[vsnet-grb-info 18921] Swift Trigger 730874 is not astrophysical

[GCN Circulars]

TITLE:   GCN CIRCULAR
NUMBER:  20391
SUBJECT: Swift Trigger 730874 is not astrophysical
DATE:    17/01/07 14:11:48 GMT
FROM:    Boris Sbarufatti at INAF-OAB/IASFPA  <boris.sbarufatti at brera.inaf.it>

S. D. Barthelmy (GSFC), P. D'Avanzo (INAF-OAB), J. A. Kennea (PSU),
A. Y. Lien (GSFC/UMBC) and B. Sbarufatti (INAF-OAB/PSU) report on
behalf of the Swift Team:

At 13:37:25 UT, the Swift Burst Alert Telescope (BAT) detected a
peak in an image at a location near to the known source NGC 1023 
(trigger=730874).  Swift slewed immediately to the location. 
The BAT on-board calculated location is 
RA, Dec 40.038, +39.132 which is 
   RA(J2000) = 02h 40m 09s
   Dec(J2000) = +39d 07' 57"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  The BAT light curve showed no obvious structure,
as usual to an image trigger. 

The XRT began observing the field at 13:39:27.2 UT, 121.7 seconds after
the BAT trigger. No source was detected in 1.2 ks of promptly
downlinked data. 

UVOT took a finding chart exposure of 150 seconds with the White filter
starting 125 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
0.06. 

Given that this is a low significance detection in BAT (5.9 sigma) and that there is
no XRT source, we think this is a non-astrophysical fluctuation in the BAT image
that happens to be close the NGC1023 source.