TITLE: GCN CIRCULAR NUMBER: 12389 SUBJECT: GRB 110921A: Swift/UVOT Observations DATE: 11/09/23 14:23:26 GMT FROM: Alice Breeveld at MSSL-UCL A. A. Breeveld (MSSL-UCL) and W. H. Baumgartner (GSFC/UMBC) report on behalf of the Swift/UVOT team: The Swift/UVOT began settled observations of the field of GRB 110921A 126 s after the BAT trigger (Baumgartner et al., GCN Circ. 12373). A source consistent with the optical position (Xin et al., GCN Circ. 12377 and Xu et al., GCN Circ. 12385) is detected at 3.5 sigma in the summed white data, but is not seen in any of the other filters, nor in any of the white exposures individually. Preliminary magnitudes and 3-sigma upper limits using the UVOT photometric system (Poole et al. 2008, MNRAS, 383, 627) for the first finding chart (FC) exposure and subsequent exposures are: Filter T_start(s) T_stop(s) Exp(s) Mag white_FC 126 276 147 >21.3 u_FC 284 534 246 >20.6 white 126 1704 411 21.4 ± 0.3 v 614 5307 308 >20.3 b 540 1679 117 >20.7 u 284 1654 343 >20.9 w1 664 1629 117 >21.1 m2 639 1604 117 >19.7 w2 590 1725 132 >20.6 The magnitudes in the table are not corrected for the Galactic extinction due to the reddening of E(B-V) = 0.14 in the direction of the burst (Schlegel et al. 1998).