TITLE:   GCN CIRCULAR
NUMBER:  8790
SUBJECT: GRB 090107B: Swift/UVOT Observations
DATE:    09/01/07 21:41:56 GMT
FROM:    Frank Marshall at GSFC  <marshall@milkyway.gsfc.nasa.gov>

F.E. Marshall (NASA/GSFC)
reports on behalf of the Swift/UVOT team:

The Swift/UVOT observed the field of GRB 090107B starting 4396 s after
the INTEGRAL trigger (Gotz et al., GCN Circ. 8786). No optical afterglow
is detected in the initial UVOT exposures at the position
of the candidate X-ray afterglow (Perri et al., GCN Circ. 8789).
There is also no obvious optical afterglow candidate
elsewhere in the INTEGRAL error circle.

The 3-sigma upper limits in the UVOT photometric system 
(Poole et al. 2008, MNRAS, 383, 627) for detecting
a source at the position of the XRT candidate are:

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

white             5420         5535          113        >20.9
v                 4396         4595          197        >19.6
b                 5216         5415          197        >20.3
u                 5010         5209          197        >20.1
uvw1              4806         5005          197        >20.3
uvm2              4600         4799          197        >20.3
uvw2              4191         4390          197        >20.6

The values quoted above are not corrected for the Galactic extinction due
to the reddening of E(B-V) = 0.046 in the direction of the burst 
(Schlegel et al. 1998).