Diluted Axion Star Collisions with Neutron Stars

TL;DR

This paper investigates the collision between diluted axion stars and neutron stars, modeling their dynamics and potential radio signals, suggesting that such events could help detect QCD axions with current radio telescopes.

Contribution

It provides a combined numerical and semi-analytic analysis of axion star evolution during collisions, highlighting the conditions for resonance conversion and detection prospects.

Findings

Self-gravity and quantum pressure are negligible after entering Roche radius.

Free-fall approximation accurately models particle trajectories.

Detection of collision events can probe QCD axion parameter space.

Abstract

Diluted axion star, a self-gravitating object with the quantum pressure balancing gravity, has been predicted in many models with a QCD axion or axion-like particle. It can be formed in the early universe and composes a sizable fraction of dark matter. One could detect the transient radio signals when it passes by a magnetar with the axion particle converted into photon in the magnetic field. Using both numerical and semi-analytic approaches, we simulate the axion star's dynamic evolution and estimate the fraction of axion particles that can have a resonance conversion during such a collision event. We have found that both self-gravity and quantum pressure are not important after the diluted axion star enters the Roche radius. A free-fall approximate can capture individual particle trajectories very well. With some optimistic cosmological and astrophysical assumptions, the QCD axion parameter space can be probed from detecting such a collision event by radio telescopes.