Polarization evolution accompanying the very early sharp decline of GRB 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 behaviors in the X-ray afterglows of {\it Swift} gamma-ray bursts (GRBs), such as energetic flares and the plateau followed by a sharp drop, might by highly linearly-polarized because the outflows powering these behaviors may be Poynting-flux dominated. Furthermore, the broken-down of the symmetry of the visible emitting region may be hiding in current X-ray data and will give rise to interesting polarization signatures. In this work 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 VS. Optical breaks. An {\it XRT}-like detector but with polarization capability on board a {\em Swift}-like satellite would be suitable to test our predictions.