TITLE: GCN CIRCULAR NUMBER: 22665 SUBJECT: GRB 180418A: Swift/UVOT Detection DATE: 18/04/19 22:05:17 GMT FROM: Mike Siegel at PSU/Swift MOC M. H. Siegel (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 180418A 3086 s after the BAT trigger (D'Elia et al., GCN Circ. 22646). A fading source consistent with the XRT position (Goad et al. GCN Circ. 22650) is detected in the initial UVOT exposures, confirming the previously reported optical afterglows (Zheng & Filippenko, GCN Circ. 22647; Guidorzi et al., GCN Circ. 22648; Troja et al., GCN Circ. 22652; Fong et al., GCN Circ. 22659; Malesani et al., GCN Circ. 22660; Schady et al., GCN Circ. 22662; Misra et al., GCN Circ. 22663). The preliminary UVOT position is: RA (J2000) = 11:20:29.21 = 170.12172 (deg.) Dec (J2000) = +24:55:59.2 = 24.93311 (deg.) with an estimated uncertainty of 0.49 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) 3087 3236 147 19.19+-0.11 white 3087 4470 344 19.45+-0.09 white 49806 72093 4850 >22.62 v 3243 4835 347 >19.53 b 4064 4264 196 >20.19 b 15510 16114 590 >20.82 u (fc) 3859 4059 196 19.35+-0.24 u 14598 31858 894 >20.73 uvw1 3655 3855 196 19.19+-0.28 uvw1 9720 27635 1206 20.02+-0.21 uvm2 3450 3649 196 18.93+-0.27 uvm2 8814 26975 2165 20.65+-0.26 uvw2 4476 4676 196 >19.69 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).