[vsnet-grb-info 18916] Swift Trigger 730513 is not an astrophysical event

[GCN Circulars]

TITLE:   GCN CIRCULAR
NUMBER:  20378
SUBJECT: Swift Trigger 730513 is not an astrophysical event
DATE:    17/01/06 08:56:53 GMT
FROM:    David Palmer at LANL  <palmer at lanl.gov>

P. D'Avanzo (INAF-OAB), A. Melandri (INAF-OAB),
K. L. Page (U Leicester), D. M. Palmer (LANL) and
B. Sbarufatti (INAF-OAB/PSU) report on behalf of the Swift Team:

At 08:31:01 UT, the Swift Burst Alert Telescope (BAT) detected a
peak in a waiting mode image at a location near to the known 
source SS Cyg (trigger=730513).  Swift slewed immediately to the location. 
The BAT on-board calculated location is 
RA, Dec 325.783, +43.422 which is 
   RA(J2000) = 21h 43m 08s
   Dec(J2000) = +43d 25' 17"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  The BAT light curve shows no obvious activity. 

The XRT began observing the field at 08:33:34.4 UT, 152.9 seconds after
the BAT trigger. No source was detected in 461 s of promptly downlinked
data. 

UVOT took a finding chart exposure of 250 seconds with the U filter starting
153 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
0.46. 

Swift followed up on this image peak despite its low significance 
(5.9 sigma) due to its rough proximity to a known source.  
However, the 10 arcmin offset is well outside the error radius
for this point source.  This, combined with the lack of an BAT
rate trigger or an X-ray detection, leads us to conclude that this
was a statistical fluctuation in the image plane and not
an astrophysical event.