TITLE: GCN CIRCULAR NUMBER: 30767 SUBJECT: GRB 210901A: Swift/UVOT Upper Limits DATE: 21/09/05 00:48:10 GMT FROM: Mike Siegel at PSU/Swift MOC <mhs18@psu.edu> M. H. Siegel (PSU) and S. L. Laha (GSFC/UMBC/CRESST) report on behalf of the Swift/UVOT team: The Swift/UVOT began settled observations of the field of GRB 210901A 158 s after the BAT trigger (Laha et al., GCN Circ. 30746). No optical afterglow consistent with the XRT position (Osborne et al. GCN Circ. 30749) 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 158 308 147 >20.9 u_FC 316 566 246 >19.6 white 158 974 289 >20.9 v 646 839 39 >17.8 b 572 764 39 >18.8 u 316 740 265 >19.8 w1 695 715 19 >17.8 The magnitudes in the table are not corrected for the Galactic extinction due to the reddening of E(B-V) = 0.200 in the direction of the burst (Schlegel et al. 1998).