GCN Circular 40760

K. L. Page (U Leicester), R. Gupta (NASA GSFC), A. Y. Lien (U Tampa),
M. J. Moss (GSFC), D. M. Palmer (LANL), T. Sakamoto (AGU),
M. H. Siegel (PSU) and M. A. Williams (PSU) report on behalf of the
Neil Gehrels Swift Observatory Team:

At 21:01:50.11 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 250617B (trigger=1325580). Swift did not slew immediately to the burst.
The BAT on-board calculated location is RA, Dec 332.902, +32.727 which is 
   RA(J2000) = 22h 11m 37s
   Dec(J2000) = +32d 43' 38"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  The currently available BAT light curve 
showed a complex structure with a duration of at least 10 sec. 
However, the light curve data from ~T+8 s to ~T+100 s is unavailable 
in the immediate data downlink.  The peak count rate was ~900 counts/sec 
(15-350 keV), at ~0 sec after the trigger.

The XRT began observing the field at 21:15:03.4 UT, 793.4 seconds after
the BAT trigger. Using promptly downlinked data we find an uncatalogued
X-ray source with an enhanced position: RA, Dec 332.89735, 32.73270
which is equivalent to:
   RA(J2000)  = 22h 11m 35.36s
   Dec(J2000) = +32d 43' 57.7"
with an uncertainty of 1.9 arcseconds (radius, 90% containment). This
location is 24 arcseconds from the BAT onboard position, within the BAT
error circle. This position may be improved as more data are received;
the latest position is available at https://www.swift.ac.uk/sper.  We
cannot determine whether the source is fading at the present time. 

A power-law fit to a spectrum formed from promptly downlinked event
data gives a column density in excess of the Galactic value (9.66 x
10^20 cm^-2, Willingale et al. 2013), with an excess column of 5.7
(+4.15/-3.46) x 10^21 cm^-2 (90% confidence).

UVOT took a finding chart exposure of 150 seconds with the White filter
starting 796 seconds after the BAT trigger. There is a candidate afterglow in
the rapidly available 2.7'x2.7' sub-image at
  RA(J2000)  =	22:11:35.25 = 332.89686
  DEC(J2000) = +32:43:56.9  =  32.73247
with a 90%-confidence error radius of about 0.62 arc sec. This position is 1.9
arc sec. from the center of the XRT error circle. The estimated magnitude is
18.17 with a 1-sigma error of about  0.15. No correction has been made for the
expected extinction corresponding to E(B-V) of 0.087.

Burst Advocate for this burst is K. L. Page (klp5 AT leicester.ac.uk).
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/)