TITLE: GCN CIRCULAR NUMBER: 30686 SUBJECT: GRB 210820A: Swift/UVOT Detection DATE: 21/08/22 19:54:45 GMT FROM: Sam LaPorte at PSU GRB 210820A: Swift/UVOT Detection S. J. LaPorte (PSU) and A. P. Beardmore (U. Leicester) report on behalf of the Swift/UVOT team: The Swift/UVOT began settled observations of the field of GRB 210820A 131 s after the BAT trigger (Beardmore et al., GCN Circ. 30664). A source consistent with the XRT position (Beardmore et al., GCN Circ. 30664) is detected in the initial UVOT exposures. The preliminary UVOT position is: RA (J2000) = 01:22:33.48 = 20.63952 (deg.) Dec (J2000) = +04:36:28.3 = 4.60786 (deg.) with an estimated uncertainty of 0.45 arc sec. (radius, 90% confidence). 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 v 437 1684 156 18.86 +/- 0.24 b 387 555 39 18.57 +/- 0.19 u 131 381 246 17.60 +/- 0.06 w1 784 1733 58 >19.0 The magnitudes in the table are not corrected for the Galactic extinction due to the reddening of E(B-V) = 0.030 in the direction of the burst (Schlegel et al. 1998).