X-ray plateaus in $\gamma$-ray bursts explained by structured jets

Abstract

The follow-up of $\gamma$-ray bursts (GRBs) by the X-ray telescope (XRT, 0.3-10 keV) on board the Neil Gehrels Swift Observatory led to the discovery of a shallow decay phase (the so-called plateau) of the X-ray emission in a good fraction of GRBs. This unexpected temporal behaviour does not fit the standard GRB afterglow expectation. Thus, in the last years, many models emerged, that invoke energy injection into the external shock, requiring long-lasting activity of the central engine of GRBs. We discuss a new alternative, comprehensive model: the plateau phase originates from the high latitude emission (i.e., the radiation observed from larger angles relative to the line of sight, after the prompt emission from a curved surface is switched off) when the jet exhibits bulk motion and intensity structure. This model enables us to reproduce not only the temporal behaviour of the X-ray light curves, but also the diversity of joint optical-to-X-ray emission.