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

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          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.

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          Mixing of the photon with low-mass particles

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            Evidence for a new light spin-zero boson from cosmological gamma-ray propagation?

            Recent findings by Imaging Atmospheric Cherenkov Telescopes indicate a large transparency of the Universe to gamma rays, which can be hardly explained within the current models of extragalactic background light. We show that the observed transparency is naturally produced by an oscillation mechanism -- which can occur inside intergalactic magnetic fields -- whereby a photon can become a new spin-zero boson with mass m << 10^(-10) eV. Because the latter particle travels unimpeded throughout the Universe, photons can reach the observer even if the distance from the source considerably exceeds their mean free path. We compute the expected flux of gamma rays from blazar 3C279 at different energies. Our predictions can be tested in the near future by the gamma-ray telescopes H.E.S.S., MAGIC, CANGAROO and VERITAS. Moreover, our result provides an important observational test for models of dark energy wherein quintessence is coupled to the photon through an effective dimension-five operator.
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              The Milky Way as a Kiloparsec-Scale Axionscope

              Very high energy gamma-rays are expected to be absorbed by the extragalactic background light over cosmological distances via the process of electron-positron pair production. Recent observations of cosmologically distant gamma-ray emitters by ground based gamma-ray telescopes have, however, revealed a surprising degree of transparency of the universe to very high energy photons. One possible mechanism to explain this observation is the oscillation between photons and axion-like-particles (ALPs). Here we explore this possibility further, focusing on photon-ALP conversion in the magnetic fields in and around gamma-ray sources and in the magnetic field of the Milky Way, where some fraction of the ALP flux is converted back into photons. We show that this mechanism can be efficient in allowed regions of the ALP parameter space, as well as in typical configurations of the Galactic Magnetic Field. As case examples, we consider the spectrum observed from two HESS sources: 1ES1101-232 at redshift z=0.186 and H 2356-309 at z=0.165. We also discuss features of this scenario which could be used to distinguish it from standard or other exotic models.
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                Author and article information

                Journal
                05 February 2013
                2013-08-26
                Article
                10.1103/PhysRevD.87.035027
                1302.1208
                2ea30263-96e4-4fda-a277-adab276ccf7f

                http://arxiv.org/licenses/nonexclusive-distrib/1.0/

                History
                Custom metadata
                Phys. Rev. D 87, 035027 (2013)
                Figures and references updated, typos in formulas corrected
                astro-ph.HE hep-ph

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