TITLE:   GCN CIRCULAR
NUMBER:  5936
SUBJECT: Swift triggers (#251863) on a possible burst
DATE:    06/12/18 04:33:40 GMT
FROM:    Scott Barthelmy at NASA/GSFC  <scott@lheamail.gsfc.nasa.gov>

S. Zane (UCL-MSSL), S. D. Barthelmy (GSFC), D. N. Burrows (PSU),
M. M. Chester (PSU), N. Gehrels (NASA/GSFC), J. A. Kennea (PSU),
F. E. Marshall (NASA/GSFC), K. M. McLean (LANL/UTD),
S. R. Oates (UCL-MSSL), C. Pagani (PSU), D. M. Palmer (LANL) and
M. Stamatikos (NASA/ORAU) report on behalf of the Swift Team:

At 04:05:05 UT, the Swift Burst Alert Telescope (BAT) triggered and
located trigger=251863.  Swift slewed immediately to the location. 
The BAT on-board calculated location is 
RA, Dec 149.223, -35.146 which is 
   RA(J2000)  =  09h 56m 53s
   Dec(J2000) = -35d 08' 45"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  The BAT light curve shows a possible
very weak pulse lasting about 10 sec, overlying the much stronger 
periodic variation of Vela X-1 in the same field.  The peak count 
rate was ~500 counts/sec (15-350 keV), at ~1 sec before the trigger. 

The XRT began taking data at 04:06:51 UT, 106 seconds after the BAT
trigger.  The XRT on-board centroid algorithm did not find a source in the
image and no prompt position is available. No source is detected in 
275s of quicklook photon counting mode data. 

UVOT took a finding chart exposure of 100 seconds with the White
(160-650 nm) filter starting 110 seconds after the BAT trigger, and a
finding chart exposure of nominal 400 seconds with the V filter
starting 216 seconds after the BAT trigger. No 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 18.5 mag (in White). 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
0.125