TITLE:   GCN CIRCULAR
NUMBER:  39066
SUBJECT: GRB 250129A: Swift detection of a burst with an optical counterpart
DATE:    25/01/29 05:12:01 GMT
FROM:    K.L. Page at U Leicester <klp5@leicester.ac.uk>

A. P. Beardmore (U Leicester), S. Dichiara (PSU) and
K. L. Page (U Leicester) report on behalf of the Neil Gehrels Swift
Observatory Team:

At 04:45:09 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 250129A (trigger=1285812).  Swift slewed immediately to the burst.
The BAT on-board calculated location is 
RA, Dec 198.689, +5.039 which is 
   RA(J2000) = 13h 14m 45s
   Dec(J2000) = +05d 02' 19"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  As is typical for an image trigger, the BAT light curve  
shows no significant peaks.

The XRT began observing the field at 04:47:47.6 UT, 158.6 seconds after
the BAT trigger. Using promptly downlinked data we find a bright,
uncatalogued X-ray source with an enhanced position: RA, Dec 198.67654,
5.03019 which is equivalent to:
   RA(J2000)  = 13h 14m 42.37s
   Dec(J2000) = +05d 01' 48.7"
with an uncertainty of 1.9 arcseconds (radius, 90% containment). This
location is 54 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. No
spectrum from the promptly downlinked event data is yet available to
determine the column density.

The initial flux in the 0.1 s image was 1.18e-08 erg cm^-2 s^-1 (0.2-10
keV).

UVOT took a finding chart exposure of nominal 150.000 seconds with the White
filter starting 1184 seconds after the BAT trigger. There is a candidate
afterglow in the list of sources generated on-board at
  RA(J2000)  =	13:14:42.42 = 198.67673
  DEC(J2000) = +05:01:50.3  =	5.03063
with a 90%-confidence error radius of about 1.10 arc sec. This position is 2.7
arc sec. from the center of the XRT error circle. The estimated magnitude is
17.88. No correction has been made for the expected extinction corresponding to
E(B-V) of 0.037.

Burst Advocate for this burst is A. P. Beardmore (apb AT star.le.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/)