TITLE: GCN CIRCULAR NUMBER: 28745 SUBJECT: GRB 201021C: Swift/UVOT Detection DATE: 20/10/22 13:16:47 GMT FROM: Samantha Oates at MSSL S. R. Oates (U.Birmingham) and A. Y. Lien (GSFC/UMBC) report on behalf of the Swift/UVOT team: The Swift/UVOT began settled observations of the field of GRB 201021C 105 s after the BAT trigger (Lien et al., GCN Circ. 28730). A source consistent with the XRT position (Beardmore et al. GCN Circ. 28736) is detected in the initial UVOT exposures. The preliminary UVOT position is: RA (J2000) = 00:50:06.93 = 12.52886 (deg.) Dec (J2000) = -55:51:56.8 = -55.86577 (deg.) with an estimated uncertainty of 0.42 arc sec. (radius, 90% confidence). Preliminary detections using the UVOT photometric system (Breeveld et al. 2011, AIP Conf. Proc. 1358, 373) for the early exposures are: Filter T_start(s) T_stop(s) Exp(s) Mag white (FC) 105 255 147 16.66 +/- 0.03 white 598 617 19 16.22 +/- 0.05 v 647 667 19 16.49 +/- 0.16 b 573 593 20 16.80 +/- 0.10 u (FC) 317 567 246 15.76 +/- 0.03 u 722 742 19 16.06 +/- 0.10 uvm2 672 692 19 16.43 +/- 0.24 uvw2 623 643 19 16.68 +/- 0.22 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).