TITLE: GCN CIRCULAR NUMBER: 17673 SUBJECT: GRB 150403A: Swift/UVOT Detection DATE: 15/04/04 13:51:07 GMT FROM: Paul Kuin at MSSL N. P. M. Kuin (UCL-MSSL) and A. Y. Lien (GSFC/UMBC) report on behalf of the Swift/UVOT team: The Swift/UVOT began settled observations of the field of GRB 150403A 85 s after the BAT trigger (Lien et al., GCN Circ. 17665). A source consistent with the XRT position (Beardmore et al. GCN Circ. 17666) is detected in the initial UVOT exposures. The preliminary UVOT position is: RA (J2000) = 20:46:01.15 = 311.50479 (deg.) Dec (J2000) = -62:42:41.0 = -62.71139 (deg.) with an estimated uncertainty of 0.42 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 85 235 147 14.89 +/- 0.02 v 627 647 20 16.49 +/- 0.13 b 553 573 20 16.65 +/- 0.08 u 298 547 246 16.14 +/- 0.03 w1 676 5286 294 19.68 +/- 0.21 m2 6317 23414 1178 >22.2 w2 5907 7536 387 >20.6 The magnitudes in the table are not corrected for the Galactic extinction due to the reddening of E(B-V) = 0.05 in the direction of the burst (Schlegel et al. 1998).