GCN Circular 15920

F. E. Marshall (NASA/GSFC) and P. A. Evans (U. Leicester)
report on behalf of the Swift/UVOT team:

The Swift/UVOT began settled observations of the field of GRB 140304A
138 s after the BAT trigger (Evans et al., GCN Circ. 15915
Not found
 
 
).
No optical afterglow consistent with the optical position
(Gorbovskoy et al. GCN Circ. 15914
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

u_FC               138          388          246         >20.4
v                  420         1561          165         >19.5
u                  138         1519          529         >20.9
w1                 469         1495          156         >19.7
m2                 570         1470          117         >20.1
w2                 396         1546          156         >20.0

The magnitudes in the table are not corrected for the Galactic extinction
due to the reddening of E(B-V) = 0.08 in the direction of the burst
(Schlegel et al. 1998).

In view of the bright (16.5 mag) afterglow reported by
Gorbovskoy et al., it is likely that the GRB has large
intrinsic extinction or is at a redshift greater than
about 3.5.