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.]