TITLE:   GCN CIRCULAR
NUMBER:  42290
SUBJECT: Swift Trigger 1404295: possible GRB 251015A
DATE:    25/10/15 14:44:43 GMT
FROM:    K.L. Page at U Leicester <klp5@leicester.ac.uk>

N. J. Klingler (GSFC/UMBC/CRESSTII), R. Brivio (INAF-OAB),
S. Dichiara (PSU), M. Ferro (INAF-OAB), R. Gupta (NASA GSFC),
N. P. M. Kuin (UCL-MSSL), A. Mei (INAF-OAB), K. L. Page (U Leicester),
D. M. Palmer (LANL), T. Sbarrato (INAF-OAB) and M. H. Siegel (PSU)
report on behalf of the Neil Gehrels Swift Observatory Team:

At 14:17:23 UT, the Swift Burst Alert Telescope (BAT) triggered and
located a possible GRB 251015A (trigger=1404295).  Swift slewed
immediately to the location.
The BAT on-board calculated location is 
RA, Dec 245.007, -54.853 which is 
   RA(J2000) = 16h 20m 02s
   Dec(J2000) = -54d 51' 10"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  The BAT light curve showed a complex
structure with a duration of about 10 sec.  The peak count rate
was ~800 counts/sec (15-350 keV), at ~0 sec after the trigger.

The XRT began observing the field at 14:19:31.3 UT, 127.6 seconds after
the BAT trigger. No source was detected in 877 s of promptly downlinked
data. We are waiting for the full dataset to detect and localise the
XRT counterpart.

UVOT took a finding chart exposure of 150 seconds with the White filter
starting 870 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. Because of the density of catalogued stars, further
analysis is required to report an upper limit for any afterglow in the
sub-image. The 8'x8' region for the list of sources generated on-board covers
0.00% of the BAT error circle. No correction has been made for the large, but
uncertain, extinction expected.

Due to the marginal detection in the BAT image (6.53 sigma) and
the lack of an XRT counterpart, we believe that this is most likely
to be a noise fluctuation, but might be a GRB.  Analysis
of the complete downlinked dataset will be required to determine
the reality of this event.

Burst Advocate for this burst is N. J. Klingler (noelklin AT umbc.edu).
Please contact the BA by email if you require additional information
regarding Swift followup of this burst. In extremely urgent cases, after
trying the Burst Advocate, you can contact the Swift PI by phone (see
Swift TOO web site for information: http://www.swift.psu.edu/)