TITLE: GCN CIRCULAR NUMBER: 39948 SUBJECT: GRB 250330A: Swift/UVOT Upper Limits DATE: 25/03/30 17:15:45 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 250330A 3944 s after the BAT trigger (Cenko et al., GCN Circ. 39938). No optical afterglow consistent with the XRT position (Osborne et al., GCN Circ. 39947) 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 3944 4094 147 >21.2 white 3944 4094 147 >21.2 v 4101 4300 197 >20.0 b 4921 5109 185 >20.9 u 4716 4915 197 >20.0 w1 4511 4711 197 >21.3 m2 4306 4505 197 >19.8 The magnitudes in the table are not corrected for the Galactic extinction due to the reddening of E(B-V) = 0.147 in the direction of the burst (Schlegel et al. 1998). View this GCN Circular online at https://gcn.nasa.gov/circulars/39948. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0LW...