[vsnet-grb-info 24909] Swift Trigger 954840 is not a GRB

[GCN Circulars]

TITLE:   GCN CIRCULAR
NUMBER:  27008
SUBJECT: Swift Trigger 954840 is not a GRB
DATE:    20/02/07 03:42:44 GMT
FROM:    David Palmer at LANL  <palmer at lanl.gov>

J.D. Gropp (PSU), J. A. Kennea (PSU), N. J. Klingler (PSU),
S. Laha (GSFC/UMBC/CRESST), A. Y. Lien (GSFC/UMBC),
F. E. Marshall (NASA/GSFC), K. L. Page (U Leicester),
B. Sbarufatti (PSU) and M. H. Siegel (PSU) report on behalf of the
Neil Gehrels Swift Observatory Team:

At 03:02:18 UT, the Swift Burst Alert Telescope (BAT) triggered on a rate
increase and found a marginal significance peak in the image plane
(trigger=954840).  Swift slewed immediately to the location. 
The BAT on-board calculated location is 
RA, Dec 163.623, -49.581 which is 
   RA(J2000) = 10h 54m 30s
   Dec(J2000) = -49d 34' 52"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  The BAT light curve showed a steady rate
increase due to Swift's approach and entry into the SAA. 

The XRT began observing the field at 03:03:44.9 UT, 86.8 seconds after
the BAT trigger. No source was detected in 594 s of promptly downlinked
data. 

UVOT took a finding chart exposure of 150 seconds with the White filter
starting 1628 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 8%
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 expected extinction corresponding to
E(B-V) of 0.38. 

Because the BAT rate increase was due to the SAA, the image
significance was marginal (6.5 sigma) and much below that implied by
the rate increase, and XRT did not detect any afterglow, we
believe that this trigger was a statistical fluctuation in
the image plane and not an astrophysical event.  The full downlinked
dataset will be used to verify the nature of this trigger.