GCN Circular 28623

E. Ambrosi (INAF-IASFPA), S. D. Barthelmy (GSFC),
A. D'Ai (INAF-IASFPA), V. D'Elia (SSDC), J.D. Gropp (PSU),
J. A. Kennea (PSU), N. P. M. Kuin (UCL-MSSL), A. Y. Lien (GSFC/UMBC),
D. M. Palmer (LANL), B. Sbarufatti (PSU),
M. Stamatikos (OSU/NASA/GSFC) and A. Tohuvavohu (U Toronto) report on
behalf of the Neil Gehrels Swift Observatory Team:

At 22:48:38 UT, the Swift Burst Alert Telescope (BAT) found a peak in 
a waiting mode image (trigger=1000255).  Swift slewed immediately to 
the location. 
The BAT on-board calculated location is 
RA, Dec 20.847, +27.692 which is 
   RA(J2000) = 01h 23m 23s
   Dec(J2000) = +27d 41' 32"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  As is typical for an image trigger, no
obvious variation is apparent in the immediately available lightcurve. 

No source was detected in 666 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 176 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
0.08. 

Due to the marginal detection in the BAT image (7.4 sigma), the lack
of a rate trigger, and the non-detection of an afterglow by XRT,
we believe that this is probably not an astrophysical event. 
Determination of the nature of this trigger will require the
full downlinked dataset.