Cosmological Neutron Stars Produce Diffuse Axion X-Ray Signatures

TL;DR

This paper investigates the potential X-ray signatures produced by axion-like particles in cosmological neutron stars, using observational data to place new constraints on axion properties and rule out certain explanations for observed X-ray excesses.

Contribution

It introduces a novel analysis of cosmological neutron star populations to constrain axion-like particles via their X-ray signatures, covering new parameter space.

Findings

No evidence for axion-induced X-ray signals was found.

New constraints on axion-photon and axion-nucleon couplings were established.

The axion explanation for the Magnificent Seven X-ray excess is ruled out.

Abstract

Axion-like particles can be abundantly produced through scattering processes in the cores of neutron stars (NSs). If they are ultralight ($m_a \lesssim 10^{-4}$ eV), then they can efficiently convert to detectable photons in the external NS magnetospheres, and if they are heavy ($m_a \gtrsim 1$ eV), then they can decay into photons before reaching Earth. In this work, we search for the resulting X-ray signatures from both of these channels summing over the $\textit{cosmological}$ NS population. We compare the predicted axion-induced X-ray signal to the cosmic X-ray background today as measured by a number of instruments such as NuSTAR, HEAO, Swift, and INTEGRAL. We model the axion-induced signal using NS cooling simulations and magnetic field evolution models. We find no evidence for axions and derive strong constraints for both ultralight and heavy axion scenarios, covering new parameter space for the axion-photon and axion-nucleon couplings. Our results rule out the axion-explanation of the Magnificent Seven X-ray excess from nearby isolated NSs.