Polarization of impulsive relativistic jets propagating in a stratified medium

Abstract

Gamma-ray bursts (GRBs) and at least some tidal disruption events (TDEs) are highly energetic transients involving impulsive relativistic jets. As these jets propagate into their surrounding media, they drive a relativistic forward shock into it, which radiates multi-wavelength polarized synchrotron emission, known as an afterglow. Analyzing the afterglow light curve along with its polarization curve can shed light on the geometrical properties of the GRB or TDE system such as the magnetic field structure behind the shock, the jet angular structure and even distinguish between proposed scenarios of delayed radio emission seen in some TDEs. While most afterglow polarization measurements are for long GRBs, whose massive star progenitor winds are expected to form a stratified external medium, the polarization from such environments is largely unexplored. Similarly, stratified media surround jetted TDEs. In this work we explore how shock propagation into a stratified medium affects the observed afterglow polarization from impulsive relativistic jets. We find that for both top hat and structured jets, in most cases the polarization levels are reduced and its temporal signature evolves on longer time scales compared to a uniform external medium. The polarization peaks at times close to a geometrical break in the light curve, indicating measurements during this time probe the maximal levels of polarization possible for the system. In addition, observationally capturing the polarization evolution time scales can assist in constraining our viewing angle and the external medium stratification. Such composite models, that account for more realistic systems that are motivated by light curve fittings, will allow us to complement afterglow light curves better and promote joint modeling of these observables.