GCN Circular 35388

N. P. M. Kuin (UCL-MSSL) and Sbarrato (INAF-OAB)
report on behalf of the Swift/UVOT team:

The Swift/UVOT began settled observations of the field of GRB 231216A
191 s after the BAT trigger (Sbarrato et al., GCN Circ. 35378
Not found
 
 
).
No optical afterglow consistent with the XRT position was 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

u_FC               191          441          246         >20.2
v                  497          814           58         >18.5
b                  447         1028           63         >20.0
u                  191         1018          432         >20.8
w1                 546          864           58         >19.5
m2                 819          839           19         >18.3
w2                 473          790           58         >19.6

The magnitudes in the table are not corrected for the Galactic extinction
due to the reddening of E(B-V) = 0.082 in the direction of the burst
(Schlegel et al. 1998).