Extragalactic photon-ALP conversion at CTA energies

TL;DR

This paper develops a detailed statistical model for photon-ALP conversion in extragalactic magnetic fields at CTA energies, accounting for different regimes of oscillation and magnetic field coherence, with implications for gamma-ray observations.

Contribution

It derives exact and analytical evolution equations for photon-ALP transfer functions in non-adiabatic and quasi-adiabatic regimes, advancing understanding of photon-ALP mixing in astrophysical contexts.

Findings

Derived exact statistical evolution equations for non-adiabatic regime.

Provided analytical predictions for quasi-adiabatic regime.

Results are relevant for CTA observations and models with non-zero ALP masses.

Abstract

Magnetic fields in extragalactic space between galaxy clusters may induce conversions between photons and axion-like particles (ALPs), thereby shielding the photons from absorption on the extragalactic background light. For TeV gamma rays, the oscillation length ($l_{\rm osc}$) of the photon-ALP system becomes inevitably of the same order as the coherence length of the magnetic field ($l$) and the length over which the field changes significantly (transition length $l_{\rm t}$) due to refraction on background photons. We derive exact statistical evolution equations for the mean and variance of the photon and ALP transfer functions in the non-adiabatic regime ($l_{\rm osc} \sim l \gg l_{\rm t}$). We also make analytical predictions for the transfer functions in the quasi-adiabatic regime ($l_{\rm osc} \ll l, l_{\rm t}$). Our results are important in light of the upcoming Cherenkov Telescope Array (CTA), and may also be applied to models with non-zero ALP masses.