The X-ray Asynchronous Optical Afterglow of GRB 060912A and Tentative
  Evidence of a 2175-A Host Dust Extinction Feature
  We present optical photometry of the GRB 060912A afterglow obtained with
ground-based telescopes, from about 100 sec after the GRB trigger till about
0.3 day later, supplemented with the Swift optical afterglow data released in
its official website. The optical light curve (LC) displays a smooth single
power-law decay throughout the observed epochs, with a power-law index of about
-1 and no significant color evolution. This is in contrast to the X-ray LC
which has a plateau phase between two normal power-law decays of a respective
index of about -1 and -1.2. It is shown by our combined X-ray and optical data
analysis that this asynchronous behavior is difficult to be reconciled with the
standard afterglow theory and energy injection hypothesis. We also construct an
optical-to-X-ray spectral energy distribution at about 700 sec after the GRB
trigger. It displays a significant flux depression in the B-band, reminding us
of the possibility of a host-galaxy (at z=0.937) 2175-A dust absorption similar
to the one that characterizes the Milky Way extinction law. Such an
identification, although being tentative, may be confirmed by our detailed
analysis using both template extinction laws and the afterglow theory. So far
the feature is reported in very few GRB afterglows. Most seem to have a host
galaxy either unusually bright for a GRB, just like this one, or of an early
type, supporting the general suggestion of an anti-correlation between the
feature and star-forming activities.
