[vsnet-grb-info 21923] Swift Trigger 895503 is not a GRB

[GCN Circulars]

TITLE:   GCN CIRCULAR
NUMBER:  24022
SUBJECT: Swift Trigger 895503 is not a GRB
DATE:    19/03/27 23:43:56 GMT
FROM:    Scott Barthelmy at NASA/GSFC  <scott at milkyway.gsfc.nasa.gov>

S. D. Barthelmy (GSFC), A. P. Beardmore (U Leicester),
N. J. Klingler (PSU), A. Y. Lien (GSFC/UMBC),
F. E. Marshall (NASA/GSFC), M. J. Moss (George Washington University),
K. L. Page (U Leicester), D. M. Palmer (LANL) and A. Tohuvavohu (PSU)
report on behalf of the Neil Gehrels Swift Observatory Team:

At 23:16:13 UT, the Swift Burst Alert Telescope (BAT) triggered and
located a noise peak (trigger=895503).  Swift slewed immediately to the location. 
The BAT on-board calculated location is 
RA, Dec 297.816, +39.070, which is 
   RA(J2000)  =  19h 51m 16s
   Dec(J2000) = +39d 04' 12"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  The BAT light curve shows no obviously
significant structure, which is unusual for a rate trigger.  The 
peak count rate was ~700 counts/sec (15-350 keV), at ~0 sec 
after the trigger. 

The XRT began observing the field at 23:17:20.2 UT, 66.4 seconds after
the BAT trigger. No source was detected in 749 s of promptly downlinked
data, which covered 95% of the BAT error circle. 

UVOT took a finding chart exposure of 150 seconds with the White
filter  starting 69 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. We note that this is a
crowded field. No correction has been made for the  expected
extinction corresponding to E(B-V) of 0.25. 

Due to the marginal significance of both the rate trigger (6.2 sigma)
and the image peak (6.6 sigma, declining to 5.7 sigma in ground analysis),
and the lack of a clear XRT or UVOT counterpart, we believe that this 
trigger is probably a statistical fluctuation and not an astrophysical 
event.  A final determination of the reality of this event will use
the full ground-linked dataset.