. M. Palmer (LANL),
B. Sbarufatti (INAF-OAB/PSU) and M. H. Siegel (PSU) report on behalf
of the Swift Team:

At 02:27:02 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 121102A (trigger=537266).  Swift slewed immediately to the burst. 
The BAT on-board calculated location is 
RA, Dec 270.903, -16.966 which is 
   RA(J2000)  = 18h 03m 37s
   Dec(J2000) = -16d 57' 56"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  The BAT light curve shows a single FRED-like peak
with a duration of about 25 sec.  The peak count rate
was ~3500 counts/sec (15-350 keV), at ~6 sec after the trigger. 

The XRT began observing the field at 02:27:50.4 UT, 47.7 seconds after
the BAT trigger. Using promptly downlinked data we find a bright,
fading, uncatalogued X-ray source with an enhanced position: RA, Dec
270.9009, -16.9578 which is equivalent to:
   RA(J2000)  = 18h 03m 36.22s
   Dec(J2000) = -16d 57' 28.2"
with an uncertainty of 2.1 arcseconds (radius, 90% containment). This
location is 30 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 in excess of the Galactic value (5.33 x
10^21 cm^-2, Kalberla et al. 2005), with an excess column of 3.1
(+3.61/-1.55) x 10^21 cm^-2 (90% confidence). 

The initial flux in the 2.5 s image was 3.45e-09 erg cm^-2 s^-1 (0.2-10
keV). 

UVOT took a finding chart exposure of 150 seconds with the White
filter  starting 57 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 8'x8'
region for the list of sources generated on-board covers  100% of the
XRT error circle. Because of the density of catalogued stars,  further
analysis is required to report an upper limit for any afterglow in the
 region. No correction has been made for the large, but uncertain
extinction  expected. 

This event has all appearances of a GRB: hard BAT spectrum,
bright XRT initial detection and fading.  Still, we note that 
the galactic coordinates are l=12 deg, b=2.5 deg, consistent 
with a galactic origin.  We believe it is a GRB, but can not 
rule out a new galactic transient. 

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.)