TITLE:   GCN CIRCULAR
NUMBER:  14345
SUBJECT: Swift Trigger 552142: a possible GRB
DATE:    13/03/28 02:49:14 GMT
FROM:    Scott Barthelmy at NASA/GSFC  <scott@milkyway.gsfc.nasa.gov>

H. A. Krimm (CRESST/GSFC/USRA), S. D. Barthelmy (GSFC),
W. H. Baumgartner (GSFC/UMBC), J. A. Kennea (PSU),
N. P. M. Kuin (UCL-MSSL), V. Mangano (INAF-IASFPA),
A. Maselli (INAF-IASFPA), D. M. Palmer (LANL),
J. L. Racusin (NASA/GSFC), M. H. Siegel (PSU),
E. Troja (NASA/GSFC/UMCP) and T. N. Ukwatta (MSU) report on behalf of
the Swift Team:

At 02:13:57 UT, the Swift Burst Alert Telescope (BAT) triggered and
located a possible GRB (trigger=552142).  Swift slewed immediately to the burst. 
The BAT on-board calculated location is 
RA, Dec 270.747, +21.327, which is 
   RA(J2000)  =  18h 02m 59s
   Dec(J2000) = +21d 19' 38"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  The BAT light curve shows nothing significant
as is typical for an image trigger. 

The XRT began observing the field at 02:16:10.7 UT, 133.1 seconds after
the BAT trigger. No source was detected in 1.5 ks of promptly
downlinked data. We are waiting for the full dataset to detect and
localise the XRT counterpart. 

UVOT took a finding chart exposure of 150 seconds with the White filter
starting 136 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 25% of
the BAT 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
BAT 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
0.11. 

Ground reprocessing of the immediately available BAT data found
a decreased significance at the on-board detection location. 
This, along with the non-detection of a source in the XRT and
UVOT data means that we cannot confirm that this is a
true astrophysical event.  A final determination of the
reality of this source will require analysis of the full 
ground-linked data set. 

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.)