No evidence for axions from Chandra observation of magnetic white dwarf

Understand

Ultralight axions with axion-photon couplings $g_{a\gamma\gamma} \sim {\rm few} \times 10^{-11}$ GeV$^{-1}$ may resolve a number of astrophysical anomalies, such as unexpected ~TeV transparency, anomalous stellar cooling, and X-ray excesses from nearby neutron stars.

• We show, however, that such axions are severely constrained by the non-observation of X-rays from the magnetic white dwarf (MWD) RE J0317-853 using ~40 ks of data acquired from a dedicated observation with the Chandra X-ray Observatory.
• Axions may be produced in the core of the MWD through electron bremsstrahlung and then convert to X-rays in the magnetosphere.
• The non-observation of X-rays constrains the axion-photon coupling to $g_{a\gamma\gamma} \lesssim 5.5 \times 10^{-13} \sqrt{C_{a\gamma\gamma}/C_{aee}}$ GeV$^{-1}$ at 95% confidence for axion masses $m_a \lesssim 5 \times 10^{-6}$ eV, with $C_{aee}$ and $C_{a\gamma\gamma}$ the dimensionless coupling constants to electrons and photons.