GCN Circular 22570

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

The Swift/UVOT began settled observations of the field of GRB
180331A 116 s after the BAT trigger (Markwardt et al., GCN Circ.
22568
Not found
 
 
).  No optical afterglow consistent with the XRT position
(Evans et al. GCN Circ. 22569
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           116          266          147         >20.2
u_FC               274          524          246         >19.9
white              116         1373          371         >20.4
v                  604         1250           78         >18.6
b                  530         1350           78         >19.4
u                  274          524          246         >19.9

The magnitudes in the table are not corrected for the Galactic extinction
due to the reddening of E(B-V) = 0.60 in the direction of the burst
(Schlegel et al. 1998).