TITLE: GCN CIRCULAR NUMBER: 18161 SUBJECT: GRB 150818A: Swift/UVOT Detection DATE: 15/08/18 20:39:33 GMT FROM: Marissa McCaule at PSU L. M. McCauley (PSU), L. M. Z. Hagen (PSU), and V. D'Elia (ASDC) report on behalf of the Swift/UVOT team: The Swift/UVOT began settled observations of the field of GRB 150818A 94 s after the BAT trigger (D'Elia et al., GCN Circ. 18152). A source consistent with the XRT position (Beardmore et al. GCN Circ. 18159) is detected in the initial UVOT exposures. The preliminary UVOT position is: RA (J2000) = 15:21:25.43 = 230.35594 (deg.) Dec (J2000) = +68:20:33.0 = 68.34251 (deg.) with an estimated uncertainty of 0.66 arc sec. (radius, 90% confidence). Preliminary detections and 3-sigma upper limits using the UVOT photometric system (Breeveld et al. 2011, AIP Conf. Proc. 1358, 373) for the early exposures are: Filter T_start(s) T_stop(s) Exp(s) Mag white_fc 94 121 27 19.39+-0.25 white 3414 4790 344 19.92+-0.17 white 10222 10826 590 19.68+-0.07 v 3572 5201 393 >19.36 b 4385 4585 196 >20.16 b 5821 10217 927 20.56+-0.33 u 4180 4379 196 >19.66 u 5616 5816 196 19.63+-0.25 uvw1 3976 4175 195 18.78+-0.20 uvw1 5411 5611 196 18.95+-0.20 uvm2 3776 3976 192 18.81+-0.23 uvm2 5206 5406 196 18.89+-0.24 uvw2 4796 4996 196 18.90+-0.19 The magnitudes in the table are not corrected for the Galactic extinction due to the reddening of E(B-V) = 0.02 in the direction of the burst (Schlegel et al. 1998).