TITLE: GCN CIRCULAR NUMBER: 11955 SUBJECT: GRB 110420B: Swift/UVOT Upper Limits DATE: 11/04/21 14:29:56 GMT FROM: Samantha Oates at MSSL S. R. Oates (MSSL-UCL) & B. Sbarufatti (INAF-OAB/IASFPA) report on behalf of the Swift-UVOT team The Swift/UVOT began settled observations of the field of GRB 110420B 2562 s after the BAT trigger (Sbarufatti et al., GCN Circ. 11944). No optical afterglow is found within the BAT error circle. 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 2562 2711 147 >20.6 white 2562 4765 540 >20.5 v 3541 16124 1322 >19.9 b 2926 22368 621 >20.4 u 2720 28151 1730 >20.0 w1 3950 28007 2844 >19.6 m2 3745 27103 2416 >19.5 w2 3336 15211 1279 >19.7 ############################################################### The values quoted above are not corrected for the Galactic extinction due to the reddening of E(B-V) = 0.03 in the direction of the burst (Schlegel et al. 1998).