{
  "eventId": "GRB 251221A",
  "createdOn": 1766365458027,
  "bibcode": "2025GCN.43177....1S",
  "submittedHow": "web",
  "format": "text/plain",
  "body": "M.H. Siegel (PSU) and Ambrosi (INAF-IASFPA) report on behalf of the Swift/UVOT team:\n\nThe Swift/UVOT began settled observations of the field of GRB 251221A 143 s after the BAT trigger (Ambrosi et al., GCN Circ. 43166). No optical afterglow consistent with the XRT position (Goad et al., GCN Circ 43175) 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\nfinding chart (FC) exposure and subsequent exposures are:\n\nFilter         T_start(s)   T_stop(s)      Exp(s)         Mag\n\nwhite_FC           143          292          147         >20.5\nwhite              143         1541          366         >21.1\nv                  631         1584           65         >19.6\nb                  570         1521           45         >19.4\nu                  301         1497          169         >19.5\nw1                 680         1647           40         >19.1\nm2                 655          847           39         >19.5\nw2                 606         1572           78         >19.6\n\nThe magnitudes in the table are not corrected for the Galactic extinction\ndue to the reddening of E(B-V) = 1.088 in the direction of the burst\n(Schlegel et al. 1998).",
  "circularId": 43177,
  "submitter": "Mike Siegel at PSU/Swift MOC <mhs18@psu.edu>",
  "subject": "GRB 251221A: Swift/UVOT Upper Limits"
}