Search for Axions from Magnetic White Dwarfs with Chandra

Abstract: Low mass axion-like 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 $\sim$40 ks each in the energy range $\sim$1 - 10 keV towards the magnetic white dwarfs (MWDs) WD 1859+148 and PG 0945+246. We find no evidence for axions, which constrains the axion-electron times axion-photon coupling to $|g_{a\gamma \gamma} g_{aee}| \lesssim 1.54 \times 10^{-25}$ ($3.54 \times 10^{-25}$) GeV$^{-1}$ for PG 0945+246 (WD 1859+148) at 95% confidence for axion masses $m_a \lesssim 10^{-6}$ eV. We find an excess of low-energy X-rays between 1 - 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 non-thermal emission in the MWD magnetosphere.