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

Abstract

GRB 061007 had the brightest optical afterglow observed by UVOT so far and a highly unusual afterglow light curve. It had an usually smooth and steep panchromatic temporal decay, which, from the start of the Swift observations, decayed as a power with a slope of alpha = 1.65 +/- 0.02 in the X-ray and UV/optical bands. If we interpret the data in terms of a spherical blastwave, the energy budget that is implied is enormous, challenging the possible models for the progenitors. The alternative explanation is that with GRB 061007 we are witnessing the most collimated outflow ever observed. If this is correct if implies the occurrence of a jet break within 80 s of the prompt emission. This event has therefore remarkable consequences for the GRB communicate, since it may imply that jet breaks can occur much earlier in the afterglow light curves than hitherto expected.