GCN Circular 41303

T. M. Parsotan (GSFC), R. Brivio (INAF-OAB), J. J. DeLaunay (PSU),
S. Dichiara (PSU), R. Gupta (NASA GSFC) and D. M. Palmer (LANL) report
on behalf of the Neil Gehrels Swift Observatory Team:

At 16:53:19 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 250809A (trigger=1340954).  Swift slewed immediately to the burst.
The BAT on-board calculated location is 
RA, Dec 233.015, -53.289 which is 
   RA(J2000) = 15h 32m 04s
   Dec(J2000) = -53d 17' 18"
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 40 sec.  The peak count rate
was ~1000 counts/sec (15-350 keV), at ~11 sec after the trigger.

The XRT began observing the field at 16:55:25.0 UT, 125.5 seconds after
the BAT trigger. Using promptly downlinked data we find a bright,
uncatalogued X-ray source located at RA, Dec 233.00282, -53.29640 which
is equivalent to:
   RA(J2000)  = 15h 32m 00.68s
   Dec(J2000) = -53d 17' 47.0"
with an uncertainty of 4.3 arcseconds (radius, 90% containment). This
location is 37 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 8.04
x 10^21 cm^-2 (Willingale et al. 2013). 

The initial flux in the 2.5 s image was 5.16e-10 erg cm^-2 s^-1 (0.2-10
keV).

UVOT took a finding chart exposure of 250 seconds with the U filter starting
128 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 100% of the
XRT error circle. Because of the density of catalogued stars, further analysis
is required to report an upper limit for any afterglow in the sub-image. The
8'x8' region for the list of sources generated on-board covers 100% of the XRT
error circle. Because of the density of catalogued stars, further analysis is
required to report an upper limit for any afterglow in the region. No
correction has been made for the large, but uncertain, extinction expected.

Burst Advocate for this burst is T. M. Parsotan (tyler.parsotan AT nasa.gov).
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/)