TITLE: GCN CIRCULAR NUMBER: 21913 SUBJECT: GRB 170921A: Swift/UVOT Upper Limits DATE: 17/09/22 14:56:56 GMT FROM: Sam LaPorte at PSU GRB 170921A: Swift/UVOT Upper Limits S. J. LaPorte (PSU) and A. D'Ai (INAF-IASFPA) report on behalf of the Swift/UVOT team: (Beardmore et al. GCN Circ. 21906) The Swift/UVOT began settled observations of the field of GRB 170921A 101 s after the BAT trigger (D'Ai et al., GCN Circ. 21901). No optical afterglow consistent with the XRT position 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 101 251 147 >20.7 u_FC 259 509 246 >20.4 white 101 6816 766 >21.4 v 590 7186 451 >19.9 b 516 6611 471 >20.6 u 259 12926 1346 >21.1 w1 639 12273 1171 >20.6 m2 1412 11366 1102 >21.0 w2 566 16512 687 >21.0 The magnitudes in the table are not corrected for the Galactic extinction due to the reddening of E(B-V) = 0.05 in the direction of the burst (Schlegel et al. 1998).