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We present optical photometry of the GRB 060912A afterglow obtained withground-based telescopes, from about 100 sec after the GRB trigger till about0.3 day later, supplemented with the Swift optical afterglow data released inits official website. The optical light curve (LC) displays a smooth singlepower-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 LCwhich has a plateau phase between two normal power-law decays of a respectiveindex of about -1 and -1.2. It is shown by our combined X-ray and optical dataanalysis that this asynchronous behavior is difficult to be reconciled with thestandard afterglow theory and energy injection hypothesis. We also construct anoptical-to-X-ray spectral energy distribution at about 700 sec after the GRBtrigger. It displays a significant flux depression in the B-band, reminding usof the possibility of a host-galaxy (at z=0.937) 2175-A dust absorption similarto the one that characterizes the Milky Way extinction law. Such anidentification, although being tentative, may be confirmed by our detailedanalysis using both template extinction laws and the afterglow theory. So farthe feature is reported in very few GRB afterglows. Most seem to have a hostgalaxy either unusually bright for a GRB, just like this one, or of an earlytype, supporting the general suggestion of an anti-correlation between thefeature and star-forming activities.

The X-ray Asynchronous Optical Afterglow of GRB 060912A and Tentative Evidence of a 2175-A Host Dust Extinction Feature