Probing ALP-photon couplings in Neutron Stars: Scalar versus pseudoscalar cases

TL;DR

This paper models the distribution of axion-like particles in magnetized neutron stars, analyzing how their couplings to electromagnetic fields affect their spatial profiles and potential observational signatures.

Contribution

It introduces a detailed semi-analytical framework for studying axion distributions in neutron stars with complex magnetic and electric fields, including effects of magnetic flux tubes.

Findings

Axion distribution is sensitive to magnetic and electric field couplings.

Axions may accumulate in the crust, forming condensates.

Magnetic flux tubes could enhance photon conversion.

Abstract

We investigate the distribution of an interacting axion-like massive field within a magnetized Neutron Star. For this we consider the effect of an intense density-dependent axially symmetric stellar magnetic field ${\bf B}(r,\theta)$ adding another much weaker, but non-vanishing, electric field. We particularize the latter for the case when a finite chiral charge density is present. The axion field is thus coupled to a generic function $Q(F_0,G_0)$ depending on Lorentz invariants $F_0, G_0$ which can be constructed from these electromagnetic fields. From this, the static axion field equations are solved as function of stellar radial coordinate and angular direction, $a(r,\theta)$, using a prescribed linear form for $Q$. In addition, we use a semi-analytical approach to calculate the stellar structure in this hybrid system where pressure components are treated under a perturbative scheme, provided induced deformations with respect to spherical symmetry are tiny. Our results show that the axion couplings to magnetic and electric fields along with its mass, critically determine the axion spatial distribution. Furthermore, we focus on the possibility that the axion field might accumulate in specific outer regions of the star, particularly within the crust, where it could form a condensate. We explore the possible presence of magnetic flux tubes from superconductor phases in this outer layers and qualitatively show they may enhance local conversion into photons. We explore prospects of detectability through indirect methods.