Using eclipse attenuation in the LMC X-4 neutron star binary, the authors set g_aγ ≤ 1.44 × 10^{-10} GeV^{-1} at 90% CL, surpassing current light-shining-through-walls bounds.
Photon-photon dispersion of TeV gamma rays and its role for photon-ALP conversion
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abstract
The propagation of TeV gamma rays can be strongly modified by B-field induced conversion to axionlike particles (ALPs). We show that, at such high energies, photon dispersion is dominated by background photons - the only example where photon-photon dispersion is of practical relevance. We determine the refractive index for all energies and find that, for fixed energy density, background photons below the pair-production threshold dominate. The cosmic microwave background alone provides an "effective photon mass" of (m_gamma)^2 = -(1.01 neV * E/TeV)^2 for E < 1000 TeV. The extragalactic background light is subdominant, but local radiation fields in the galaxy or the source regions provide significant contributions. Photon-photon dispersion is small enough to leave typical scenarios of photon-ALP oscillations unscathed, but big enough to worry about it case by case.
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Neutron Star Eclipses as Axion Laboratories
Using eclipse attenuation in the LMC X-4 neutron star binary, the authors set g_aγ ≤ 1.44 × 10^{-10} GeV^{-1} at 90% CL, surpassing current light-shining-through-walls bounds.