No Evidence for Axions from <i>Chandra</i> Observation of the Magnetic White Dwarf RE J0317-853
Christopher Dessert, Andrew J. Long, Benjamin R. Safdi
Abstract
Axions with couplings ${g}_{a\ensuremath{\gamma}\ensuremath{\gamma}}\ensuremath{\sim}\mathrm{few}\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}11}\text{ }\text{ }{\mathrm{GeV}}^{\ensuremath{-}1}$ to electromagnetism may resolve a number of astrophysical anomalies, such as unexpected $\ensuremath{\sim}\mathrm{TeV}$ transparency, anomalous stellar cooling, and x-ray excesses from nearby neutron stars. We show, however, that such axions are severely constrained by the nonobservation of x rays from the magnetic white dwarf (MWD) RE J0317-853 using $\ensuremath{\sim}40\text{ }\text{ }\mathrm{ks}$ 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 nonobservation of x rays constrains the axion-photon coupling to ${g}_{a\ensuremath{\gamma}\ensuremath{\gamma}}\ensuremath{\lesssim}5.5\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}13}\sqrt{{C}_{a\ensuremath{\gamma}\ensuremath{\gamma}}/{C}_{aee}}\text{ }\text{ }{\mathrm{GeV}}^{\ensuremath{-}1}$ at 95% confidence for axion masses ${m}_{a}\ensuremath{\lesssim}5\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}6}\text{ }\text{ }\mathrm{eV}$, with ${C}_{aee}$ and ${C}_{a\ensuremath{\gamma}\ensuremath{\gamma}}$ the dimensionless coupling constants to electrons and photons. Considering that ${C}_{aee}$ is generated from the renormalization group, our results robustly disfavor ${g}_{a\ensuremath{\gamma}\ensuremath{\gamma}}\ensuremath{\gtrsim}4.4\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}11}\text{ }\text{ }{\mathrm{GeV}}^{\ensuremath{-}1}$ even for models with no ultraviolet contribution to ${C}_{aee}$.