http://arxiv.org/abs/2104.12772
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. Considering that $C_{aee}$ is generated from the renormalization group, our results robustly disfavor $g_{a\gamma\gamma} \gtrsim 4.4 \times 10^{-11}$ GeV$^{-1}$ even for models with no ultraviolet contribution to $C_{aee}$.
C. Dessert, A. Long and B. Safdi
Wed, 28 Apr 21
48/60
Comments: 7+11 pages, 4+7 figures
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