31 - Central engine afterglow from GRBs and the polarization signature

Summary

There are two kinds of gamma-ray burst (GRB) afterglows. One is the fireball afterglow that is the radiation of the external shock(s) driven by the GRB remnant. The other is the emission from the late ejecta launched by the prolonged activity of the central engine, i.e. the central engine afterglow. The former seems to be only weakly polarized and thus is not suitable for the upcoming X-ray polarimetry. For the latter, the polarization property is less clear. Some central engine afterglows, such as energetic flares and the plateau followed by a sharp drop, might be highly polarized because the outflows powering these behaviors may be Poynting-flux dominated. Furthermore, the breakdown of the symmetry of the visible emitting region may be hiding in some X-ray data and will give rise to interesting polarization signatures. For example, in the high latitude emission model for the sharp X-ray decline strong polarization evolution is possible. An XRT-like detector but with polarization capability on board a Swift-like satellite would be suitable to detect these possible signals.

The central engine afterglow

In the context of the standard fireball model of GRBs, the prompt γ-rays and the afterglow emission are powered by internal shocks and external shocks, respectively (see for a review). Before 2004, most of the afterglow data were collected hours after the prompt γ-ray emission and were found to be consistent with the external forward shock model, though at times energy injection, a wind medium profile, or a structured/patchy jet were needed.