TITLE: GCN CIRCULAR NUMBER: 9397 SUBJECT: GRB 090518: Swift/UVOT Upper Limits DATE: 09/05/19 11:50:39 GMT FROM: Frank Marshall at GSFC F.E. Marshall (NASA/GSFC) and P. Evans (U. Leicester) report on behalf of the Swift/UVOT team: The Swift/UVOT observed the field of GRB 090518 starting 77 s after the BAT trigger (Evans et al., GCN Circ. 9386). No optical afterglow is detected in the initial UVOT exposures at the position of the X-ray afterglow (Evans et al., GCN Circ. 9387). Three-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) 77 227 147 >20.7 white 77 1367 373 >21.3 v 620 1417 97 >19.3 b 545 1341 78 >19.7 u 290 1316 304 >20.0 uvw1 669 1466 97 >19.7 uvm2 644 1442 97 >21.2 uvw2 596 1392 97 >20.0 The values quoted above are not corrected for the Galactic extinction due to the reddening of E(B-V) = 0.04 in the direction of the burst (Schlegel et al. 1998).