TITLE: GCN CIRCULAR NUMBER: 30664 SUBJECT: GRB 210820A: Swift detection of a burst with an optical counterpart DATE: 21/08/20 17:52:12 GMT FROM: David Palmer at LANL A. P. Beardmore (U Leicester), J.D. Gropp (PSU), J. A. Kennea (PSU), N. J. Klingler (GSFC/UMBC/CRESSTII), N. P. M. Kuin (UCL-MSSL), F. E. Marshall (NASA/GSFC), K. L. Page (U Leicester), D. M. Palmer (LANL) and A. Tohuvavohu (U Toronto) report on behalf of the Neil Gehrels Swift Observatory Team: At 17:38:00 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 210820A (trigger=1069551). Swift slewed immediately to the burst. The BAT on-board calculated location is RA, Dec 20.629, +4.632 which is RA(J2000) = 01h 22m 31s Dec(J2000) = +04d 37' 55" 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 ~500 counts/sec (15-350 keV), at ~8 sec after the trigger. The XRT began observing the field at 17:40:05.7 UT, 125.0 seconds after the BAT trigger. XRT found a bright, uncatalogued X-ray source located at RA, Dec 20.6376, 4.6071 which is equivalent to: RA(J2000) = 01h 22m 33.02s Dec(J2000) = +04d 36' 25.6" with an uncertainty of 11.1 arcseconds (radius, 90% containment). This location is 94 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 3.75e-09 erg cm^-2 s^-1 (0.2-10 keV). UVOT took a finding chart exposure of 250 seconds with the U filter starting 130 seconds after the BAT trigger. There is a candidate afterglow in the rapidly available 2.7'x2.7' sub-image at RA(J2000) = 01:22:33.48 = 20.63952 DEC(J2000) = +04:36:28.2 = 4.60783 with a 90%-confidence error radius of about 0.76 arc sec. This position is 7.4 arc sec. from the center of the XRT error circle. The estimated magnitude is 17.53 with a 1-sigma error of about 0.15. No correction has been made for the expected extinction corresponding to E(B-V) of 0.031. Burst Advocate for this burst is A. P. Beardmore (apb AT star.le.ac.uk). 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/)