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

نویسنده

  • D. A. Perley
چکیده

We report on observations of gamma-ray burst (GRB061126) 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 intermediateto 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 GRB061126 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. Subject headings: gamma rays: bursts, gamma-ray bursts: individual: 061126

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تاریخ انتشار 2007