[vsnet-grb-info 13363] Swift Trigger 556533 is probably not an astrophysical event

[GCN Circulars]

TITLE:   GCN CIRCULAR
NUMBER:  14699
SUBJECT: Swift Trigger 556533 is probably not an astrophysical event
DATE:    13/05/24 04:23:05 GMT
FROM:    David Palmer at LANL  <palmer at lanl.gov>

A. P. Beardmore (U Leicester), H. A. Krimm (CRESST/GSFC/USRA),
C. J. Mountford (U Leicester), K. L. Page (U Leicester),
D. M. Palmer (LANL) and M. H. Siegel (PSU) report on behalf of the
Swift Team:

At 04:03:55 UT, the Swift Burst Alert Telescope (BAT) triggered and
located trigger 556533.  Swift slewed immediately to the location. 
The BAT on-board calculated location is 
RA, Dec 118.580, -28.134 which is 
   RA(J2000) = 07h 54m 19s
   Dec(J2000) = -28d 08' 03"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  The BAT light curve showed no significant 
structure.  The peak count rate was ~1400 counts/sec (15-350 keV), 
at ~4 sec after the trigger. 

The XRT began observing the field at 04:05:29.5 UT, 94.7 seconds after
the BAT trigger. No source was detected in 574 s of promptly downlinked
data, which covered 92% of the BAT error circle. We are waiting for the
full dataset to detect and localise the XRT counterpart. 

UVOT took a finding chart exposure of nominal 250.000 seconds with the
U filter  starting 154 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 large, but uncertain extinction 
expected. 

This was a marginal detection (5.85 sigma) in an image
at a location which was almost 12 arcminutes from
the known source A0753-2802.  Under these circumstances,
Swift attempts to verify or refute the reality of the
possible event using the narrow field instruments.  In
this case, we believe that this was due to a statistical
fluctuation and not an astrophysical event.