First lower limits on the photon-axion-like particle coupling from very high energy gamma-ray observation

TL;DR

This study uses very high energy gamma-ray observations to establish the first lower bounds on the photon-axion-like particle coupling, suggesting potential new physics and guiding future experimental searches.

Contribution

It provides the first lower limits on the photon-ALP coupling constant based on VHE gamma-ray spectra, considering various magnetic field scenarios and energy dependencies.

Findings

Lower limit of ~10^{-12} GeV^{-1} for optimistic magnetic field models.

More conservative models yield a lower limit of 2×10^{-11} GeV^{-1}.

Results are within reach of future ALP detection experiments.

Abstract

The intrinsic flux of very high energy (VHE, Energy > 100 GeV) gamma-rays from extragalactic sources is attenuated due to pair production in the interaction with photons of the extragalactic background light (EBL). Depending on the distance of the source, the Universe should be opaque to VHE photons above a certain energy. However, indications exist that the Universe is more transparent than previously thought. A recent statistical analysis of a large sample of VHE spectra shows that the correction for absorption with current EBL models is too strong for the data points with the highest attenuation. An explanation might be the oscillation of VHE photons into hypothetical axion-like particles (ALPs) in ambient magnetic fields. This mechanism would decrease the opacity as ALPs propagate unimpeded over cosmological distances. Here, a large sample of VHE gamma-ray spectra obtained with imaging air Cherenkov telescopes is used to set, for the first time, lower limits on the photon-ALP coupling constant over a large range of ALP masses. The conversion in different magnetic field configurations, including intra-cluster and intergalactic magnetic fields together with the magnetic field of the Milky Way, is investigated taking into account the energy dependence of the oscillations. For optimistic scenarios of the intervening magnetic fields, a lower limit on the photon-ALP coupling of the order of $10^{-12}\mathrm{GeV}^{-1}$ is obtained whereas more conservative model assumptions result in $2\times10^{-11}\mathrm{GeV}^{-1}$. The latter value is within reach of future dedicated ALP searches.