The remarkable afterglow of GRB 061007: Implications for optical flashes and GRB fireballs

We present a multiwavelength analysis of Swift GRB 061007. The 2 m robotic Faulkes Telescope South began observing 137 s after the onset of the γ-ray emission, when the optical counterpart was already decaying from R ∼ 10.3 mag, and continued observing for the next 5.5 hr. These observations begin during the final γ-ray flare and continue through and beyond a long, soft tail of γ-ray emission whose flux shows an underlying simple power-law decay identical to that seen at optical and X-ray wavelengths, with temporal slope α ∼ 1.7 (F ∝ t^-α). This remarkably simple decay in all of these bands is rare for Swift bursts, which often show much more complex light curves. We suggest the afterglow emission begins as early as 30-100 s and is contemporaneous with the ongoing variable prompt emission from the central engine, but originates from a physically distinct region dominated by the forward shock. The observed multiwavelength evolution of GRB 061007 is explained by an expanding fireball whose optical, X-ray, and late-time γ-ray emission is dominated by emission from a forward shock with typical synchrotron frequency, ν_m, that is already below the optical band as early as t = 137 s and a cooling frequency, ν_c, above the X-ray band to at least t = 10⁵ s. In contrast, the typical frequency of the reverse shock lies in the radio band at early time. We suggest that the unexpected lack of bright optical flashes from the majority of Swift GRBs may be explained with a low ν_m originating from small microphysics parameters, ε_e and ε_B. Finally, the optical light curves imply a minimum jet opening angle θ = 4.7°, and no X-ray jet break before t ∼ 10⁶ s makes GRB 061007 a secure outlier to spectral energy correlations.