TITLE:   GCN CIRCULAR
NUMBER:  38796
SUBJECT: GRB 250103B: Swift detection of a burst
DATE:    25/01/03 15:58:32 GMT
FROM:    P.A. Evans at U. Leicester <pae9@leicester.ac.uk>

R. A. J. Eyles-Ferris (U Leicester), S. Dichiara (PSU),
P. A. Evans (U Leicester), R. Gupta (NASA GSFC),
N. P. M. Kuin (UCL-MSSL), M. J. Moss (GSFC), K. L. Page (U Leicester),
D. M. Palmer (LANL) and M. A. Williams (PSU) report on behalf of the
Neil Gehrels Swift Observatory Team:

At 15:44:17 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 250103B (trigger=1278865).  Swift slewed immediately to the burst.
The BAT on-board calculated location is 
RA, Dec 54.651, -33.772 which is 
   RA(J2000) = 03h 38m 36s
   Dec(J2000) = -33d 46' 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 30 sec.  The peak count rate
was ~600 counts/sec (15-350 keV), at ~3 sec after the trigger.

The XRT began observing the field at 15:46:48.8 UT, 151.2 seconds after
the BAT trigger. Using promptly downlinked data we find a fading,
uncatalogued X-ray source with an enhanced position: RA, Dec 54.66403,
-33.74965 which is equivalent to:
   RA(J2000)  = 03h 38m 39.37s
   Dec(J2000) = -33d 44' 58.7"
with an uncertainty of 1.9 arcseconds (radius, 90% containment). This
location is 89 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 (9.99 x
10^19 cm^-2, Willingale et al. 2013), with an excess column of 4.1
(+3.96/-3.24) x 10^21 cm^-2 (90% confidence).

UVOT took a finding chart exposure of 150 seconds with the White filter
starting 155 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 0.008.

Burst Advocate for this burst is R. A. J. Eyles-Ferris (raje1 AT leicester.ac.uk).
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/)