TITLE: GCN CIRCULAR NUMBER: 20210 SUBJECT: GRB 161129A: Swift detection of a burst DATE: 16/11/29 07:32:25 GMT FROM: Scott Barthelmy at NASA/GSFC D. Kocevski (NASA/GSFC/ORAU), S. D. Barthelmy (GSFC), J. R. Cummings (NASA/UMBC), B. Mingo (U Leicester), K. L. Page (U Leicester), B. Sbarufatti (INAF-OAB/PSU) and M. H. Siegel (PSU) report on behalf of the Swift Team: At 07:11:39 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 161129A (trigger=724438). Swift slewed immediately to the burst. The BAT on-board calculated location is RA, Dec 316.193, +32.125 which is RA(J2000) = 21h 04m 46s Dec(J2000) = +32d 07' 30" with an uncertainty of 3 arcmin (radius, 90% containment, including systematic uncertainty). The BAT light curve shows multiple peaks with a duration of about 50 sec. The peak count rate was ~4400 counts/sec (15-350 keV), at ~0 sec after the trigger. The XRT began observing the field at 07:13:01.8 UT, 82.0 seconds after the BAT trigger. Using promptly downlinked data we find an uncatalogued X-ray source located at RA, Dec 316.22890, 32.13574 which is equivalent to: RA(J2000) = 21h 04m 54.94s Dec(J2000) = +32d 08' 08.7" with an uncertainty of 3.6 arcseconds (radius, 90% containment). This location is 116 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 http://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 (2.50 x 10^21 cm^-2, Willingale et al. 2013), with an excess column of 2.5 (+2.84/-2.37) x 10^21 cm^-2 (90% confidence). UVOT took a finding chart exposure of 150 seconds with the White filter starting 86 seconds after the BAT trigger. No credible afterglow candidate has been found in the initial data products. The 2.7'x2.7' sub-image does not cover 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 0.21. Burst Advocate for this burst is D. Kocevski (dankocevski AT gmail.com). 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/too.html.)