{
  "submittedHow": "web",
  "circularId": 40279,
  "submitter": "Mike Siegel at PSU/Swift MOC <mhs18@psu.edu>",
  "format": "text/plain",
  "bibcode": "2025GCN.40279....1S",
  "subject": "GRB 250427A: Swift/UVOT Detection",
  "eventId": "GRB 250427A",
  "createdOn": 1745953147560,
  "body": "M. H. Siegel (PSU) reports on behalf of the Swift UVOT team:\n\nSwift/UVOT observed the field of GRB 250427A detected by Einstein Probe (Wang et al., GCN Circ. 40257) starting 22.9 ks after the burst.\n\nA 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).\n\nThe preliminary detection and 3-sigma upper limits calculated using the UVOT photometric system\n(Breeveld et al. 2011, AIP Conf. Proc. 1358, 373) for the initial and subsequent exposures are:\n\nFilter         T_start(s)   T_stop(s)      Exp(s)           Mag\n\nu               22960        28938         1622            18.74+/-0.07\nu              136546       153119         1721           >20.47\n\nThe magnitudes in the table are not corrected for the Galactic extinction\ndue to the reddening of E(B-V) = 0.209 in the direction of the burst\n(Schlegel et al. 1998)."
}