TITLE:   GCN CIRCULAR
NUMBER:  23549
SUBJECT: Swift Trigger 878504 is not astrophysical
DATE:    18/12/22 22:14:41 GMT
FROM:    Scott Barthelmy at NASA/GSFC  <scott@milkyway.gsfc.nasa.gov>

S. D. Barthelmy (GSFC), M. G. Bernardini (INAF-OAB),
P. D'Avanzo (INAF-OAB), J.D. Gropp (PSU), J. A. Kennea (PSU),
N. P. M. Kuin (UCL-MSSL), A. Y. Lien (GSFC/UMBC),
F. E. Marshall (NASA/GSFC), A. Melandri (INAF-OAB),
M. J. Moss (George Washington University), B. Sbarufatti (PSU) and
T. N. Ukwatta (LANL) report on behalf of the Neil Gehrels Swift
Observatory Team:

At 21:42:09 UT, the Swift Burst Alert Telescope (BAT) triggered and
located a possible source (trigger=878504).  Swift slewed immediately
to the location.  The BAT on-board calculated location is 
RA, Dec 131.375, -5.488, which is 
   RA(J2000)  =  08h 45m 30s
   Dec(J2000) = -05d 29' 15"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  As is typical for image triggers, the realtime
light curve does not show anything significant. 

The XRT began observing the field at 21:44:34.7 UT, 144.9 seconds after
the BAT trigger. No source was detected in 997 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 147 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
0.02. 

Given the ground analysis of the scaled-map shows a
significant decrease of the image significance to a value below 
4 sigma, and the lack of a new source in the XRT data, 
we believe that this event is a spurious statistical fluctuation 
in the image. Further ground data analysis will be required to 
confirm the nature of this event.