TITLE: GCN CIRCULAR NUMBER: 6934 SUBJECT: GRB 071010A: Late-time Keck/LRIS photometry - possible host galaxy and jet break DATE: 07/10/18 21:59:38 GMT FROM: Daniel Perley at U.C. Berkeley D. A. Perley, A. V. Filippenko, J. M. Silverman, R. J. Foley, M. Modjaz, D. Kocevski, and J. S. Bloom report: We acquired an additional series of imaging observations of the field of GRB 071010A (Moretti et al., GCN 6859) with Keck I + LRIS starting at 5:01 UT, 2007-10-16 (6.05 days after the trigger), in g and R filters. The optical afterglow (Klotz et al. GCN 6860) has faded substantially. An object is observed at the GRB position, resolvable into two regions: a brighter, redder source to the east and a fainter, bluer source to the west. Comparison with our previous Keck imaging shows the afterglow position to be consistent only with the fainter, western source. The two sources may be a bright elongated host galaxy, a compact host galaxy with the afterglow offset from the center, or a foreground star with the afterglow coincidentally located very nearby. An image of the field is posted at: http://lyra.berkeley.edu/~dperley/071010a/071010a_keck.png Aperture photometry shows the combined complex of both sources to have a magnitude of R=22.5, using the same calibration system in previous circulars. The contribution from the afterglow is limited to R>23.3, depending on the uncertain contribution of a possible bright host galaxy. Refined photometry of our imaging starting at 2007-10-11 UT 04:47 (GCN 6885) shows the afterglow magnitude at that time to be R = 19.82+/-0.02. This indicates that the afterglow decay underwent a sharp break, from alpha < 0.5 between the first and second night to a minimum of alpha > 1.7 over the following five days. Comparison with the XRT light curve at http://astro.berkeley.edu/~nat/swift/00293707/bat_xrt.jpg shows the X-ray afterglow to have undergone a break at 10^5 seconds (roughly coincident with our measurement on 2007-10-11) from approximately flat evolution before this point to a rapidly decaying power law of alpha ~ 1.8 afterward. This suggests that this sharp break may be achromatic, and possibly indicative of a jet break. [GCN OPS NOTE(18oct07): The "and jet break" was added back on to the Subject-line of this circular. It was chopped off during processing because the mail sending/delivery system chopps wrapped Subject lines.]