{
  "bibcode": "2021GCN.29259....1B",
  "body": "A. A. Breeveld (UCL-MSSL) and S. L. Laha (GSFC/UMBC/CRESST) report on behalf of the Swift/UVOT team:\n\nThe Swift/UVOT began settled observations of the field of GRB 210104B 84 s after the BAT trigger \n(Laha et al., GCN Circ. 29243).\nNo optical afterglow consistent with the XRT position (Evans et al. GCN Circ. 29253) is detected in \nthe initial UVOT exposures. Preliminary 3-sigma upper limits using the UVOT photometric system \n(Breeveld et al. 2011, AIP Conf. Proc. 1358, 373) for the first finding chart (FC) exposure and \nsubsequent exposures are:\n\nFilter         T_start(s)   T_stop(s)      Exp(s)         Mag\n\nwhite_FC            84          234          147         >20.5\nu_FC               296          546          246         >19.8\nwhite               84          753          172         >20.7\nv                  625          645           19         >17.6\nb                  551          743           39         >19.1\nu                  296          719          265         >19.9\n\nThe magnitudes in the table are not corrected for the Galactic extinction due to the reddening of \nE(B-V) = 0.79 in the direction of the burst (Schlegel et al. 1998).",
  "circularId": 29259,
  "createdOn": 1609856336000,
  "email": "a.breeveld@ucl.ac.uk",
  "subject": "GRB 210104B: Swift/UVOT Upper Limits",
  "submitter": "Alice Breeveld at MSSL-UCL  <a.breeveld@ucl.ac.uk>",
  "eventId": "GRB 210104B"
}