TITLE: GCN CIRCULAR NUMBER: 28307 SUBJECT: GRB 200829A: Swift detection of a burst with an optical afterglow DATE: 20/08/29 14:18:33 GMT FROM: Boris Sbarufatti at PSU M. H. Siegel (PSU), J.D. Gropp (PSU), J. A. Kennea (PSU), N. P. M. Kuin (UCL-MSSL), S. Laha (GSFC/UMBC/CRESST) and B. Sbarufatti (PSU) report on behalf of the Neil Gehrels Swift Observatory Team: At 13:59:34 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 200829A (trigger=993768). Swift slewed immediately to the burst. The BAT on-board calculated location is RA, Dec 251.212, +72.335 which is RA(J2000) = 16h 44m 51s Dec(J2000) = +72d 20' 07" 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 30 sec. The peak count rate was ~1335 counts/sec (15-350 keV), at ~6 sec after the trigger. The XRT began observing the field at 14:01:43.1 UT, 128.7 seconds after the BAT trigger. XRT found a bright, uncatalogued X-ray source located at RA, Dec 251.2096, 72.3273 which is equivalent to: RA(J2000) = 16h 44m 50.30s Dec(J2000) = +72d 19' 38.3" with an uncertainty of 4.7 arcseconds (radius, 90% containment). This location is 27 arcseconds from the BAT onboard position, within the BAT error circle. No event data are yet available to determine the column density using X-ray spectroscopy. The initial flux in the 2.5 s image was 9.30e-09 erg cm^-2 s^-1 (0.2-10 keV). UVOT took a finding chart exposure of 150 seconds with the White filter starting 138 seconds after the BAT trigger. There is a candidate afterglow in the rapidly available 2.7'x2.7' sub-image at RA(J2000) = 16:44:49.05 = 251.20439 DEC(J2000) = +72:19:45.3 = 72.32924 with a 90%-confidence error radius of about 0.61 arc sec. This position is 9.0 arc sec. from the center of the XRT error circle. The estimated magnitude is 14.28 with a 1-sigma error of about 0.14. No correction has been made for the expected extinction corresponding to E(B-V) of 0.04. Burst Advocate for this burst is M. H. Siegel (siegel AT swift.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/)