GCN Circular 35208

J.D. Gropp (PSU), N. J. Klingler (GSFC/UMBC/CRESSTII),
D. M. Palmer (LANL), M. H. Siegel (PSU) and A. Tohuvavohu (U Toronto)
report on behalf of the Neil Gehrels Swift Observatory Team:

At 05:05:59 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 231129A (trigger=1199764).  Swift slewed immediately to the burst. 
The BAT on-board calculated location is 
RA, Dec 317.536, +41.544 which is 
   RA(J2000) = 21h 10m 09s
   Dec(J2000) = +41d 32' 37"
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 120 sec.  The peak count rate
was ~3700 counts/sec (15-350 keV), at ~90 sec after the trigger. 

The XRT began observing the field at 05:07:51.1 UT, 112.0 seconds after
the BAT trigger. Using promptly downlinked data we find a bright,
uncatalogued X-ray source located at RA, Dec 317.54581, 41.52062 which
is equivalent to:
   RA(J2000)  = 21h 10m 10.99s
   Dec(J2000) = +41d 31' 14.2"
with an uncertainty of 4.0 arcseconds (radius, 90% containment). This
location is 88 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 3.05e-08 erg cm^-2 s^-1 (0.2-10
keV). 

UVOT took a finding chart exposure of 150 seconds with the White filter
starting 120 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 19.6 mag. 
Results from the list of sources generated on-board are not available at this
time. No correction has been made for the large, but uncertain, extinction
expected. 

Burst Advocate for this burst is J.D. Gropp (jdg44 AT psu.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/)