TITLE: GCN CIRCULAR NUMBER: 10781 SUBJECT: GRB 100518A: Swift/UVOT Upper Limits DATE: 10/05/19 11:00:50 GMT FROM: Peter Curran at MSSL P.A. Curran (MSSL-UCL) and M. Stamatikos (OSU/NASA/GSFC) report on behalf of the Swift/UVOT team: During Swift target of opportunity observations of GRB 100518A (Stamatikos et al. GCN Circ. 10775), Swift/UVOT began settled observations starting 9551 seconds after the INTEGRAL trigger (Mereghetti et al. GCN Circ. 10772). No optical afterglow consistent with the enhanced XRT position (Beardmore et al. GCN Circ. 10780) or the optical counterpart (Cano et al. GCN Circ. 10776) is detected in the 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 9551 9701 147 >21.4 (FC) white 9551 11529 1033 >22.4 v 16324 17231 885 >20.7 b 9711 10618 885 >21.7 m2 17236 17899 652 >20.9 w2 11536 16318 1137 >21.6 The values quoted above are not corrected for the Galactic extinction due to the reddening of E(B-V) = 0.07 in the direction of the burst (Schlegel et al. 1998).