Jet breaks at the end of the plateau phase of Swift GRB lightcurves

Abstract

An increasing sample of Swift GRBs show evidence of 'chromatic breaks', i.e. breaks that are present in the X-ray but not in the optical. Modelling the X-ray and optical data, we find that in a significant fraction of these GRB afterglows the component producing the X-ray emission cannot be responsible for the optical emission too. We propose that these afterglow lightcurves are the result of a two-component jet, in which both components are energy-injected for the whole observation and the X-ray break is due to a jet break in the narrow outflow. Bursts with chromatic breaks provide us with an explanation for another surprising finding, the paucity of late achromatic breaks. We show that our model may explain the behaviour of GRB emission in both X-ray and optical bands. Therefore, it can be a radical and noteworthy alternative to the current interpretation for the 'canonical' XRT and UVOT lightcurves, bearing fundamental implications for GRB physics.