TITLE: GCN CIRCULAR NUMBER: 43177 SUBJECT: GRB 251221A: Swift/UVOT Upper Limits DATE: 25/12/22 01:04:18 GMT FROM: Mike Siegel at PSU/Swift MOC <mhs18@psu.edu> M.H. Siegel (PSU) and Ambrosi (INAF-IASFPA) report on behalf of the Swift/UVOT team: The 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 finding chart (FC) exposure and subsequent exposures are: Filter T_start(s) T_stop(s) Exp(s) Mag white_FC 143 292 147 >20.5 white 143 1541 366 >21.1 v 631 1584 65 >19.6 b 570 1521 45 >19.4 u 301 1497 169 >19.5 w1 680 1647 40 >19.1 m2 655 847 39 >19.5 w2 606 1572 78 >19.6 The magnitudes in the table are not corrected for the Galactic extinction due to the reddening of E(B-V) = 1.088 in the direction of the burst (Schlegel et al. 1998). View this GCN Circular online at https://gcn.nasa.gov/circulars/43177. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0LW...