TITLE:   GCN CIRCULAR
NUMBER:  45739
SUBJECT: GRB 260925A: Swift detection of a burst
DATE:    26/09/25 07:34:59 GMT
FROM:    Jimmy DeLaunay at PSU <jjd330@psu.edu>

E. Ambrosi (INAF-IASFPA), S. Dichiara (PSU), A. Melandri (INAF-OAR),
K. L. Page (U Leicester), B. Sbarufatti (INAF-OAB) and
M. H. Siegel (PSU) report on behalf of the Neil Gehrels Swift
Observatory Team:

At 07:13:02 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 260925A (trigger=1513316).  Swift slewed immediately to the burst.
The BAT on-board calculated location is 
RA, Dec 132.462, +24.971,  which is 
   RA(J2000) = 08h 49m 51s
   Dec(J2000) = +24d 58' 16"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  The BAT light curve showed a single-peaked
structure with a duration of about 15 sec.  The peak count rate
was ~1200 counts/sec (15-350 keV), at ~1 sec after the trigger.

The XRT began observing the field at 07:14:52.2 UT, 110.7 seconds after
the BAT trigger. Using promptly downlinked data we find a fading,
uncatalogued X-ray source with an enhanced position: RA, Dec 132.45674,
24.91182 which is equivalent to:
   RA(J2000)  = 08h 49m 49.62s
   Dec(J2000) = +24d 54' 42.6"
with an uncertainty of 2.3 arcseconds (radius, 90% containment). This
location is 213 arcseconds from the BAT onboard position. This position
may be improved as more data are received; the latest position is
available at https://www.swift.ac.uk/sper. This position is more than
2-sigma away from the BAT position, and so possibly unrelated to the
trigger. 

A power-law fit to a spectrum formed from promptly downlinked event
data gives a column density consistent with the Galactic value of 3.64
x 10^20 cm^-2 (Willingale et al. 2013).

UVOT took a finding chart exposure of 150 seconds with the White filter
starting 119 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 none of
the XRT error circle. The 8'x8' region for the list of sources generated
on-board covers 100% of the XRT error circle. The list of sources is typically
complete to about 18 mag. No correction has been made for the expected
extinction corresponding to E(B-V) of 0.038.

Burst Advocate for this burst is E. Ambrosi (elena.ambrosi AT inaf.it).
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/)