Dark Matter Axion Clump Resonance of Photons

TL;DR

This paper investigates the conditions under which dark matter axion condensate clumps can induce photon resonance, revealing that resonance depends on clump size, coupling strength, and angular momentum, with potential astrophysical implications.

Contribution

It provides a detailed numerical analysis of photon resonance in localized axion clumps, including non-spherical cases and the impact of angular momentum, extending previous homogeneous models.

Findings

Resonance exists for homogeneous condensates but depends on parameters for localized clumps.

Resonance is absent for standard QCD axion couplings in spherical clumps.

Resonant decay can occur in non-spherical clumps with high angular momentum.

Abstract

Recently there has been interest in the physical properties of dark matter axion condensates. Due to gravitational attraction and self-interactions, they can organize into spatial localized clumps, whose properties were examined by us in Refs. [1, 2]. Since the axion condensate is coherently oscillating, it can conceivably lead to parametric resonance of photons, leading to exponential growth in photon occupancy number and subsequent radio wave emission. We show that while resonance always exists for spatially homogeneous condensates, its existence for a spatially localized clump condensate depends sensitively on the size of clump, strength of axion-photon coupling, and field amplitude. By decomposing the electromagnetic field into vector spherical harmonics, we are able to numerically compute the resonance from clumps for arbitrary parameters. We find that for spherically symmetric clumps, which are the true BEC ground states, the resonance is absent for conventional values of the QCD axion-photon coupling, but it is present for axions with moderately large couplings, or into hidden sector photons, or from scalar dark matter with repulsive interactions. We extend these results to non-spherically symmetric clumps, organized by finite angular momentum, and find that even QCD axion clumps with conventional couplings can undergo resonant decay for sufficiently large angular momentum. We discuss possible astrophysical consequences of these results, including the idea of a pile-up of clump masses and rapid electromagnetic emission in the sky from mergers.