Probing Axions with Relativistic Jet Polarimetry

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 $g_{a γ} \sim 5 \times 10^{-15} - 5 \times 10^{-14} GeV^{-1}$, 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.

Figures (13)

• Figure 1: Relationship between axion mass and soliton core radius.
• Figure 2: Density profile for dark matter with axion mass of $10^{-21} \rm eV.$ White circle highlights the radius of the soliton core of which, density falls off as NFW profile outside this. The black hole would lie in the center of the soliton with the soliton being much larger than the black hole and extension of the jet.
• Figure 3: Synthetic Stokes maps for $m_a = 5 \times 10^{-21} eV$ and $g_{a \gamma} = 1.786 \times 10^{-14} GeV^{-1}$. The red EVPA ticks correspond to the EVPA angle without axions while the cyan-colored ticks correspond to the EVPA angle with axions. This example shows clearly the difference in EVPA angle when the jet is surrounded by the axion cloud vs. when it is not.
• Figure 4: Example stokes map for $m_a = 10^{-22} eV$ and $g_{a \gamma} = 5 \times 10^{-14} GeV^{-1}$. The example is meant to highlight the weakness of the signal in comparison for this mass despite the strongest coupling in our dataset being used.
• Figure 5: Histogram and PDF distributions for measured $\Delta \chi$ for each mass. Horizontal lines highlight the mean EVPA change with directionality taken into account, and the mean magnitude of EVPA change.