Polarization evolution accompanying the very early sharp decline of gamma-ray burst X-ray afterglows

Abstract

In the synchrotron radiation model, the polarization property depends on both the configuration of the magnetic field and the geometry of the visible emitting region. Some peculiar behaviours in the X-ray afterglows of gamma-ray bursts (GRBs) observed with Swift, such as energetic flares and a plateau followed by a sharp drop, might be highly linearly polarized because the outflows powering these behaviours may be dominated by Poynting flux. The breakdown of the symmetry of the visible emitting region may also be well hidden in the peculiar X-ray data and may give rise to interesting polarization signatures. In this paper, we focus on the polarization accompanying the very early sharp decline of GRB X-ray afterglows. We show that strong polarization evolution is possible in both the high latitude emission model and the dying central engine model, which are used to interpret this sharp X-ray decline. It is thus not easy to efficiently probe the physical origin of the very early X-ray sharp decline with future polarimetry. Strong polarization evolution is also possible in the decline phase of X-ray flares and in the shallow decline phase of X-ray light curves characterized by chromatic X-ray versus optical breaks. A detector such as the X-ray Telescope (XRT), but with polarization capability, on board a satellite like Swift would be suitable for testing our predictions.