Investigating the light curve variation of magnetic white dwarfs induced by axion-photon conversion

Axion-photon oscillation refers to the process of mutual conversion between photons and axions when they propagate in a magnetic field. This process depends on the strength of the background magnetic field, and magnetic white dwarfs provide a natural laboratory for testing this process. In this work, we study the behavior of axion-photon oscillation near magnetic white dwarfs: as the magnetic white dwarf rotates, its magnetic field structure rotates accordingly, causing a periodic change of the magnetic field along the path of photons. These variations affect the axion-photon oscillation process experienced by the photons emitted from the white dwarf, thereby inducing a periodic modulation in the intensity and polarization of the white dwarf’s thermal emission that we observe. Our study focuses on the impact of axion effects on the observed light curve variation and conducts a detailed investigation through numerical calculations. Using the light curve data of the white dwarf <a:math xmlns:a="http://www.w3.org/1998/Math/MathML" display="inline"><a:mrow><a:mi>PG</a:mi><a:mn>1015</a:mn><a:mo>+</a:mo><a:mn>014</a:mn></a:mrow></a:math> obtained from the observations by the Jacobus Kapteyn Telescope, which has a photometric precision of <c:math xmlns:c="http://www.w3.org/1998/Math/MathML" display="inline"><c:mo>∼</c:mo><c:mn>1</c:mn><c:mo>%</c:mo></c:math>, we derive the constraints on axion parameters. In the axion mass range of <e:math xmlns:e="http://www.w3.org/1998/Math/MathML" display="inline"><e:mo>≲</e:mo><e:msup><e:mn>10</e:mn><e:mrow><e:mo>−</e:mo><e:mn>8</e:mn></e:mrow></e:msup><e:mtext> </e:mtext><e:mtext> </e:mtext><e:mi>eV</e:mi></e:math>, the 95% credible interval upper limit of the axion-photon coupling <g:math xmlns:g="http://www.w3.org/1998/Math/MathML" display="inline"><g:msub><g:mi>g</g:mi><g:mrow><g:mi>a</g:mi><g:mi>γ</g:mi><g:mi>γ</g:mi></g:mrow></g:msub></g:math> is constrained to <i:math xmlns:i="http://www.w3.org/1998/Math/MathML" display="inline"><i:mrow><i:mo>&lt;</i:mo><i:mn>8.1</i:mn><i:mo>×</i:mo><i:msup><i:mrow><i:mn>10</i:mn></i:mrow><i:mrow><i:mo>−</i:mo><i:mn>12</i:mn></i:mrow></i:msup><i:mtext> </i:mtext><i:mtext> </i:mtext><i:mrow><i:msup><i:mrow><i:mi>GeV</i:mi></i:mrow><i:mrow><i:mo>−</i:mo><i:mn>1</i:mn></i:mrow></i:msup></i:mrow></i:mrow></i:math>.