[vsnet-grb-info 11847] Swift Trigger 519874 is probably a noise event

[GCN Circulars]

TITLE:   GCN CIRCULAR
NUMBER:  13238
SUBJECT: Swift Trigger 519874 is probably a noise event
DATE:    12/04/11 01:31:44 GMT
FROM:    David Palmer at LANL  <palmer at lanl.gov>

S. D. Barthelmy (GSFC), D. N. Burrows (PSU), P. D'Avanzo (INAF-OAB),
N. Gehrels (NASA/GSFC), S. T. Holland (STScI), E. A. Hoversten (PSU),
J. A. Kennea (PSU), A. Y. Lien (NASA/GSFC/ORAU),
F. E. Marshall (NASA/GSFC), A. Melandri (INAF-OAB),
D. M. Palmer (LANL), B. Sbarufatti (INAF-OAB/PSU), C. A. Swenson (PSU)
and B.-B. Zhang (PSU) report on behalf of the Swift Team:

At 01:16:39 UT, the Swift Burst Alert Telescope (BAT) triggered and
located a marginal peak (6.51 sigma) in the resulting image
(trigger=519874).  Swift slewed immediately to the location. 
The BAT on-board calculated location is 
RA, Dec 35.550, +59.746 which is 
   RA(J2000) = 02h 22m 12s
   Dec(J2000) = +59d 44' 46"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  The BAT light curve shows nothing significant
at the time of the trigger. 

The XRT began observing the field at 01:18:11.4 UT, 92.5 seconds after
the BAT trigger. No source was detected in 218 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 95 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. 

Due to the marginal nature of the BAT detection, and the lack
of any detection by XRT and UVOT despite the prompt slew,
we believe that this event was most likely a statistical
fluctuation and not an astrophysical source.