TITLE: GCN CIRCULAR NUMBER: 23360 SUBJECT: GRB 181020A: Swift/UVOT Detection DATE: 18/10/21 18:39:11 GMT FROM: Jeffrey Gropp at PSU J. D. Gropp (PSU), M. De Pasquale (U. Instanbul) and Moss (George Washington University) report on behalf of the Swift/UVOT team: The Swift/UVOT began settled observations of the field of GRB 181020A 122 s after the BAT trigger (Moss et al., GCN Circ. 23349). A fading source consistent with the XRT enhanced position (Goad et al. GCN Circ. 23351) is detected in the initial UVOT exposures. The preliminary UVOT position is: RA (J2000) = 00:55:55.77 = 13.98239 (deg.) Dec (J2000) = -47:22:51.9 = -47.38108 (deg.) with an estimated uncertainty of 0.45 arc sec. (radius, 90% confidence). This optical afterglow is also reported by VLT/X-shooter, Fynbo et al., GCN Circ. 23356; LCO Cerro Tololo, Cucchiara et al., GCN Circ. 23358; and its position is consistent with the Fermi LAT Measurement reported in, Axelsson et al., GCN Circ. 23350; Preliminary detections and 3-sigma upper limits 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 b 352 1027 97 17.18+-0.07 m2 427 745 58 >18.06 u_fc 96 6797 768 18.52+-0.09 u 96 346 245 17.60+-0.08 v 21 720 67 14.89+-0.04 w1 452 769 58 >18.20 w2 379 1053 97 >18.56 The magnitudes in the table are not corrected for the Galactic extinction due to the reddening of E(B-V) = 0.01 in the direction of the burst (Schlegel et al. 1998). We note that the non-detection in the UV filters is expected for a source at the reported redshift z=2.94 (VLT/X-shooter, Fynbo et al., GCN Circ. 23356;), given the Lyman forest absorption by intergalactic hydrogen.