TITLE: GCN CIRCULAR NUMBER: 38634 SUBJECT: GRB 241218A: Swift/UVOT Upper Limits DATE: 24/12/19 14:17:44 GMT FROM: Mike Siegel at PSU/Swift MOC <mhs18@psu.edu> M. H. Siegel (PSU) and M. Williams (PSU) report on behalf of the Swift/UVOTteam: The Swift/UVOT began settled observations of the field of GRB 241218A 152 s after the BAT trigger (Williams et al., GCN Circ. 38601). No optical afterglow consistent with the XRT position (D'Elia et al., GCN Circ. 38614) 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 152 301 147 >20.7 u_FC 310 559 246 >19.8 white 152 1014 334 >21.0 v 640 1411 97 >18.5 b 565 1337 78 >18.7 u 310 1312 304 >19.8 w1 689 709 19 >21.1 The magnitudes in the table are not corrected for the Galactic extinction due to the reddening of E(B-V) = 0.024 in the direction of the burst (Schlegel et al. 1998). View this GCN Circular online at https://gcn.nasa.gov/circulars/38634. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0LW...