Extreme properties of GRB061007: a highly energetic or a highly collimated burst?

GRB061007 was the brightest gamma-ray burst (GRB) to be detected by Swift and was accompaniedbyanexceptionallyluminousafterglowthathada V-bandmagnitude <11.1at80s after the prompt emission. From the start of the Swift observations the afterglow decayed as a power law with a slope of αX = 1.66 ± 0.01 in the X-ray and αopt = 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 100s 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 GRB061007, 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 80s of the prompt emission, which places an upper limit on the jet opening angle of θj = 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 Ecorr γ = 1050 erg, consistent with other GRBs.