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

The intrinsic flux of very high-energy (VHE, energy 100 GeV) γ 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 axionlike 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 γ-ray spectra obtained with imaging air Cherenkov telescopes is used to set, for the first time, lower limits on the photon-ALP coupling constant g_aγ 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 g_aγ of the order of 10^-12 GeV^-1 is obtained, whereas more conservative model assumptions result in g_aγ2×10^-11 GeV^-1. The latter value is within reach of future dedicated ALP searches.