No Evidence for Axions from Chandra Observation of the Magnetic White Dwarf RE J0317-853

Abstract

Axions with couplings g_{a γ γ}∼few ×10^-11 GeV^-1 to electromagnetism 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 nonobservation 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 nonobservation of x rays constrains the axion-photon coupling to g_{a γ γ}≲5.5 ×10^-13√{C_{a γ γ}/C_{a e e} } GeV^-1 at 95% confidence for axion masses m_a≲5 ×10^-6 eV , with C_{a e e} and C_{a γ γ} the dimensionless coupling constants to electrons and photons. Considering that C_{a e e} is generated from the renormalization group, our results robustly disfavor g_{a γ γ}≳4.4 ×10^-11 GeV^-1 even for models with no ultraviolet contribution to C_{a e e}.