[vsnet-grb-info 11300] Trigger 510075 is not a GRB

[GCN Circulars]

TITLE:   GCN CIRCULAR
NUMBER:  12712
SUBJECT: Trigger 510075 is not a GRB
DATE:    11/12/21 02:47:35 GMT
FROM:    David Palmer at LANL  <palmer at lanl.gov>

W. H. Baumgartner (GSFC/UMBC), D. N. Burrows (PSU),
M. M. Chester (PSU), J. R. Cummings (NASA/UMBC), D. Grupe (PSU),
E. A. Hoversten (PSU), J. A. Kennea (PSU),
H. A. Krimm (CRESST/GSFC/USRA), C. B. Markwardt (NASA/GSFC),
F. E. Marshall (NASA/GSFC), D. M. Palmer (LANL),
B. Sbarufatti (INAF-OAB/PSU), M. H. Siegel (PSU), C. A. Swenson (PSU)
and B.-B. Zhang (PSU) report on behalf of the Swift Team:

At 02:14:47 UT, the Swift Burst Alert Telescope (BAT) triggered and
located trigger 510075.  Swift slewed immediately to the location. 
The BAT on-board calculated location is RA, Dec 91.175, -23.979 which is 
   RA(J2000) = 06h 04m 42s
   Dec(J2000) = -23d 58' 43"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  The BAT light curve is potentially confused 
by the presence of Vela X-1 in the Field of View.  Although the 
on-board significance was strong enough to report a trigger (6.74 sigma), 
a refined ground analysis produces a significance of 6.0 sigma, which
is below the standard BAT triggering threshold. 

The XRT began observing the field at 02:17:00.1 UT, 132.5 seconds after
the BAT trigger. No source was detected in 428 s of promptly downlinked
data. 

UVOT took a finding chart exposure of 150 seconds with the White filter
starting 135 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
0.04. 

Due to the low significance in the ground re-analysis of the BAT map
and the non-detection by XRT, we believe that this event is not a GRB.