Axion-photon Propagation in Magnetized Universe

TL;DR

This paper investigates how continuous variations in intergalactic magnetic fields affect axion-photon conversion probabilities, revealing significant differences from previous models and implications for astrophysical constraints on ALPs.

Contribution

It introduces a more realistic model of magnetic field variation along photon paths, showing that conversion probabilities can be much higher and energy-dependent, impacting ALP constraints.

Findings

Conversion probability can reach 100% in realistic magnetic field models.

Energy dependence of conversion probability differs from previous models.

Implications for astrophysical constraints on axion-like particles.

Abstract

Oscillations between photons and axion-like particles (ALP) travelling in intergalactic magnetic fields have been invoked to explain a number of astrophysical phenomena, or used to constrain ALP properties using observations. One example is the anomalous transparency of the universe to TeV gamma-rays. The intergalactic magnetic field is usually modeled as patches of coherent domains, each with a uniform magnetic field, but the field orientation changes randomly from one domain to the next ("discrete-$\varphi$ model"). We show in this paper that in more realistic situations, when the magnetic field direction varies continuously along the propagation path, the photon-to-ALP conversion probability $P$ can be significantly different from the discrete-$\varphi$ model. In particular, $P$ has a distinct dependence on the photon energy and ALP mass, and can be as large as 100 percent. This result may affect previous constraints on ALP properties based on ALP-photon propagation in intergalactic magnetic fields.