TITLE: GCN CIRCULAR NUMBER: 31136 SUBJECT: GRB 211129A: Swift detection of a burst DATE: 21/11/29 10:11:32 GMT FROM: Phil Evans at U of Leicester J.D. Gropp (PSU), S. D. Barthelmy (GSFC), P. A. Evans (U Leicester), N. J. Klingler (GSFC/UMBC/CRESSTII), K. L. Page (U Leicester) and B. Sbarufatti (PSU) report on behalf of the Neil Gehrels Swift Observatory Team: At 09:51:06 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 211129A (trigger=1085430). Swift slewed immediately to the burst. The BAT on-board calculated location is RA, Dec 274.557, +31.776 which is RA(J2000) = 18h 18m 14s Dec(J2000) = +31d 46' 33" with an uncertainty of 3 arcmin (radius, 90% containment, including systematic uncertainty). The BAT light curve shows 3 major peaks with a duration of about 200 sec. The peak count rate was ~1400 counts/sec (15-350 keV), at ~3 sec after the trigger. The XRT began observing the field at 09:52:19.7 UT, 73.5 seconds after the BAT trigger. Using promptly downlinked data we find a bright, uncatalogued X-ray source located at RA, Dec 274.58129, 31.78820 which is equivalent to: RA(J2000) = 18h 18m 19.51s Dec(J2000) = +31d 47' 17.5" with an uncertainty of 4.0 arcseconds (radius, 90% containment). This location is 86 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/1085430. 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 2.5 s image was 1.41e-09 erg cm^-2 s^-1 (0.2-10 keV). UVOT took a finding chart exposure of 150 seconds with the White filter starting 83 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. 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 0.082. 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/)