GCN Circular 21049

N. P. M. Kuin (UCL-MSSL) and A. P. Beardmore (U. Leicester) report
on behalf of the Swift/UVOT team:

The Swift/UVOT began settled observations of the field of GRB 170428A
740 s after the BAT trigger (Beardmore et al., GCN Circ. 21042
Not found
 
 
).
No optical afterglow consistent with the XRT position (Beardmore
et al., GCN Circ. 21047
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 initial exposures
are:

Filter         T_start(s)   T_stop(s)      Exp(s)         Mag

v                  740        16576         1101         >20.0
b                  838        10823         1101         >21.0
u                  813         5412          299         >19.5
w1                 789         5207          216         >19.5
m2                 764        17481         1102         >20.1
w2                4392        11731         1082         >20.0

The magnitudes in the table are not corrected for the Galactic extinction
due to the reddening of E(B-V) = 0.06 in the direction of the burst
(Schlegel et al. 1998).