Axion-like particles explain the unphysical redshift-dependence of AGN gamma-ray spectra

Giorgio Galanti, Marco Roncadelli, Alessandro De Angelis, Giovanni F. Bignami

Published 2015-03-15Version 1

Blazars are a class of AGN known to be powerful very-high-energy (VHE, 100 GeV - 100 TeV) celestial gamma-ray emitters. At the time of writing, 41 blazars, spread all over the sky and with known redshift in the range $0.0215 \leq z \leq 0.635$ have been observed in the VHE band by the Imaging Atmospheric Cherenkov Telescopes H.E.S.S., MAGIC and VERITAS. Thus, they represent an isotropic and relatively local extragalactic sample, unaffected by significant cosmological evolution. The blazar emitted spectra are well fitted by a power law with index $\Gamma_{\rm em}$. We show that the $\Gamma_{\rm em}$ distribution exhibits an unexpected and previously unnoticed unphysical redshift-dependence. We demonstrate that this result is not due to any selection effect. It is difficult to imagine an intrinsic mechanism which could lead to such a spectral variation, and so this result seriously challenges the conventional view. We propose that such a behaviour is explained by oscillations between the VHE gamma-rays and Axion-Like Particles (ALPs), taking place in extragalactic magnetic fields. We recall that ALPs are predicted by several extensions of the Standard Model and especially by those based on superstring theories. Moreover, they are attracting growing interest being also good candidates for cold dark matter. As a consequence of the photon-ALP oscillation mechanism, the $\Gamma_{\rm em}$ distribution becomes redshift-independent, indeed in agreement with the physical expectation. This is a highly nontrivial fact, which therefore provides a preliminary evidence for the existence of ALPs. Thus, besides physics laboratory data, astrophysical VHE data from e.g. the upcoming CTA can settle this issue. Our Universe may in this way be offering us a compelling reason to push physics beyond the Standard Model along a very specific direction and can shed light on the nature of cold dark matter.