Magnetohydrodynamics predicts heavy-tailed distributions of axion-photon conversion

TL;DR

This study compares magnetohydrodynamic (MHD) simulated magnetic fields with Gaussian random fields to understand their impact on axion-photon conversion, revealing heavy-tailed distributions in realistic turbulent environments.

Contribution

First systematic analysis of ALP-photon conversion in realistic MHD magnetic fields, highlighting deviations from Gaussian models and implications for astrophysical ALP searches.

Findings

MHD models follow exponential distribution for typical conversions

Heavy tails in MHD models indicate rare large conversions

Non-Gaussian features cause deviations from simple models

Abstract

The interconversion of axionlike particles (ALPs) and photons in magnetised astrophysical environments provides a promising route to search for ALPs. The strongest limits to date on light ALPs use galaxy clusters as ALP-photon converters. However, such studies traditionally rely on simple models of the cluster magnetic fields, with the state-of-the-art being Gaussian random fields (GRFs). We present the first systematic study of ALP-photon conversion in more realistic, turbulent fields from dedicated magnetohydrodynamic (MHD) simulations, which we compare with GRF models. For GRFs, we analytically derive the distribution of conversion ratios at fixed energy and find that it follows an exponential law. We find that the MHD models agree with the exponential law for typical, small-amplitude mixings but exhibit distinctly heavy tails for rare and large mixings. We explain how non-Gaussian features, e.g.~coherent structures and local spikes in the MHD magnetic field, are responsible for the heavy tail. Our results suggest that limits placed on ALPs using GRFs are robust.