TITLE: GCN CIRCULAR NUMBER: 6557 SUBJECT: Swift/UVOT detection of GRB070616 optical afterglow. DATE: 07/06/19 20:31:32 GMT FROM: Massimiliano de Pasquale at MSSL-UCL 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), 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.