TITLE: GCN CIRCULAR NUMBER: 40279 SUBJECT: GRB 250427A: Swift/UVOT Detection DATE: 25/04/29 18:59:07 GMT FROM: Mike Siegel at PSU/Swift MOC <mhs18@psu.edu> M. H. Siegel (PSU) reports on behalf of the Swift UVOT team: Swift/UVOT observed the field of GRB 250427A detected by Einstein Probe (Wang et al., GCN Circ. 40257) starting 22.9 ks after the burst. A fading optical source was detected in the U-band consistent with the position of the uncatalogued X-ray source detected by Swift/XRT (D'Elia et al., GCN Circ 40268) and the optical afterglow detected by various facilities (Perez-Garcia et al., GCN Circ 40259; Liu et al., GCN Circ. 40260, Becerra et al,. GCN Circ. 40261; Brivio et al., GCN Circ 40264; Pérez-Fournon et al., GCN Circ. 40267; Xi et al., GCN Circ. 40269; Swain et al., GCN Circ 40270; de Wet et al., GCN Circ 40272; Busmann et al., GCN Circ. 40273; Amram et al., GCN Circ 40275). The preliminary detection and 3-sigma upper limits calculated using the UVOT photometric system (Breeveld et al. 2011, AIP Conf. Proc. 1358, 373) for the initial and subsequent exposures are: Filter T_start(s) T_stop(s) Exp(s) Mag u 22960 28938 1622 18.74+/-0.07 u 136546 153119 1721 >20.47 The magnitudes in the table are not corrected for the Galactic extinction due to the reddening of E(B-V) = 0.209 in the direction of the burst (Schlegel et al. 1998). View this GCN Circular online at https://gcn.nasa.gov/circulars/40279. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0LW...