Evidence for an axion-like particle from PKS 1222+216?

The surprising discovery by Major Atmospheric Gamma Imaging Cherenkov Telescope (MAGIC) of an intense, rapidly varying emission in the energy range 70-400 GeV from the flat spectrum radio quasar PKS 1222+216 represents a challenge for all interpretative scenarios. Indeed, in order to avoid absorption of γ rays in the dense ultraviolet radiation field of the broad line region, one is forced to invoke some unconventional astrophysical picture, like for instance the existence of a very compact (r∼1014cm) emitting blob at a large distance (R∼1018cm) from the jet base. We offer the investigation of a scenario based on the standard blazar model for PKS 1222+216 where γ rays are produced close to the central engine, but we add the new assumption that inside the source photons can oscillate into axion-like particles (ALPs), which are a generic prediction of several extensions of the Standard Model of elementary particle interactions. As a result, a considerable fraction of very-high-energy photons can escape absorption from the broad line region through the mechanism of photon-ALP oscillations much in the same way as they largely avoid absorption from extragalactic background light when propagating over cosmic distances in the presence of large-scale magnetic fields in the nG range. In addition we show that the above Major Atmospheric Gamma Imaging Cherenkov Telescope (MAGIC) observations and the simultaneous Fermi/Large Area Telescope (LAT) observations in the energy range 0.3-3 GeV can both be explained by a standard spectral energy distribution for experimentally allowed values of the model parameters. In particular, we need a very light ALP just like in the case of photon-ALP oscillations in cosmic space. Moreover, we find it quite tantalizing that the most favorable value of the photon-ALP coupling happens to be the same in both situations. Although our ALPs cannot contribute to the cold dark matter, they are a viable candidate for the quintessential dark energy. An astrophysical test of our scenario is proposed and an independent laboratory check for the existence of an ALP with the properties required by our picture will be performed with the planned upgrade of the photon regeneration experiment ALPS at Deutsches Elektronen-Synchrotron (DESY) and with the next generation of solar axion detectors like International Axion Observatory (IAXO). © 2012 American Physical Society.