Impact of oscillations of photons into axion-like particles on the very-high energy gamma-ray spectrum of the blazar PKS1424+240

TL;DR

This paper investigates how photon-ALP oscillations could affect the very-high energy gamma-ray spectrum of the distant blazar PKS1424+240, potentially reducing universe opacity and altering observed spectra.

Contribution

It demonstrates the impact of photon-ALP conversions on the VHE gamma-ray spectrum of PKS1424+240 and derives photon-ALP couplings under specific magnetic field and EBL models.

Findings

Photon-ALP oscillations can lead to a less attenuated gamma-ray spectrum.

A lower limit on the blazar's redshift is established at z ≥ 0.6035.

Derived photon-ALP couplings suggest possible spectral modifications.

Abstract

Very high energy (VHE) gamma-rays undergo pair production with low energy photons of background radiation fields. This leads to an attenuation of the primary gamma-ray flux of extragalactic sources in the interaction with the extragalactic background light (EBL) which stretches from ultraviolet to far-infrared wavelengths. In the presence of magnetic fields, gamma-rays could oscillate into hypothetical axion-like particles (ALPs). This might lead to a reduced opacity of the Universe for VHE gamma-rays, as ALPs circumvent pair production. Here, the impact of photon-ALP conversions on the spectrum of PKS1424+240 is demonstrated. A lower limit on the redshift of this blazar was recently determined to be $z \geqslant 0.6035$, making it the farthest source ever observed at VHE energies. Under the assumption of a specific magnetic field scenario and EBL model, photon-ALP couplings are derived that lead to an overall concave intrinsic blazar spectrum.