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

Abstract

Axions with couplings gaγγ∼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 gaγγ≲5.5×10-13Caγγ/Caee GeV-1 at 95% confidence for axion masses ma≲5×10-6 eV, with Caee and Caγγ the dimensionless coupling constants to electrons and photons. Considering that Caee is generated from the renormalization group, our results robustly disfavor gaγγ≳4.4×10-11 GeV-1 even for models with no ultraviolet contribution to Caee.