[vsnet-grb-info 16855] Swift Trigger 652046 is probably not a GRB

[GCN Circulars]

TITLE:   GCN CIRCULAR
NUMBER:  18136
SUBJECT: Swift Trigger 652046 is probably not a GRB
DATE:    15/08/13 05:19:13 GMT
FROM:    David Palmer at LANL  <palmer at lanl.gov>

A. D'Ai (INAF-IASFPA), N. Gehrels (NASA/GSFC), A. Y. Lien (GSFC/UMBC),
F. E. Marshall (NASA/GSFC), A. Maselli (INAF-IASFPA),
K. L. Page (U Leicester), D. M. Palmer (LANL),
B. Sbarufatti (INAF-OAB/PSU) and E. Troja (NASA/GSFC/UMCP) report on
behalf of the Swift Team:

At 04:54:28 UT, the Swift Burst Alert Telescope (BAT) triggered and
located a possible, but unlikely, GRB (trigger=652046). 
Swift slewed immediately to the location. 
The BAT on-board calculated location is 
RA, Dec 216.661, -7.541 which is 
   RA(J2000) = 14h 26m 39s
   Dec(J2000) = -07d 32' 27"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  The BAT light curve showed a single high
time bin with a duration of about 0.128 sec.  The peak count rate
was ~1200 counts/sec (15-350 keV), at ~0.1 sec after the trigger. 

The XRT began observing the field at 04:55:25.5 UT, 57.3 seconds after
the BAT trigger. No source was detected in 955 s 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 60 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
0.05. 

Due to the marginal image significance of this event (6.5 sigma),
and the lack of an X-ray detection, we believe that this is probably
a statistical fluctuation rather than an astrophysical event. 

Determination of the reality of this event will require the
full downlinked dataset.