Polarization signals from axion-photon resonant conversion in a neutron star magnetosphere

Neutron stars provide ideal astrophysical laboratories for probing new physics beyond the Standard Model. If axions exist, then 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 <a:math xmlns:a="http://www.w3.org/1998/Math/MathML" display="inline"><a:mrow><a:mi mathvariant="normal">B</a:mi><a:mn>0656</a:mn><a:mo>+</a:mo><a:mn>14</a:mn></a:mrow></a:math>, 4U <d:math xmlns:d="http://www.w3.org/1998/Math/MathML" display="inline"><d:mrow><d:mn>0142</d:mn><d:mo>+</d:mo><d:mn>61</d:mn></d:mrow></d:math>, and the benchmark polarization measurement in the midinfrared 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 <f:math xmlns:f="http://www.w3.org/1998/Math/MathML" display="inline"><f:msup><f:mn>10</f:mn><f:mrow><f:mo>−</f:mo><f:mn>11</f:mn></f:mrow></f:msup><f:mo>≲</f:mo><f:msub><f:mi>m</f:mi><f:mi>a</f:mi></f:msub><f:mo>≲</f:mo><f:msup><f:mn>10</f:mn><f:mrow><f:mo>−</f:mo><f:mn>3</f:mn></f:mrow></f:msup><f:mtext> </f:mtext><f:mtext> </f:mtext><f:mi>eV</f:mi></f:math>.