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

We report on observations of gamma-ray burst (GRB 061126) with an extremely bright (R ~ 12 mag at peak) early-time optical afterglow. The optical afterglow is already fading as a power-law 22 seconds after the trigger, with no detectable prompt contribution in our first exposure, which was coincident with a large prompt-emission pulse. The optical-IR photometric SED is an excellent fit to a power-law but exhibits a moderate red-to-blue evolution in the spectral index at about 500 sec. This color change is contemporaneous with a switch from a relatively fast decay to slower decay. The rapidly decaying early afterglow is broadly consistent with synchrotron emission from a reverse shock, but a bright forward shock component predicted by the intermediate- to late-time X-ray observations under the assumptions of standard afterglow models is not observed. Indeed, despite its remarkable early-time brightness this burst would qualify as a dark burst at later times on the basis of its nearly flat optical-to-X-ray spectral index. Our photometric SED provides no evidence of host extinction, requiring either large quantities of grey dust in the host system (at z=1.1588 +/- 0.0006, based upon our late-time Keck spectroscopy) or separate physical origins for the X-ray and optical afterglows. In either case, events like GRB 061126 may represent a significant fraction of observed dark bursts with faint or absent optical afterglows, suggesting a need for redress of the interpretations concerning the origin of these events, and possibly of afterglows in general.