[vsnet-grb-info 19665] Swift Trigger 766161 is not a GRB

[GCN Circulars]

TITLE:   GCN CIRCULAR
NUMBER:  21409
SUBJECT: Swift Trigger 766161 is not a GRB
DATE:    17/08/04 05:40:10 GMT
FROM:    David Palmer at LANL  <palmer at lanl.gov>

S. J. LaPorte (PSU), A. Y. Lien (GSFC/UMBC), K. L. Page (U Leicester),
D. M. Palmer (LANL) and B. Sbarufatti (INAF-OAB/PSU) report on behalf
of the Swift Team:

At 05:08:36 UT, the Swift Burst Alert Telescope (BAT) detected a marginal
peak in the image domain (trigger=766161). Swift slewed immediately to the
location. The BAT on-board calculated location is 
RA, Dec 230.833, -19.547 which is 
   RA(J2000) = 15h 23m 20s
   Dec(J2000) = -19d 32' 49"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  As is usual with an image trigger, the available
BAT light curve shows no significant structure. 

The XRT began observing the field at 05:11:04.0 UT, 147.5 seconds after
the BAT trigger. No source was detected in 1.4 ks of promptly
downlinked data, which covered 98% of the BAT error circle. 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 150 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 expected extinction corresponding to E(B-V) of
0.14. 

Ground re-analysis of the detector histogram shows a greatly reduced
significance (from 7.3 to 4.2 sigma).  This, combined with the lack
of a rate trigger and the non-detection of a counterpart by XRT,
leads us to conclude that this is probably a statistical fluctuation 
in the image plane and not an astrophysical source.