TITLE:   GCN CIRCULAR
NUMBER:  21251
SUBJECT: Swift Trigger 756953 is probably not an astrophysical source
DATE:    17/06/12 21:14:40 GMT
FROM:    Scott Barthelmy at NASA/GSFC  <scott@milkyway.gsfc.nasa.gov>

S. D. Barthelmy (GSFC), A. Cholden-Brown (PSU), A. D'Ai (INAF-IASFPA),
V. D'Elia (ASDC), J. A. Kennea (PSU), F. E. Marshall (NASA/GSFC) and
B. Sbarufatti (INAF-OAB/PSU) report on behalf of the Swift Team:

At 20:55:08 UT, the Swift Burst Alert Telescope (BAT) triggered on noise near
IGR17303-0601 (trigger=756953).  Swift slewed immediately to the location. 
The BAT on-board calculated location is 
RA, Dec 262.731, -5.979, which is 
   RA(J2000)  =  17h 30m 56s
   Dec(J2000) = -05d 58' 42"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  As is typical for image triggers, 
there is nothing in the real-time light curve. 

The XRT began observing the field at 20:59:25.9 UT, 257.1 seconds after
the BAT trigger. No source was detected in 631 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 261 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. 
Results from the list of sources generated on-board are not available at this
time. No correction has been made for the expected extinction corresponding to
E(B-V) of 0.59. 

Due to the low significance of the image peak (5.98 sigma), the 
large distance to the potential associated source (8.5 arcminutes),
the lack of activity in the BAT count rates, and the non-detection 
by XRT, we believe that this is merely a noise fluctuation in
the image plane and not an astrophysical source.