TITLE: GCN CIRCULAR NUMBER: 18238 SUBJECT: GRB 150902A: Swift/UVOT Upper Limits DATE: 15/09/04 15:37:25 GMT FROM: Alice Breeveld at MSSL-UCL A. A. Breeveld (UCL-MSSL) and L. Hagen (PSU) report on behalf of the Swift/UVOT team: The Swift/UVOT began settled observations of the field of GRB 150902A 59510 s after the LAT trigger (Arimoto et al., GCN Circ. 18228). No optical afterglow consistent with the XRT Source 1 position given in McCauley et al. (GCN Circ. 18237) is detected in the initial UVOT exposures. Source 2 is outside the UVOT field of view. Preliminary 3-sigma upper limits using the UVOT photometric system (Breeveld et al. 2011, AIP Conf. Proc. 1358, 373) for the initial exposures for source 1 are: Filter T_start(s) T_stop(s) Exp(s) Mag white 60331 60972 631 >21.7 v 61152 61997 828 >20.3 u 59510 66787 1184 >20.7 The magnitudes in the table are not corrected for the Galactic extinction due to the reddening of E(B-V) = 0.40 in the direction of the burst (Schlegel et al. 1998).