TITLE:   GCN CIRCULAR
NUMBER:  22570
SUBJECT: GRB 180331A: Swift/UVOT Upper Limits
DATE:    18/03/31 11:26:21 GMT
FROM:    Paul Kuin at MSSL  <npkuin@gmail.com>

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).  No optical afterglow consistent with the XRT position
(Evans et al. GCN Circ. 22569) 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).