TITLE: GCN CIRCULAR NUMBER: 20626 SUBJECT: GRB 170206B: Swift/UVOT Upper Limits DATE: 17/02/07 21:35:31 GMT FROM: Sam Emery at MSSL-UCL S.W.K. Emery (UCL-MSSL) 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 170206B 106 s after the BAT trigger (Lien et al., GCN Circ. 20618). No optical afterglow consistent with the XRT position (Evans et al. GCN Circ. 20619) is detected in the initial UVOT exposures. Preliminary 3-sigma upper limits using the UVOT photometric system (Breeveld et al. 2011, AIP Conf. Proc. 1358, 373) for the first finding chart (FC) exposure and subsequent exposures are: Filter T_start(s) T_stop(s) Exp(s) Mag white_FC 106 255 147 >20.3 u_FC 264 513 246 >19.6 white 106 1534 392 >20.7 v 594 1411 97 >18.2 b 519 1510 97 >19.1 u 226 1484 343 >19.7 w1 643 1460 97 >18.6 w2 569 1386 97 >19.1 The magnitudes in the table are not corrected for the Galactic extinction due to the reddening of E(B-V) = 0.146 in the direction of the burst (Schlegel et al. 1998).