TITLE: GCN CIRCULAR NUMBER: 35208 SUBJECT: GRB 231129A: Swift detection of a burst DATE: 23/11/29 05:19:12 GMT FROM: Aaron Tohuvavohu at University of Toronto 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/)