GCN Circular 13047

A. A. Breeveld (MSSL-UCL) 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 120311B
3072 s after the BAT trigger (Marshall et al., GCN Circ. 13038
Not found
 
 
).
No optical afterglow consistent with the XRT position (Osborne et al., 
GCN Circ. 13042
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 first
finding chart (FC) exposure and subsequent exposures are:

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

white_FC          3072         3222          147         >21.4
white             3072        16503         2114         >22.8
v                 3228        21300         2013         >20.3
b                 4048        15591         1233         >20.7
u                 3843         5479          393         >19.7
w1                3638        22969         1136         >20.9
m2                3433        22206         1082         >20.8
w2                4460        17192         1754         >21.9

The magnitudes in the table are not corrected for the Galactic 
extinction due to the reddening of E(B-V) = 0.52 in the direction of the 
burst (Schlegel et al. 1998).