  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 B0531+21, PSR B0656+14, and 4U
0142+61, we derive new strong constraints on the axion-photon coupling in a
broad axion mass range $10^{-11}\lesssim m_a \lesssim 10^{-3}$ eV.
