GCN Circular 23014

N. P. M. Kuin (UCL-MSSL) and B. Sbarufatti (PSU)
report on behalf of the Swift/UVOT team:

The Swift/UVOT began settled observations of the field of GRB 180721A
120 s after the BAT trigger (Sbarufatti et al., GCN Circ. 22994
Not found
 
 
).
No optical afterglow consistent with the XRT position (Goad et al, GCN
Circ. 22997
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

u_FC               120          370          246         >20.1
v                  425         1693          156         >19.1
b                  375         1642          175         >19.9
u                  120         1617          529         >20.1
w1                 474         1742          156         >19.6
m2                1697         1717           19         >18.3
w2                 401         1668          156         >20.5

The magnitudes in the table are not corrected for the Galactic extinction
due to the reddening of E(B-V) = 0.07 in the direction of the burst
(Schlegel et al. 1998).