TITLE: GCN CIRCULAR NUMBER: 10640 SUBJECT: GRB 100420A: Swift/UVOT Upper Limits DATE: 10/04/21 04:21:50 GMT FROM: Margaret Chester at PSU M. M. Chester (PSU) and W. H. Baumgartner (GSFC/UMBC) report on behalf of the Swift/UVOT team: The Swift/UVOT began settled observations of the field of GRB 100420A 134 s after the BAT trigger (Baumgartner et al., GCN Circ. 10628). No optical afterglow consistent with the enhanced XRT position (Beardmore et al., GCN Circ. 10638) is detected in the initial UVOT exposures. Preliminary 3-sigma upper limits using the UVOT photometric system (Poole et al. 2008, MNRAS, 383, 627) for the first finding chart (FC) exposure and subsequent exposures are: Filter T_start(s) T_stop(s) Exp(s) Mag white_FC 134 284 147 >20.8 u_FC 292 542 246 >20.0 white 134 13589 1503 >22.7 v 624 30201 1434 >20.5 b 548 19368 1911 >22.2 u 292 18659 2409 >21.7 w1 674 17744 1435 >21.6 m2 648 30928 1260 >21.5 w2 598 29287 1434 >21.4 The values quoted above are not corrected for the Galactic extinction due to the reddening of E(B-V) = 0.11 in the direction of the burst (Schlegel et al. 1998).