Magnetar Hard X-Ray Emission from Axion-like Particle Conversion

TL;DR

This paper investigates whether axion-like particles produced in magnetar cores and converting into photons in the magnetosphere can explain the observed hard X-ray spectra and address related heating and lifetime issues.

Contribution

It introduces a novel scenario where ALP-photon conversion accounts for magnetar X-ray emissions and impacts their thermal evolution, which has not been extensively studied before.

Findings

ALP conversion could explain hard X-ray spectra of magnetars.

This mechanism may reduce core temperatures and neutrino emissions.

The study suggests further detailed investigation is warranted.

Abstract

We explore the possibility that axion-like-particles (ALPs), which would be produced in the core of magnetars and would then convert in the magnetosphere into photons, can explain magnetar hard X-ray spectra. We remark that this scenario would also provide answers to some questions related to magnetar heating. Indeed, considering that magnetars have: 1) hard X-ray spectra that are difficult to explain with known mechanisms; 2) large photon luminosities that force high core temperatures; 3) high core temperatures that imply large neutrino emissivities; 4) and large neutrino emissivities that lead to small magnetar lifetimes in contradiction to observations -- explaining the hard X-ray spectra with ALPs could decrease the core temperatures and thus the neutrino emissivities, allowing for longer magnetar lifetimes as expected from observations. In this work, we initiate the study of this scenario for three magnetars with extreme luminosities, and conclude that the general idea is likely worth investigating in more detail.