Constraining Axion-Like-Particles with Hard X-ray Emission from Magnetars

TL;DR

This paper models the conversion of axion-like particles into X-ray photons in magnetar environments, deriving constraints on ALP properties by comparing predicted signals with observed magnetar luminosities.

Contribution

It introduces an efficient numerical method for calculating ALP-photon conversion probabilities in magnetar magnetospheres and derives new constraints on ALP parameters from magnetar observations.

Findings

Constraints on ALP mass and coupling are competitive with helioscope experiments.

Developed an analytic formalism for multi-state oscillation systems.

Provided detailed dependence of conversion probability on magnetar and ALP parameters.

Abstract

Axion-like particles (ALPs) produced in the core of a magnetar will convert to photons in the magnetosphere, leading to possible signatures in the hard X-ray band. We perform a detailed calculation of the ALP-to-photon conversion probability in the magnetosphere, recasting the coupled differential equations that describe ALP-photon propagation into a form that is efficient for large scale numerical scans. We show the dependence of the conversion probability on the ALP energy, mass, ALP-photon coupling, magnetar radius, surface magnetic field, and the angle between the magnetic field and direction of propagation. Along the way, we develop an analytic formalism to perform similar calculations in more general $n$-state oscillation systems. Assuming ALP emission rates from the core that are just subdominant to neutrino emission, we calculate the resulting constraints on the ALP mass versus ALP-photon coupling space, taking SGR 1806-20 as an example. In particular, we take benchmark values for the magnetar radius and core temperature, and constrain the ALP parameter space by the requirement that the luminosity from ALP-to-photon conversion should not exceed the total observed luminosity from the magnetar. The resulting constraints are competitive with constraints from helioscope experiments in the relevant part of ALP parameter space.