TITLE: GCN CIRCULAR NUMBER: 40002 SUBJECT: GRB 250331B: Swift/UVOT Upper Limits DATE: 25/04/01 13:04:29 GMT FROM: Mike Siegel at PSU/Swift MOC <mhs18@psu.edu> M. H. Siegel (PSU) and S. B. Cenko (GSFC) report on behalf of the Swift/UVOT team: The Swift/UVOT began settled observations of the field of GRB 250331B 317 s after the BAT trigger (Cenko et al., GCN Circ. 39958). No optical afterglow consistent with the XRT position (Osborne et al., GCN Circ. 39974) 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 317 467 147 >19.5 white 317 1020 334 >19.9 v 474 1070 78 >18.3 b 572 764 39 >18.3 u 547 1144 58 >18.6 w1 523 1120 58 >18.0 m2 670 690 19 >17.7 w2 621 1046 58 >18.4 The magnitudes in the table are not corrected for the Galactic extinction due to the reddening of E(B-V) = 0.979 in the direction of the burst (Schlegel et al. 1998). View this GCN Circular online at https://gcn.nasa.gov/circulars/40002. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0LW...