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Upper limit on the axion-photon coupling from magnetic white dwarf polarization
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abstract
Polarization measurements of thermal radiation from magnetic white dwarf (MWD) stars have been proposed as a probe of axion-photon mixing. The radiation leaving the surface of the MWD is unpolarized, but if low-mass axions exist then photons polarized parallel to the direction of the MWD's magnetic field may convert into axions, which induces a linear polarization dependent on the strength of the axion-photon coupling $g_{a\gamma\gamma}$. We model this process by using the formalism of axion-photon mixing in the presence of strong-field vacuum birefringence to show that of all stellar types MWDs are the most promising targets for axion-induced polarization searches. We then consider linear polarization data from multiple MWDs, including SDSS J135141 and Grw+70$^\circ$8247, to show that after rigorously accounting for astrophysical uncertainties the axion-photon coupling is constrained to $|g_{a\gamma\gamma}| \lesssim 5.4 \times 10^{-12}$ GeV$^{-1}$ at 95% confidence for axion masses $m_a \lesssim 3 \times 10^{-7}$ eV. This upper limit puts in tension the previously-suggested explanation of the anomalous transparency of the Universe to TeV gamma-rays in terms of axions. We identify MWD targets for which future data and modeling efforts could further improve the sensitivity to axions.
Forward citations
Cited by 6 Pith papers
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Resonant axion and dark photon production in magnetic white dwarfs
Magnetic fields broaden the resonant conversion of photons to axions and dark photons inside magnetic white dwarfs, enabling emission where none occurred before.
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Gravitational Wave Scattering on Magnetic Fields
In a 3D treatment of the inverse Gertsenshtein effect, an isotropic unpolarized gravitational wave background scattering off a dipolar magnetic field yields partially linearly polarized radio emission, with peak inten...
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Time-Domain Axion Searches with Magnetic White Dwarfs
The apparent axion-photon conversion signal in PG 1015+014 TESS data is absorbed by the second harmonic of the star's intrinsic light curve, yielding no evidence for axions and a conservative coupling limit.
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Axiverse Lampposts
In a hierarchical multi-axion theory with random couplings, axion field ranges shrink with 1/sqrt(N), generic axion–SM couplings are suppressed, but the QCD axion's coupling is unsuppressed.
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Axion lines from nuclear de-excitations in galactic stellar populations
No 14.4 keV axion line from 57Fe de-excitations is seen by NuSTAR toward M87, M82, M31, or the Galactic Center, yielding |g_ann x g_aγγ| < 1.1e-22 GeV^-1 for m_a < 1e-10 eV.
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Circular polarization effects induced by photon-axion mixing in astrophysical environments
The paper derives analytic circular-polarization signals from photon-axion mixing and uses the blazar S4 0954+65 optical circular-polarization limit to bound g_aγγ around 10^-12 to 10^-11 GeV^-1 for ultralight axion masses.
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