GCN Circular 41358

R. Caputo (GSFC), A. P. Beardmore (U Leicester),
P. A. Evans (U Leicester), R. A. J. Eyles-Ferris (U Leicester),
C. Gronwall (PSU), A. Y. Lien (U Tampa) and K. L. Page (U Leicester)
report on behalf of the Neil Gehrels Swift Observatory Team:

At 07:34:59 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 250814A (trigger=1342439).  Swift slewed immediately to the burst.
The BAT on-board calculated location is 
RA, Dec 306.329, +48.391 which is 
   RA(J2000) = 20h 25m 19s
   Dec(J2000) = +48d 23' 27"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  The BAT light curve showed a complex structure
structure with a duration of about 20 sec. The peak at ~T+26 s in 
the 15-25 keV band raw light curve is likely due to detector noise, 
though further ground data is required to confirm this. 
The peak count rate was ~500 counts/sec (15-350 keV), at ~10 sec after 
the trigger.

The XRT began observing the field at 07:37:54.1 UT, 174.3 seconds after
the BAT trigger. Using promptly downlinked data we find a bright,
fading, uncatalogued X-ray source with an enhanced position: RA, Dec
306.30802, 48.39903 which is equivalent to:
   RA(J2000)  = 20h 25m 13.92s
   Dec(J2000) = +48d 23' 56.5"
with an uncertainty of 2.2 arcseconds (radius, 90% containment). This
location is 57 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 6.51
x 10^21 cm^-2 (Willingale et al. 2013). 

The initial flux in the 2.5 s image was 7.47e-10 erg cm^-2 s^-1 (0.2-10
keV).

UVOT took a finding chart exposure of 250 seconds with the U filter starting
179 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. The typical 3-sigma upper limit has been about 19.2 mag. 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 18.0 mag. No
correction has been made for the expected extinction corresponding to E(B-V) of
1.066.

This GRB is temporally and spatially coincident with the subthreshold
GW trigger S250814bg which occurred 6 s after the BAT trigger. The XRT
position is towards the edge of the GW localisation. Given the low
significance of the GW event (a false alarm rate of ~10/day) this is
probably a chance coincidence; however, follow up of this GRB is
advised.

Burst Advocate for this burst is R. Caputo (regina.caputo 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/)