QCD Axion Conversion in Magnetospheres of Neutron Stars

TL;DR

This paper investigates how strong magnetic fields in neutron stars affect axion emission and conversion to photons, providing insights into axion properties and neutron star cooling mechanisms.

Contribution

It analyzes the impact of intense magnetic fields on axion emission spectra and neutron star cooling, incorporating magnetic effects into axion-photon conversion models at 15 meV axion mass.

Findings

Magnetic fields significantly alter axion luminosity and spectra.

Bremsstrahlung dominates over PBF at lower axion energies.

Magnetic effects are less pronounced at lower axion energies.

Abstract

The axion-converted-photons flux is a principal window for searching QCD axions as a dark matter (DM) candidate. In addition to solving the strong CP problem, these may explain the properties of the mysterious DM. Neutron star (NS) cooling by neutrino/axion emissions rate constrains the astrophysical properties of superdense matter. We attempt to analyse the impact of strong magnetic fields on the emission properties of NS by employing the FPS equation of State (EoS). We use the Tolman Oppenheimer Volkoff (TOV) equations by considering effects of strong fields and generating profiles. We assume Cooper-pair-breaking formation (PBF) and the Bremsstrahlung process occur in the core of NS. We adopt a polynomial fit function of radial profile to analyse the effects of strong magnetic field. Our entire analysis is at an axion mass of $15$ meV and central magnetic field $B_{c}$=$10^{17}$ G. Our work assumes the core comprises hadronic matter of the spherically symmetric magnetized NSs. We have present the results for the energy spectrum of axions and their subsequent conversion to photons. We show that the cooling rate and the luminosity of axions for NSs change significantly due to the intense magnetic field. We report that the energy spectrum of axions from the Bremsstrahlung process dominates over the PBF process at lesser axion energies, within the possible axion mass range for PSR J1356-6429 NS. Our results reveal that the impact of the magnetic field is less at lower axion energies, indicating the necessity for including a magnetic field in axion-to-photon conversion mechanisms.