TITLE:   GCN CIRCULAR
NUMBER:  41714
SUBJECT: GRB 250905A: Swift detection of a burst
DATE:    25/09/05 02:25:34 GMT
FROM:    David Palmer at LANL <palmer@lanl.gov>

R. Brivio (INAF-OAB), S. Dichiara (PSU), D. M. Palmer (LANL),
C. Salvaggio (INAF-OAB), T. Sbarrato (INAF-OAB),
B. Sbarufatti (INAF-OAB) and M. H. Siegel (PSU) report on behalf of
the Neil Gehrels Swift Observatory Team:

At 02:09:54 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 250905A (trigger=1347265).  Swift slewed immediately to the burst.
The BAT on-board calculated location is 
RA, Dec 233.047, +0.777 which is 
   RA(J2000) = 15h 32m 11s
   Dec(J2000) = +00d 46' 39"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  Due to a telemetry gap, the BAT lightcurve
is not immediately available.

The XRT began observing the field at 02:12:11.4 UT, 136.6 seconds after
the BAT trigger. Using promptly downlinked data we find an uncatalogued
X-ray source with an enhanced position: RA, Dec 233.00629, 0.79395
which is equivalent to:
   RA(J2000)  = 15h 32m 01.51s
   Dec(J2000) = +00d 47' 38.2"
with an uncertainty of 3.4 arcseconds (radius, 90% containment). This
location is 158 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 consistent with the Galactic value of 5.50
x 10^20 cm^-2 (Willingale et al. 2013).

UVOT took a finding chart exposure of 150 seconds with the White filter
starting 140 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 0.054.

Burst Advocate for this burst is R. Brivio (riccardo.brivio 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/)