The Troublesome Broadband Evolution of GRB 061126: Does a Gray Burst Imply Gray Dust?

We report on observations of a gamma-ray burst (GRB 061126) with an extremelybright (R ~ 12 mag at peak) early-time optical afterglow. The optical afterglowis already fading as a power law 22 seconds after the trigger, with nodetectable prompt contribution in our first exposure, which was coincident witha large prompt-emission gamma-ray pulse. The optical--infrared photometricspectral energy distribution is an excellent fit to a power law, but itexhibits a moderate red-to-blue evolution in the spectral index at about 500 safter the burst. This color change is contemporaneous with a switch from arelatively fast decay to slower decay. The rapidly decaying early afterglow isbroadly consistent with synchrotron emission from a reverse shock, but a brightforward-shock component predicted by the intermediate- to late-time X-rayobservations under the assumptions of standard afterglow models is notobserved. Indeed, despite its remarkable early-time brightness, this burstwould qualify as a dark burst at later times on the basis of its nearly flatoptical-to-X-ray spectral index. Our photometric spectral energy distributionprovides no evidence of host-galaxy extinction, requiring either largequantities of grey dust in the host system (at redshift 1.1588 +/- 0.0006,based upon our late-time Keck spectroscopy) or separate physical origins forthe X-ray and optical afterglows.Comment: Revised version submitted to ApJ. Contains significantly expanded discussion, an additional figure, and numerous other change