Polarization Signals from Axion-Photon Resonant Conversion in Neutron Star Magnetosphere

Abstract: Neutron stars provide ideal astrophysical laboratories for probing new physics beyond the Standard Model. If axions exist, photons can develop linear polarization during photon-axion conversion in the magnetic field of a neutron star. We find that the plasma in the neutron star magnetosphere could dramatically enhance the polarization through the resonant conversion effect. With the polarization measurements from PSR B0656+14, 4U 0142+61, and the benchmark polarization measurement in the mid-infrared band, we demonstrate that optical and infrared polarization from neutron stars can provide strong constraints on the axion-photon coupling over a broad axion mass range $10^{-11}\lesssim m_a \lesssim 10^{-3}$ eV.