Emission of Photons and Relativistic Axions from Axion Stars

TL;DR

This paper investigates how axion stars lose axions through inelastic reactions, producing photons or relativistic axions, and identifies potential observable signals such as monochromatic radio-frequency emissions.

Contribution

It calculates the rates of axion loss via inelastic reactions in axion stars and highlights the conditions under which these processes dominate, providing potential observational signatures.

Findings

Axion decay into two photons is dominant in dilute axion systems.

Four-axion scattering into relativistic axions dominates in dense axion systems.

Monochromatic radio signals at odd harmonics could indicate dense axion stars.

Abstract

The number of nonrelativistic axions can be changed by inelastic reactions that produce photons or relativistic axions. Any odd number of axions can annihilate into two photons. Any even number of nonrelativistic axions can scatter into two relativistic axions. We calculate the rate at which axions are lost from axion stars from these inelastic reactions. In dilute systems of axions, the dominant inelastic reaction is axion decay into two photons. In sufficiently dense systems of axions, the dominant inelastic reaction is the scattering of four nonrelativistic axions into two relativistic axions. The scattering of odd numbers of axions into two photons produces monochromatic radio-frequency signals at odd-integer harmonics of the fundamental frequency set by the axion mass. This provides a unique signature for dense systems of axions, such as a dense axion star or a collapsing dilute axion star.