Relevance of axion-like particles for very-high-energy astrophysics

TL;DR

This paper explores how axion-like particles (ALPs) could influence very-high-energy astrophysics by affecting photon spectra from blazars, potentially reducing cosmic dimming effects and explaining spectral variations.

Contribution

It demonstrates that ALPs lighter than 5 x 10^{-10} eV can increase photon survival probabilities, offering a testable prediction for upcoming gamma-ray observatories.

Findings

Photon-ALP oscillations can make blazar spectra harder at high energies.

The spread in observed spectral indices is mainly due to source distance variations.

ALPs could explain the reduced dimming of VHE photons from distant sources.

Abstract

Many extensions of the Standard Model predict the existence of ALPs, which are very light spin-zero bosons with a two-photon coupling. Photon-ALP oscillations occur in the presence of an external magnetic field, and ALPs can lead to observable effects on the measured photon spectrum of astrophysical sources. An intriguing situation arises when blazars are observed with the Cherenkov Telescopes H.E.S.S., MAGIC, CANGAROO III and VERITAS. The extragalactic background light (EBL) produced by galaxies during cosmic evolution gives rise to a source dimming which becomes important in the VHE band. This dimming can be considerably reduced by photon-ALP oscillations in the large-scale magnetic fields, and the resulting blazar spectra become harder than expected. We find that for ALPs lighter than 5 x 10^{-10} eV the photon survival probability is larger than predicted by conventional physics above a few hundred GeV. This is a clear-cut prediction which can be tested with the planned Cherenkov Telescope Array and HAWC. Moreover, we offer a new interpretation of the VHE blazars detected so far, according to which the large spread in the values of the observed spectral index is mainly due to the wide spread in the source distances rather than to large variations of their internal physical properties.