[vsnet-grb-info 17572] Swift Trigger 672897 is probably not an astrophysical event

[GCN Circulars]

TITLE:   GCN CIRCULAR
NUMBER:  19002
SUBJECT: Swift Trigger 672897 is probably not an astrophysical event
DATE:    16/02/08 00:19:49 GMT
FROM:    David Palmer at LANL  <palmer at lanl.gov>

A. D'Ai (INAF-IASFPA), J. A. Kennea (PSU), F. E. Marshall (NASA/GSFC),
A. Maselli (INAF-IASFPA), D. M. Palmer (LANL) and
B. Sbarufatti (INAF-OAB/PSU) report on behalf of the Swift Team:

At 23:49:52 UT, the Swift Burst Alert Telescope (BAT) found a marginal
significance fluctuation in an untriggered image (trigger=672897). 
Swift slewed immediately to the location. 
The BAT on-board calculated location is 
RA, Dec 156.059, +19.770 which is 
   RA(J2000) = 10h 24m 14s
   Dec(J2000) = +19d 46' 14"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  As is usual for an image trigger, there
is no obvious variation in the immediately-available BAT lightcurve. 

The XRT began observing the field at 23:52:49.7 UT, 177.5 seconds after
the BAT trigger. No source was detected in 235 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 250 seconds with the U filter starting
339 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 19.2 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 18.0 mag. No
correction has been made for the expected extinction corresponding to E(B-V) of
0.02. 

As part of a search for nearby sources, Swift slews
to confirm or refute marginal significance events that are near 
the line-of-sight to nearby galaxies.  Based on the lack of a BAT
rate trigger, the low significance of the original detection 
(5.9 sigma) the lack of an XRT detection, and the distance to the 
suggested host galaxy (12 arcmin) we believe that this event 
is merely a noise fluctuation in the BAT imager and 
not an astrophysical event.