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Gamma-ray spectral modulations induced by photon-ALP-dark photon oscillations

Recently it has been noticed that the Fermi-LAT data of gamma-rays from some galactic pulsars and supernova remnants reveal spectral modulations that might be explained by the conversion of photons to ALPs (axion-like particles) induced by the conventional ALP coupling to photon in the presence of galactic magnetic fields. However the corresponding ALP mass and coupling are in a severe tension with the observational constraints from CAST, SN1987A, and other gamma-ray observations. Motivated by this, we examine an alternative possibility that those spectral modulations are explained by other type of ALP coupling involving both the ordinary photon and a massless dark photon, when nonzero background dark photon gauge fields are assumed. We find that our scheme results in oscillations among the photon, ALP, and dark photon, which can explain the gamma-ray spectral modulations of galactic pulsars or supernova remnants, while satisfying the known observational constraints.