TITLE:   GCN CIRCULAR
NUMBER:  23152
SUBJECT: GRB 180818B: Swift/UVOT Upper Limits
DATE:    18/08/18 18:08:33 GMT
FROM:    Paul Kuin at MSSL  <npkuin@gmail.com>

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 180818B
212 s after the BAT trigger (Marshall et al., GCN Circ. 23149).
No optical afterglow consistent with the XRT position
(Beardmore et al. GCN Circ. 23151)
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           212          362          147         >19.9
white              212        11082         1482         >21.1
v                  369        12690         1148         >19.8
b                  467         6919          471         >19.9
u                  442         6714          469         >19.6
w1                 418         6509          468         >19.9
m2                 393         6304          471         >20.5
w2                 517        11988         1160         >20.3

The magnitudes in the table are not corrected for the Galactic extinction
due to the reddening of E(B-V) = 0.09 in the direction of the burst
(Schlegel et al. 1998).