UCL Discovery

Abstract

There is an increasing sample of Swift GRBs that manifest evidence of chromatic breaks, i.e. breaks that are present in the X-ray but absent 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 the same component responsible for the optical emission. We propose that these afterglow lightcurves are produced by a two-component jet model, in which both components are energy-injected for the whole observation and the X-ray break is a jet break in the narrow outflow. This study has important consequences for the GRB community, since bursts with chromatic breaks provide us with an explanation for another surprising finding, the paucity of achromatic breaks at late times. We show that our model can explain the behaviour of both X-ray and optical band emissions and can therefore be a noteworthy and radical alternative to the current interpretation for the canonical X-ray light curve, with fundamental implications for GRB collimation.