GCN Circular 23260

S. D. Barthelmy (GSFC), C. Gronwall (PSU), A. Y. Lien (GSFC/UMBC),
D. M. Palmer (LANL) and T. Sakamoto (AGU) report on behalf of the Neil
Gehrels Swift Observatory Team:

At 00:01:30 UT, the Swift Burst Alert Telescope (BAT) triggered on an image
fluctuation which was misidentified as IGR 17511-3057 (trigger=863053) due to
incorrect attitude information.  Swift slewed immediately to the target. 
The BAT on-board calculated location of the image fluctuations is 
RA, Dec 267.574, -31.767, which is 
   RA(J2000)  =  17h 50m 18s
   Dec(J2000) = -31d 46' 01"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  As is typical for an image trigger, there is nothing
significant in the real-time light curve. 

The XRT began observing the field at 00:16:46.0 UT, 915.7 seconds after
the BAT trigger. No source was detected in 305 s of promptly downlinked
data. 

UVOT took a finding chart exposure of 150 seconds with the White filter
starting 925 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 large, but uncertain, extinction expected. 

This event was due to the misidentification of an image fluctuation 
with a known source, as a result of a known race condition 
in the on-board software.  It is not due to an astrophysical source.