TITLE:   GCN CIRCULAR
NUMBER:  41898
SUBJECT: GRB 250920B: Swift detection of a burst with a bright optical counterpart.
DATE:    25/09/20 09:01:06 GMT
FROM:    K.L. Page at U Leicester <klp5@leicester.ac.uk>

R. Gupta (NASA GSFC), C. Gronwall (PSU), N. P. M. Kuin (UCL-MSSL) and
K. L. Page (U Leicester) report on behalf of the Neil Gehrels Swift
Observatory Team:

At 08:46:30 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 250920B (trigger=1351008).  Swift slewed immediately to the burst.
The BAT on-board calculated location is 
RA, Dec 187.337, +47.844 which is 
   RA(J2000) = 12h 29m 21s
   Dec(J2000) = +47d 50' 39"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  The BAT light curve showed multiple peaks, with a
complex structure with a duration of about 150 sec.  The peak count rate
was ~8200 counts/sec (15-350 keV), at ~5 sec after the trigger.

The XRT began observing the field at 08:48:01.1 UT, 90.6 seconds after
the BAT trigger. XRT found a bright, uncatalogued X-ray source located
at RA, Dec 187.3179, 47.8372 which is equivalent to:
   RA(J2000)  = 12h 29m 16.30s
   Dec(J2000) = +47d 50' 13.9"
with an uncertainty of 4.7 arcseconds (radius, 90% containment). This
location is 52 arcseconds from the BAT onboard position, within the BAT
error circle. No event data are yet available to determine the column
density using X-ray spectroscopy.

The initial flux in the 2.5 s image was 8.20e-09 erg cm^-2 s^-1 (0.2-10
keV).

UVOT took a finding chart exposure of 150 seconds with the White filter
starting 98 seconds after the BAT trigger. There is a candidate afterglow in
the rapidly available 2.7'x2.7' sub-image at
  RA(J2000)  =	12:29:16.07 = 187.31697
  DEC(J2000) = +47:50:14.4  =  47.83734
with a 90%-confidence error radius of about 0.74 arc sec. This position is 2.3
arc sec. from the center of the XRT error circle. The estimated magnitude is
13.75 with a 1-sigma error of about  0.14. No correction has been made for
extinction.

Burst Advocate for this burst is R. Gupta (rahulbhu.c157 AT gmail.com).
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/)