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).