X-Ray and Gamma-Ray Polarization in Leptonic and Hadronic Jet Models of Blazars

TL;DR

This paper compares X-ray and gamma-ray polarization signatures in leptonic and hadronic blazar models, providing predictions to help distinguish between these models using current and future polarimetric observations.

Contribution

It offers detailed theoretical predictions of polarization degrees in blazar models, aiding observational efforts to differentiate leptonic and hadronic emission mechanisms.

Findings

Hadronic models show higher polarization in low-synchrotron-peaked blazars.

Leptonic and hadronic models predict similar X-ray polarization in high-synchrotron-peaked blazars.

Hadronic models predict higher gamma-ray polarization than leptonic models.

Abstract

We present a theoretical analysis of the expected X-ray and gamma-ray polarization signatures resulting from synchrotron self-Compton emission in leptonic models, compared to the polarization signatures from proton synchrotron and cascade synchrotron emission in hadronic models for blazars. Source parameters resulting from detailed spectral-energy-distribution modeling are used to calculate photon-energy-dependent upper limits on the degree of polarization, assuming a perfectly organized, mono-directional magnetic field. In low-synchrotron-peaked blazars, hadronic models exhibit substantially higher maximum degrees of X-ray and gamma-ray polarization than leptonic models, which may be within reach for existing X-ray and gamma-ray polarimeters. In high-synchrotron-peaked blazars (with electron-synchrotron-dominated X-ray emission), leptonic and hadronic models predict the same degree of X-ray polarization, but substantially higher maximum gamma-ray polarization in hadronic models than leptonic ones. These predictions are particularly relevant in view of the new generation of balloon-borne X-ray polarimeters (and possibly GEMS, if revived), and the ability of Fermi-LAT to measure gamma-ray polarization at < 200 MeV. We suggest observational strategies combining optical, X-ray, gamma-ray polarimetry to determine the degree of ordering of the magnetic field and to distinguish between leptonic and hadronic high-energy emission.