GCN Circular 38847

N. J. Klingler (GSFC/UMBC/CRESSTII), R. Brivio (INAF-OAB),
J. J. DeLaunay (PSU), R. Gupta (NASA GSFC), K. L. Page (U Leicester),
C. Salvaggio (INAF-OAB) and T. Sbarrato (INAF-OAB) report on behalf of
the Neil Gehrels Swift Observatory Team:

At 10:23:06 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 250108B (trigger=1280056).  Swift slewed immediately to the burst.
The BAT on-board calculated location is 
RA, Dec 201.297, +25.613 which is 
   RA(J2000) = 13h 25m 11s
   Dec(J2000) = +25d 36' 46"
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 30s sec.  The peak count rate
was ~1210 counts/sec (15-350 keV), at ~2 sec after the trigger.

The XRT began observing the field at 10:24:54.0 UT, 107.8 seconds after
the BAT trigger. Using promptly downlinked data we find a bright,
uncatalogued X-ray source with an enhanced position: RA, Dec 201.32686,
25.61541 which is equivalent to:
   RA(J2000)  = 13h 25m 18.45s
   Dec(J2000) = +25d 36' 55.5"
with an uncertainty of 2.0 arcseconds (radius, 90% containment). This
location is 97 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 in excess of the Galactic value (1.16 x
10^20 cm^-2, Willingale et al. 2013), with an excess column of 4.6
(+2.37/-2.05) x 10^21 cm^-2 (90% confidence). 

The initial flux in the 2.5 s image was 9.22e-10 erg cm^-2 s^-1 (0.2-10
keV).

UVOT took a finding chart exposure of 250 seconds with the U filter starting
274 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
0.012.

Burst Advocate for this burst is N. J. Klingler (noelklin AT umbc.edu).
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/)