Extreme properties of GRB 061007: A highly energetic or a highly collimated burst?

Abstract

GRB 061007 was the brightest gamma-ray burst (GRB) to be detected by Swift and was accompanied by an exceptionally luminous afterglow that had a V-band magnitude <11.1 at 80 s after the prompt emission. From the start of the Swift observations the afterglow decayed as a power law with a slope of α<inf>X</inf> = 1.66 ± 0.01 in the X-ray and α<inf>opt</inf> = 1.64 ± 0.01 in the UV/optical, up to the point that it was no longer detected above background in the optical or X-ray bands. The brightness of this GRB and the similarity in the decay rate of the X-ray, optical and γ-ray emission from 100 s after the trigger distinguish this burst from others and present a challenge to the fireball model. The lack of a cooling or jet break in the afterglow up to ∼10⁵ s constrains any model that can produce the large luminosity observed in GRB 061007, which we found to require either an excessively large kinetic energy or highly collimated outflow. Analysis of the multiwavelength spectral and high-resolution temporal data taken with Swift suggests an early time jet break to be a more plausible scenario than a highly energetic GRB. This must have occurred within 80 s of the prompt emission, which places an upper limit on the jet opening angle of θ<inf>j</inf> = 0.8°. Such a highly collimated outflow resolves the energy budget problem presented in a spherical emission model, reducing the isotropic-equivalent energy of this burst to E^corr<inf>γ</inf> = 10⁵⁰ erg, consistent with other GRBs. © 2007 RAS.