TITLE:   GCN CIRCULAR
NUMBER:  23150
SUBJECT: GRB 180818A: Swift/UVOT Upper Limits
DATE:    18/08/18 14:15:56 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 180818A
83 s after the BAT trigger (Marshall et al., GCN Circ. 23146). No
optical afterglow consistent with the XRT position (Osborne et al. GCN
Circ. 23147) 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            83          232          147         >20.7
u_FC               241          490          246         >20.1
white               83         7151          785         >21.9
v                  570        11683         1198         >19.8
b                  496         6946          510         >20.8
u                  241        17153         1326         >20.9
w1                 621        13097          985         >20.1
m2                 596        12588         1199         >20.5
w2                 546         7357          510         >20.7

The magnitudes in the table are not corrected for the Galactic extinction
due to the
reddening of E(B-V) = 0.11 in the direction of the burst (Schlegel et al.
1998).