TITLE: GCN CIRCULAR NUMBER: 23167 SUBJECT: GRB 180821A: Swift/UVOT Upper Limits DATE: 18/08/22 16:24:33 GMT FROM: Frank Marshall at Swift/UVOT F. E. Marshall (NASA/GSFC) and A. Tohuvavohu (PSU) report on behalf of the Swift/UVOT team: The Swift/UVOT began settled observations of the field of GRB 180821A 219 s after the BAT trigger (Tohuvavohu et al., GCN Circ. 23158). No optical afterglow consistent with the enhanced XRT position (Evans et al. GCN Circ. 23162) is detected in the initial UVOT exposures. Preliminary 3-sigma upper limits using the UVOT photometric system (Breeveld et al. 2011, AIP Conf. Proc. 1358, 373) for the first finding chart (FC) exposure and subsequent exposures are: Filter T_start(s) T_stop(s) Exp(s) Mag white_FC 219 369 147 >21.0 white 219 495 167 >21.1 v 526 546 19 >17.7 v 4604 6039 393 >20.0 b 451 471 19 >19.3 b 3988 5624 393 >21.3 u 426 446 19 >18.6 u 3783 5418 393 >20.5 w1 402 421 19 >19.3 w1 5015 15906 241 >20.5 m2 4808 10398 812 >20.5 w2 4399 6034 393 >20.3 The magnitudes in the table are not corrected for the Galactic extinction due to the reddening of E(B-V) = 0.02 in the direction of the burst (Schlegel et al. 1998).