GCN Circular 28250

M. G. Bernardini (INAF-OAB), P. D'Avanzo (INAF-OAB),
A. Y. Lien (GSFC/UMBC), F. E. Marshall (NASA/GSFC),
A. Melandri (INAF-OAB), D. M. Palmer (LANL), T. Sbarrato (INAF-OAB)
and K. K. Simpson (PSU) report on behalf of the Neil Gehrels Swift
Observatory Team:

At 04:07:22 UT, the Swift Burst Alert Telescope (BAT) triggered and
located on an image peak that is likely due to a noisy detector element
(trigger=989458).  Swift slewed immediately to the event. 
The BAT on-board calculated location is 
RA, Dec 202.461, -22.590 which is 
   RA(J2000) = 13h 29m 51s
   Dec(J2000) = -22d 35' 22"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  The BAT light curve is difficult to interpret
due to the presence of the noisy detector element.  The full downlinked
data will allow the noisy element to be removed from analysis. 

The XRT began observing the field at 04:08:57.6 UT, 95.3 seconds after
the BAT trigger. No source was detected in 323 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 98 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 ~10% 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
0.10. 

Due to the lack of detection in XRT, the marginal significance of
the BAT detection (6.58 sigma in the on-board analysis, dropping
to 6.3 sigma in ground analysis of the scaled detector map)
and the presence of hot detector pixels in the imaged data,
we believe that this is probably not an astrophysical event. 

Final determination of the status of this trigger awaits
the full downloaded data set.