[vsnet-grb-info 20121] Swift Trigger 796115 is probably not a GRB

[GCN Circulars]

TITLE:   GCN CIRCULAR
NUMBER:  22217
SUBJECT: Swift Trigger 796115 is probably not a GRB
DATE:    17/12/09 18:19:49 GMT
FROM:    Scott Barthelmy at NASA/GSFC  <scott at milkyway.gsfc.nasa.gov>

S. D. Barthelmy (GSFC), P. D'Avanzo (INAF-OAB), A. Deich (PSU),
S. W. K Emery (UCL-MSSL), J.D. Gropp (PSU), J. A. Kennea (PSU),
N. P. M. Kuin (UCL-MSSL), S. J. LaPorte (PSU), A. Y. Lien (GSFC/UMBC),
F. E. Marshall (NASA/GSFC), A. Melandri (INAF-OAB),
D. M. Palmer (LANL), M. H. Siegel (PSU) and A. Tohuvavohu (PSU) report
on behalf of the Swift Team:

At 17:57:15 UT, the Swift Burst Alert Telescope (BAT) triggered (trigger=796115). 
Swift slewed immediately to the location.  The BAT on-board calculated location is 
RA, Dec 134.821, -55.226, which is 
   RA(J2000)  =  08h 59m 17s
   Dec(J2000) = -55d 13' 32"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  As is typical for image triggers, the light curve
does not show anything significant. 

The XRT began observing the field at 17:59:30.5 UT, 134.6 seconds after
the BAT trigger. No source was detected in 556 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 137 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. Because of the density of catalogued stars, further
analysis is required to report an upper limit for any afterglow in the
sub-image. The 8'x8' region for the list of sources generated on-board covers
100% of the BAT error circle. Because of the density of catalogued stars,
further analysis is required to report an upper limit for any afterglow in the
region. No correction has been made for the expected extinction corresponding
to E(B-V) of 0.35. 

Due to the marginal detection (7.23 sigma) by BAT, which is
reduced further in ground analysis, the lack of
a rate trigger, and the non-detection of an afterglow by XRT,
we believe that this is probably a statistical fluctuation in 
the image plane and not a real astrophysical source. 
There will be no further observations unless a source is 
detected in analysis of the full downlinked data.