GCN Circular 6557

M. De Pasquale (MSSL/UCL), R. Starling (Univ. Leicester),
F. E. Marshall (NASA/GSFC), S. Holland (CREEST/USRA/GSFC),
P. Schady (MSSL/UCL), M.J. Page (MSSL/UCL) report, on the
behalf of the UVOT team:

  After a careful investigation of the refined XRT error circle of
GRB070616 (Starling et al, GCN circ 6550
Not found
 
 
), which contains the
bright (V=14.4) star USNO-B1.0 1469-0076513, we find good evidence
to suggest that the optical afterglow is detected in the early
Swift/UVOT observations.

  The count rates in V and U band exposures between 250 and 1050
seconds after the trigger show a significant excess (at the 10
sigma level in the V band combined image and at the 5 sigma level
in the U band combined image) with respect to late observations
(T>1 day). This excess corresponds to an optical source of V=16.5
and U=16.9. The source appears to have disappeared in exposures
taken later than 1100s, thus suggesting a drop in the optical flux
suggestive of a GRB afterglow behaviour. We obtain a 3 sigma upper
limit of V>17.9 at T= 6ks after the trigger.

  The blended image of the star plus the afterglow is extended
compared to other stars in the image. Fitting it with two point
sources indicates that the afterglow is about 1.8" west and 0.9"
north of USNO-B1.0 star with a statistical uncertainty of ~0.3"
in each axis (90% confidence). A detection of the afterglow in
the U band indicates a redshift less than ~3.

  Analysis of another star in the same field of view reveals no
change of flux throughout the observations, thus ruling out
instrumental effects. While we cannot completely exclude that
the star in the XRT error circle might have varied mimicking a
GRB afterglow, observations carried out for the further 2 days
suggest a constant source. Furthermore, the UV magnitudes of the
star in the XRT error circle do not show any significant variation
corresponding to those seen in the optical filters, thus
supporting the afterglow hypothesis.