{
  "body": "M. H. Siegel (PSU) and S. B. Cenko (GSFC) report on behalf of the Swift/UVOT team:\n\nThe Swift/UVOT began settled observations of the field of GRB 250818A\n162 s after the BAT trigger (Cenko et al., GCN Circ. 41403).\nNo optical afterglow consistent with the XRT position (Osborn et al., GCN\nCirc. 41403) is detected in the initial UVOT exposures.\nPreliminary 3-sigma upper limits using the UVOT photometric system\n(Breeveld et al. 2011, AIP Conf. Proc. 1358, 373) for the first\nfinding chart (FC) exposure and subsequent exposures are:\n\nFilter         T_start(s)   T_stop(s)      Exp(s)         Mag\n\nu (fc)             162          412          246         >20.3\nb                  418          438           19         >18.6\nv                32212        32641          429.        >20.2\n\nThe magnitudes in the table are not corrected for the Galactic extinction\ndue to the reddening of E(B-V) = 1.602 in the direction of the burst\n(Schlegel et al. 1998).",
  "createdOn": 1755539417357,
  "submittedHow": "web",
  "circularId": 41416,
  "submitter": "Mike Siegel at PSU/Swift MOC <mhs18@psu.edu>",
  "eventId": "GRB 250818A",
  "format": "text/plain",
  "subject": "GRB 250818A: Swift/UVOT Upper Limits",
  "bibcode": "2025GCN.41416....1S"
}