  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 $C_{aee}$.
