Probing Axions with Relativistic Jet Polarimetry

TL;DR

This paper explores how relativistic jet polarimetry, especially observations of M87, can detect axion-like particles through polarization rotation effects, offering a new method to probe dark matter candidates beyond previous accretion disk studies.

Contribution

It introduces a novel approach using jet polarimetry to detect axions, extending beyond prior focus on accretion disk emission and providing diagnostic tools to differentiate axion signals from plasma effects.

Findings

Degree-level EVPA rotations for axion masses around 10^{-21} eV.

Sub-degree rotations for axion masses around 10^{-22} eV.

Diagnostic framework to distinguish axion birefringence from plasma Faraday rotation.

Abstract

The prospect of identifying axion signals due to axion-photon coupling induced changes to the polarization has now become a reality in view of near-horizon polarimetric observations by the Event Horizon Telescope (EHT). Axion-like particles (ALPs), motivated as dark matter candidates by the strong CP problem, induce frequency-independent birefringence in linearly polarized radiation, producing observable rotations of the electric vector position angle. While previous studies have focused exclusively on axion signatures in near-horizon accretion disk emission, the relativistic jet of M87 -- extending from 10 gravitational radii to kiloparsec scales -- remains unexplored as an axion probe despite offering extended path lengths through the putative dark matter distribution. In this study, we investigate the effects of an axion cloud around the jet in M87 on the Stokes maps of relativistic jets using a stationary, axisymmetric, self-similar model for the jet and a coherent, homogeneous soliton core in M87's galactic center for the axion background. At 230 GHz, for representative couplings in range $g_{a \gamma} \sim 5 \times 10^{-15} - 5 \times 10^{-14} GeV^{-1}$, we find that axion masses in the $10^{-21} eV $ range produce degree-level to multi-degree EVPA rotations, in some cases exceeding typical EHT measurement uncertainties, whereas masses in the $10^{-22} eV$ range yield predominantly sub-degree rotations. We identify a suite of morphological diagnostics that together constitute a framework for distinguishing axion-induced birefringence from plasma Faraday rotation in resolved jet polarimetry.