TITLE: GCN CIRCULAR NUMBER: 41864 SUBJECT: GRB 250917B: Swift/UVOT Upper Limits DATE: 25/09/18 11:35:38 GMT FROM: Alice Breeveld at MSSL-UCL <a.breeveld@ucl.ac.uk> A.A. Breeveld (UCL-MSSL) and R. Caputo (GSFC) report on behalf of the Swift/UVOT team: The Swift/UVOT began settled observations of the field of GRB 250917B 139 s after the Swift/BAT trigger (Caputo et al., GCN Circ. 41848) and the Fermi GBM trigger (Fermi GBM Team, GCN 41846). No optical afterglow consistent sources 1, 5 or 6 identified by the Swift-XRT (Kennea et al., GCN Circ. 41851) is detected in the initial UVOT exposures. Preliminary 3-sigma upper limits using the UVOT photometric system (Breeveld et al. 2011, AIP Conf. Proc. 1358, 373) for the first finding chart (FC) exposure and subsequent exposures are: Filter T_start(s) T_stop(s) Exp(s) Mag white_FC 139 289 147 >20.5 u_FC 351 601 246 >19.9 white 139 5391 492 >21.2 v 681 1059 39 >18.1 b 606 5225 236 >20.1 u 351 5020 462 >20.1 w1 730 4815 216 >18.9 m2 705 1084 39 >18.8 w2 831 1035 39 >19.0 The magnitudes in the table are not corrected for the Galactic extinction due to the reddening of E(B-V) = 1.983 in the direction of the burst (Schlegel et al. 1998). View this GCN Circular online at https://gcn.nasa.gov/circulars/41864. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0LW...