# X-ray Signatures of Axion Conversion in Magnetic White Dwarf Stars

## TL;DR

This paper proposes that axions produced in white dwarf stars can convert into X-rays in their magnetic fields, and uses observations to set new constraints on axion properties, enhancing the potential to test axion-related stellar cooling explanations.

## Contribution

It introduces a novel method to detect axions via X-ray signatures from magnetic white dwarfs and provides the strongest current constraints on axion couplings using existing X-ray data.

## Key findings

- Set the strongest constraints on axion-electron and axion-photon couplings.
- Showed that dedicated X-ray observations could significantly improve sensitivity.
- Proposed a new observational approach to test axion-like particles in stellar environments.

## Abstract

White dwarf (WD) stars may radiate keV-energy axions produced in their stellar cores. This has been extensively studied as an extra channel by which WDs may cool, with some analyses even suggesting that axions can help explain the observed WD luminosity function. We show that the radiated axions may convert into X-rays in the strong magnetic fields surrounding the WDs, leading to observable X-ray signatures. We use Suzaku observations of the WD RE J0317-853 to set the strongest constraints to date on the combination of the axion-electron ($g_{aee}$) times axion-photon ($g_{a\gamma\gamma}$) couplings, and we show that dedicated observations of magnetic WDs by telescopes such as Chandra, XMM-Newton, and NuSTAR could increase the sensitivity to $|g_{aee} g_{a\gamma\gamma}|$ by over an order of magnitude, allowing for a definitive test of the axion-like-particle explanation of the stellar cooling anomalies.