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).