GCN Circular 6534
N. Mirabal, I. D. McGreer and J. P. Halpern (Columbia U.) report on behalf of the MDM GRB follow-up team: "We have re-observed the optical afterglow of Swift GRB 070612A (Updike et al. GCN 6515, Cenko et al. GCN 6525) in the R and i' filters using the same instrumental setup as in Mirabal et al. (GCN 6526). Preliminary calibration referenced to the SDSS photometry of field stars (Cool et al. GCN 6510) shows that the afterglow has faded by 1.88 mag since the previous night. This corresponds to a decay rate steeper than ~ -2.5, which might represent the end of an optical flash from reverse shock emission (Meszaros & Rees 1997, ApJ, 306, 301; Sari & Piran 1999, ApJ, 496, L1). Alternatively, the steepening could be the signature of a "jet break". Further observations are encouraged to distinguish between various possibilities."