TITLE: GCN CIRCULAR NUMBER: 28278 SUBJECT: Swift Trigger 992444: a possible GRB 200821A DATE: 20/08/21 14:04:25 GMT FROM: David Palmer at LANL S. Dichiara (NASA/GSFC/UMCP), S. D. Barthelmy (GSFC), D. N. Burrows (PSU), P. A. Evans (U Leicester), C. Gronwall (PSU), J.D. Gropp (PSU), J. A. Kennea (PSU), N. J. Klingler (PSU), N. P. M. Kuin (UCL-MSSL), S. Laha (GSFC/UMBC/CRESST), K. L. Page (U Leicester), D. M. Palmer (LANL) and M. H. Siegel (PSU) report on behalf of the Neil Gehrels Swift Observatory Team: At 13:15:05 UT, the Swift Burst Alert Telescope (BAT) triggered and located a possible GRB 200821A (trigger=992444). Swift slewed immediately to the location. The BAT on-board calculated location is RA, Dec 353.570, -48.277 which is RA(J2000) = 23h 34m 17s Dec(J2000) = -48d 16' 37" with an uncertainty of 3 arcmin (radius, 90% containment, including systematic uncertainty). The BAT light curve shows a sequence of short peaks covering about 20 seconds. The peak count rate was ~1200 counts/sec (15-350 keV), at ~9 sec after the trigger. The XRT began observing the field at 13:17:33.3 UT, 147.9 seconds after the BAT trigger. No source was detected in 1.3 ks of promptly downlinked data. We are waiting for the full dataset to detect and localise the XRT counterpart. UVOT took a finding chart exposure of 250 seconds with the U filter starting 310 seconds after the BAT trigger. There is a candidate afterglow in the rapidly available 2.7'x2.7' sub-image at RA(J2000) = 23:34:24.51 = 353.60212 DEC(J2000) = -48:16:28.4 = -48.27456 with a 90%-confidence error radius of about 0.83 arc sec. This position is 77.5 arc sec. from the center of the BAT error circle. The estimated magnitude is 19.92 with a 1-sigma error of about 0.23. No correction has been made for the expected extinction corresponding to E(B-V) of 0.01. We note there is a known faint source at this position in the DSS. This source lies within the current (Sector 28) field-of-view of TESS camera 2. This event has a highly unusual lightcurve for a GRB in BAT. The XRT data is dominated by a single hot pixel, which is preventing rapid analysis of the data to find a counterpart. The UVOT detection may be due to the known DSS source. For these reasons we cannot confidently say whether this is an instrumental artifact or an astrophysical source. Further analysis will require the downlinked data set. Burst Advocate for this burst is S. Dichiara (dichiara AT umd.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/)