TITLE: GCN CIRCULAR NUMBER: 30618 SUBJECT: GRB 210807A: Swift/UVOT Detection DATE: 21/08/08 21:07:25 GMT FROM: Sam LaPorte at PSU GRB 210807A: Swift/UVOT Detection S. J. LaPorte (PSU) 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 210807A 210 s after the BAT trigger (Lien et al., GCN Circ. 30600). A source consistent with the XRT position (Lien et al. GCN Circ. 30600) is detected in the initial UVOT exposures. The preliminary UVOT position is: RA (J2000) = 05:06:43.89 = 76.68289 (deg.) Dec (J2000) = +58:14:59.2 = 58.24979 (deg.) with an estimated uncertainty of 0.43 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 b 489 683 38 >17.84 u 639 659 19 >17.04 v 390 583 39 16.58+-0.25 uvw1 440 633 38 >17.12 uvw2 539 721 26 >17.08 white 234 383 147 16.93+-0.04 white 234 708 184 17.19+-0.05 The magnitudes in the table are not corrected for the Galactic extinction due to the reddening of E(B-V) = 0.409 in the direction of the burst (Schlegel et al. 1998).