TITLE:   GCN CIRCULAR
NUMBER:  4114
SUBJECT: Swift-BAT triggered on a posible CR Shower or possible GRB
DATE:    05/10/21 12:51:29 GMT
FROM:    Scott Barthelmy at NASA/GSFC  <Scott@lheamail.gsfc.nasa.gov>

A. Retter (PSU), L. Barbier (GSFC), S. Barthelmy (GSFC),
A. Beardmore (U Leicester), D. Burrows (GSFC), N. Gehrels (GSFC),
S.T. Holland (GSFC/USRA), J. Kennea (PSU), F. Marshall (GSFC),
K. Page (U Leicester), J. Racusin (PSU)
on behalf of the Swift team:

At 12:12:33 UT, Swift-BAT triggered on a possible cosmic ray shower
or a possible GRB (trigger=160619).  We will not be able to know for sure
until the full data set arrives in ~2 hours.

The spacecraft slewed immediately and the XRT began observing the
field at 12:13:49 UT, 76 sec after the BAT trigger.  The on-board
detection algorithm did not centroid on a source due to insufficient
counts so no prompt X-ray position is available.  The spectrum and
light curve are inconclusive as to whether a faint source is present
in the field of view and we await further analysis of the ground
processed data.

The UVOT began observing at 12:13:47 UT, 74 sec after the BAT trigger.
The initial 200 sec V-band finding chart image has been truncated and  
covers a small fraction of the BAT error circle.  Based on comparisons
to the DSS we detect no source down to a magnitude of V_lim = 19.3 (3-sigma).