[vsnet-grb-info 1805] Swift-BAT detection of a possible GRB

[GCN Circulars]

TITLE:   GCN GRB OBSERVATION REPORT
NUMBER:  3645
SUBJECT: Swift-BAT detection of a possible GRB
DATE:    05/07/19 14:54:41 GMT
FROM:    Scott Barthelmy at NASA/GSFC  <Scott at lheamail.gsfc.nasa.gov>

S. Vaughan, A. Beardmore, M. Goad, O. Godet, J. Osborne, K. Page (U. Leicester),
S. Barthelmy (GSFC), D. Burrows (PSU), J. Cummings (GSFC/NRC), N. Gehrels (GSFC),
J. Kennea (PSU), H. Krimm (GSFC/USRA), D. Palmer (LANL), A. Blustin (MSSL),
S. Holland (GSFC/USRA), M. De Pasquale (MSSL)
on behalf of the Swift team:

At 13:58:52.85 UT, the Swift-BAT instrument triggered (trigger=146731)
on a weak event.  Using only the TDRSS lightcurve data we can not confirm
or reject that this is a real GRB.  If it is a real GRB, then it might 
be a short GRB.  There was a single spike of duration less than 128 msec
in the 100-350 keV band, and no apparent emission in the lower energy bands.
The BAT trigger duration was 32 msec which is consistent, but not definitive,
with the short nature of this event.  The rate data currently available
is also consistent with a cosmic ray event interacting in the instrument
or spacecraft and showering the BAT detector.  Such events do not produce
a point source in the image, so if that were the case, the 6.24 sigma peak
detection in the image would be an unlikely, but still feasible,
statistical deviation.  The on-board calculated position
is 213.420d,+9.444 {+14h 13m 41s, +09d 26' 38"}  (J2000) +- 3arcmin
(radius, 90%, sys+stat).  We will know more about this event when
the full data set arrives in a couple hours.

The XRT began observing the burst at 14:00:04 UT, 71 seconds after
the BAT trigger.  No source was found by the on board software due to
low flux.  The full field XRT lightcurve does not show any significant
enhancement over background, and ground processing will be necessary
to determine if a faint afterglow is detected.

The Swift Ultra Violet/Optical Telescope (UVOT) observations began
at 14:00:02 (69 seconds after the BAT trigger).  The first data taken
after the spacecraft settled was a 100 sec exposure using the V filter
with the midpoint of the observation at 119 sec after the BAT trigger.
Based on comparison with the DSS, we detect no new source within
the Genie image down to a 5 sigma upper limit magnitude of V=18.3.