Search for axions from magnetic white dwarfs with <i>Chandra</i> x-ray observations

Low-mass axionlike particles could be produced in abundance within the cores of hot, compact magnetic white dwarf (MWD) stars from electron bremsstrahlung and converted to detectable x-rays in the strong magnetic fields surrounding these systems. In this work, we constrain the existence of such axions from two dedicated Chandra x-ray observations of $\ensuremath{\sim}40\text{ }\mathrm{ks}$ each in the energy range $\ensuremath{\sim}1--10\text{ }\text{ }\mathrm{keV}$ toward the MWDs WD $1859+148$ and PG $0945+246$. We find no evidence for axions, which constrains the axion-electron times axion-photon coupling to $|{g}_{a\ensuremath{\gamma}\ensuremath{\gamma}}{g}_{aee}|\ensuremath{\lesssim}1.54\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}25}$ $(3.54\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}25})\text{ }\text{ }{\mathrm{GeV}}^{\ensuremath{-}1}$ for PG $0945+246$ (WD $1859+148$) at 95% confidence for axion masses ${m}_{a}\ensuremath{\lesssim}{10}^{\ensuremath{-}6}\text{ }\text{ }\mathrm{eV}$. We find an excess of low-energy x-rays between 1 and 3 keV for WD $1859+148$ but determine that the spectral morphology is too soft to arise from axions; instead, the soft x-rays may arise from nonthermal emission in the MWD magnetosphere.