High-Energy Polarization: Scientific Potential and Model Predictions

Haocheng Zhang

doi:10.3390/galaxies5030032

Abstract

Understanding magnetic field strength and morphology is very important for studying astrophysical jets. Polarization signatures have been a standard way to probe the jet magnetic field. Radio and optical polarization monitoring programs have been very successful in studying the space- and time-dependent jet polarization behaviors. A new era is now arriving with high-energy polarimetry. X-ray and γ -ray polarimetry can probe the most active jet regions with the most efficient particle acceleration. This new opportunity will make a strong impact on our current understanding of jet systems. This paper summarizes the scientific potential and current model predictions for X-ray and γ -ray polarization of astrophysical jets. In particular, we discuss the advantages of using high-energy polarimetry to constrain several important problems in the jet physics, including the jet radiation mechanisms, particle acceleration mechanisms, and jet kinetic and magnetic energy composition. Here we take blazars as a study case, but the general approach can be similarly applied to other astrophysical jets. We conclude that by comparing combined magnetohydrodynamics (MHD), particle transport, and polarization-dependent radiation transfer simulations with multi-wavelength time-dependent radiation and polarization observations, we will obtain the strongest constraints and the best knowledge of jet physics.

Highlights

Blazars are the most violent class of active galactic nuclei
Radio to optical polarization measurements have been a standard probe of the jet magnetic field
Recent observations of γ-ray flares with optical polarization angle swings and substantial polarization degree variations indicate the active role of the magnetic field during flares [1,2]

Summary

Introduction

Blazars are the most violent class of active galactic nuclei. To understand blazar jet physics, several key processes need to be studied—namely, the magnetic field evolution, the particle acceleration, and the radiation mechanisms. This implies that the high-energy emission comes from the most active acceleration regions with the most energetic particles. High-energy polarimetry can probe the jet magnetic field, especially in the blazar zone To understand these issues, we need to lay out robust physical modeling. We will discuss the averaged polarization degree from X-ray to γ-ray based on steady-state spectral fitting in Section 2 , time-dependent polarization signatures of shock and magnetic reconnection scenarios based on detailed particle evolution and polarization-dependent radiation transfer, and general trends of polarization signatures in the kinetic-dominated and magnetic-dominated jet emission environment, based on magnetohydrodynamics (MHD)-integrated modeling of polarization signatures We will discuss the averaged polarization degree from X-ray to γ-ray based on steady-state spectral fitting in Section 2 , time-dependent polarization signatures of shock and magnetic reconnection scenarios based on detailed particle evolution and polarization-dependent radiation transfer in Section 3, and general trends of polarization signatures in the kinetic-dominated and magnetic-dominated jet emission environment, based on magnetohydrodynamics (MHD)-integrated modeling of polarization signatures

High-Energy Polarization Degree of Leptonic and Hadronic Blazar Models

Time-Dependent High-Energy Polarization of Shock and Magnetic Reconnection

Findings

Polarization Signatures of Kinetic-Dominated and Magnetic-Dominated Jets