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.