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      Searching for spectral oscillations due to photon-ALP conversion using the Fermi-LAT observations of bright supernova remnants

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          Abstract

          Axion-like-particles (ALPs) are one promising type of dark matter candidate particles that may generate detectable effects on \(\gamma\)-ray spectra other than the canonical weakly interacting massive particles. In this work we search for such oscillation effects in the spectra of supernova remnants caused by the photon-ALP conversion, using the Fermi Large Area Telescope data. Three bright supernova remnants, IC443, W44, and W51C, are analyzed. The inclusion of photon-ALP oscillations yields an improved fit to the \(\gamma\)-ray spectrum of IC443, which gives a statistical significance of \(4.2\sigma\) in favor of such spectral oscillation. However, the best fit parameters of ALPs (\(m_{a}=6.6\,{\rm neV}\), \(g_{a\gamma}=13.4 \times 10^{-11}\,{\rm GeV}^{-1}\)) are in tension with the upper bound (\(g_{a\gamma}< 6.6 \times 10^{-11}\,{\rm GeV}^{-1}\)) set by the CAST experiment. The systematic uncertainties of the flux measurements are found to be difficult to explain the results. We speculate that the "irregularity" displayed in the spectrum of IC443 may be due to the superposition of the emission from different parts of the remnant.

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

            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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              An Improved RF Cavity Search for Halo Axions

              The axion is a hypothetical elementary particle and cold dark matter candidate. In this RF cavity experiment, halo axions entering a resonant cavity immersed in a static magnetic field convert into microwave photons, with the resulting photons detected by a low-noise receiver. The ADMX Collaboration presents new limits on the axion-to-photon coupling and local axion dark matter halo mass density from a RF cavity axion search in the axion mass range 1.9-2.3 microeV, broadening the search range to 1.9-3.3 microeV. In addition, we report first results from an improved analysis technique.
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                Author and article information

                Journal
                05 January 2018
                Article
                1801.01646

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

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                astro-ph.HE hep-ph

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