GCN Circular 8179

F.E. Marshall (NASA/GSFC) and D. Grupe (PSU)
report on behalf of the Swift/UVOT team:

The Swift/UVOT observed the field of GRB 080903 starting 75 s after
the BAT trigger (Grupe et al., GCN Circ. 8169
Not found
 
 
). A source is
detected in the initial exposure in the V filter at the position
of the optical afterglow (de Ugarte Postigo et al., GCN Circ. 8172
Not found
 
 
).
There is also a marginal (1.8 sigma) detection in the initial exposures
with the U filter. A valid detection in the U filter would
indicate a redshift of less than 3.3.

The magnitudes and 3-sigma upper limits in the UVOT photometric system 
(Poole et al. 2008, MNRAS, 383, 627) for detecting
a source in the first finding chart (FC) exposure and 
subsequent co-added exposures are:

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

v (FC)              75          475          393         19.8 +/- 0.24
v                  595        28705         2596        >21.5
b                  555        12429         1307        >21.8
u                  530          829           58         20.0 +/- 0.47
u                 4906        34528         3049        >21.8
uvw1               506         7268          551        >21.0
uvm2               481         7160          655        >20.7
uvw2               571         6750          452        >21.2

The values quoted above are not corrected for the Galactic extinction due
to the reddening of E(B-V) = 0.22 in the direction of the burst 
(Schlegel et al. 1998).