TITLE:   GCN CIRCULAR
NUMBER:  6735
SUBJECT: GRB 070810: Swift/UVOT Optical Counterpart
DATE:    07/08/10 05:25:36 GMT
FROM:    Margaret Chester at PSU  <chester@astro.psu.edu>

M.M. Chester (PSU), F.E. Marshall (NASA/GSFC), and J.R. Cummings
(NASA/UMBC) report on behalf of the Swift UVOT team:

The Swift/UVOT observed the field of GRB 070810 (Cummings et al., GCN
Circ. 6733) beginning 02:13:24 UT, 92 seconds after the BAT trigger.

We find an uncatalogued source inside the Swift/XRT error circle in the
initial 2.7' x 2.7' subimage in the White filter at:

    RA(J2000)  = 12:39:51.22 = 189.9634
    DEC(J2000) = +10:45:04.3  = 10.7512

with a 1-sigma error radius of about 0.6 arc sec. This position is 1.9
arcsec from the center of the XRT error circle, and 5.0 arcsec from the
position reported by ROTSE (Swan et al. GCN Circ. 6734).  The
counterpart is not blended in the Swift/UVOT images.  The estimated
magnitudes (1-sigma error is about 1 mag) are:

Filter   Tstart    Exp     Magnitude
            (s)     (s)
White      92      100       17.4
v         198      306       17.1

The values quoted above are not corrected for the expected Galactic
extinction corresponding to a reddening of E_{B-V} = 0.02 mag in the
direction of the burst (Schlegel et al. 1998).