GCN Circular 41959

M. H. Siegel (PSU), J. J. DeLaunay (PSU), R. Gupta (NASA GSFC),
S. Lanava (PSU), K. L. Page (U Leicester) and D. M. Palmer (LANL)
report on behalf of the Neil Gehrels Swift Observatory Team:

At 08:18:29 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 250924A (trigger=1351790).  Swift slewed immediately to the burst.
The BAT on-board calculated location is 
RA, Dec 51.865, +74.652 which is 
   RA(J2000) = 03h 27m 28s
   Dec(J2000) = +74d 39' 07"
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 30 sec.  The peak count rate
was ~1000 counts/sec (15-350 keV), at ~17 sec after the trigger.

The XRT began observing the field at 08:21:32.8 UT, 183.6 seconds after
the BAT trigger. Using promptly downlinked data we find a bright,
fading, uncatalogued X-ray source with an enhanced position: RA, Dec
51.87141, 74.65894 which is equivalent to:
   RA(J2000)  = 03h 27m 29.14s
   Dec(J2000) = +74d 39' 32.2"
with an uncertainty of 2.7 arcseconds (radius, 90% containment). This
location is 25 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. 

A power-law fit to a spectrum formed from promptly downlinked event
data gives a column density consistent with the Galactic value of 2.77
x 10^21 cm^-2 (Willingale et al. 2013). 

The initial flux in the 2.5 s image was 1.03e-09 erg cm^-2 s^-1 (0.2-10
keV).

UVOT took a finding chart exposure of 150 seconds with the White filter
starting 192 seconds after the BAT trigger. There is a candidate afterglow in
the rapidly available 2.7'x2.7' sub-image at
  RA(J2000)  =	03:27:29.62 =  51.87340
  DEC(J2000) = +74:39:34.0  =  74.65945
with a 90%-confidence error radius of about 0.62 arc sec. This position is 1.6
arc sec. from the center of the XRT error circle. The estimated magnitude is
17.74 with a 1-sigma error of about  0.14. No correction has been made for the
expected extinction corresponding to E(B-V) of 0.316.

Burst Advocate for this burst is M. H. Siegel (siegel AT swift.psu.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/)