GCN Circular 6454

F.E. Marshall (NASA/GSFC) and C. Guidorzi (U. Bicocca & INAF-OAB)
report on behalf of the Swift/UVOT team:

The Swift/UVOT observed the field of GRB 070521 starting 82 s after
the BAT trigger (Guidorzi et al., GCN Circ. 6431
Not found
 
 
).  We do not find any source
in any of the UVOT observations inside the refined XRT error circle
(Guidorzi et al., GCN Circ. 6452
Not found
 
 
). The 3-sigma upper limits for detecting 
a source in the first finding chart (FC) exposure and co-added frames are:

Filter            T_start(s)   T_stop(s)     Exp(s)     Mag (3-sigma UL)

White (FC)        82           182           98         >20.6
White             82           24605         1884       >22.3
V                 187          17181         1451       >20.4
B                 5304         23879         1967       >22.0
U                 5099         22968         1278       >21.7
UVW1              4894         18810         1111       >21.1
UWM2              4869         18085         1279       >20.8
UVW2              5713         12897         792        >21.1

The values quoted above are not corrected for the expected Galactic
extinction corresponding to a reddening of E(B-V) = 0.03 mag towards
the direction of the burst (Schlegel et al. 1998).