TITLE:   GCN CIRCULAR
NUMBER:  41848
SUBJECT: GRB 250917B: Swift detection of a short burst
DATE:    25/09/17 06:48:14 GMT
FROM:    K.L. Page at U Leicester <klp5@leicester.ac.uk>

R. Caputo (GSFC), C. Gronwall (PSU) and K. L. Page (U Leicester)
report on behalf of the Neil Gehrels Swift Observatory Team:

At 06:27:44 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 250917B (trigger=1350381).  Swift slewed immediately to the burst.
The BAT on-board calculated location is 
RA, Dec 291.378, +11.113 which is 
   RA(J2000) = 19h 25m 31s
   Dec(J2000) = +11d 06' 48"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  The BAT light curve showed a short, single peaked
structure with a duration of about 0.5 sec.  The peak count rate
was ~8000 counts/sec (15-350 keV), at ~0 sec after the trigger.

The XRT began observing the field at 06:29:58.5 UT, 134.2 seconds after
the BAT trigger. No source was detected in 681 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 138 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.
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 large, but uncertain, extinction expected.

Burst Advocate for this burst is R. Caputo (regina.caputo AT nasa.gov).
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/)