GCN Circular 40343

M. J. Moss (GSFC), C. Gronwall (PSU), R. Gupta (NASA GSFC),
J. A. Kennea (PSU), D. M. Palmer (LANL) and M. H. Siegel (PSU) report
on behalf of the Neil Gehrels Swift Observatory Team:

At 23:25:57 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 250504A (trigger=1310284).  Swift slewed immediately to the burst.
The BAT on-board calculated location is 
RA, Dec 269.612, -40.349 which is 
   RA(J2000) = 17h 58m 27s
   Dec(J2000) = -40d 20' 57"
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 60 sec.  The peak count rate
was ~4000 counts/sec (15-350 keV), at ~0 sec after the trigger.

The XRT began observing the field at 23:27:15.3 UT, 77.9 seconds after
the BAT trigger. Using promptly downlinked data we find a fading,
uncatalogued X-ray source with an enhanced position: RA, Dec 269.61691,
-40.36642 which is equivalent to:
   RA(J2000)  = 17h 58m 28.06s
   Dec(J2000) = -40d 21' 59.1"
with an uncertainty of 1.9 arcseconds (radius, 90% containment). This
location is 64 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 2.12
x 10^21 cm^-2 (Willingale et al. 2013).

UVOT took a finding chart exposure of 250 seconds with the U filter starting
137 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 0.00% of the XRT
error circle. No correction has been made for the expected extinction
corresponding to E(B-V) of 0.191.

Burst Advocate for this burst is M. J. Moss (mikejmoss3 AT gmail.com).
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/)