TITLE:   GCN CIRCULAR
NUMBER:  23014
SUBJECT: GRB 180721A: Swift/UVOT Upper Limits
DATE:    18/07/22 16:42:55 GMT
FROM:    Paul Kuin at MSSL  <npkuin@gmail.com>

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).
No optical afterglow consistent with the XRT position (Goad et al, GCN
Circ. 22997) 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).