GCN Circular 23150

N. P. M. Kuin (UCL-MSSL) and F. E. Marshall (NASA/GSFC)
report on behalf of the Swift/UVOT team:

The Swift/UVOT began settled observations of the field of GRB 180818A
83 s after the BAT trigger (Marshall et al., GCN Circ. 23146
Not found
 
 
). No
optical afterglow consistent with the XRT position (Osborne et al. GCN
Circ. 23147
Not found
 
 
) 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            83          232          147         >20.7
u_FC               241          490          246         >20.1
white               83         7151          785         >21.9
v                  570        11683         1198         >19.8
b                  496         6946          510         >20.8
u                  241        17153         1326         >20.9
w1                 621        13097          985         >20.1
m2                 596        12588         1199         >20.5
w2                 546         7357          510         >20.7

The magnitudes in the table are not corrected for the Galactic extinction
due to the
reddening of E(B-V) = 0.11 in the direction of the burst (Schlegel et al.
1998).