Kerr black holes with synchronised axionic hair

TL;DR

This paper constructs and analyzes Kerr black holes with axionic hair, exploring how axion-like particles influence black hole properties and extending previous scalar hair solutions to include non-linear potentials.

Contribution

It introduces a new class of Kerr black hole solutions with axionic hair, incorporating non-linear axion potentials and analyzing their geometric and phenomenological features.

Findings

Solutions are regular outside and on the horizon.

Non-linear effects become significant for small decay constants.

Parameter space mapping reveals diverse black hole configurations.

Abstract

We construct and analyse Kerr black holes (BHs) with synchronised axionic hair. These are the BH generalisations of the recently constructed rotating axion boson stars arXiv:2005.05982. Such BHs are stationary, axially symmetric, asymptotically flat solutions of the complex Einstein-Klein-Gordon theory with a QCD axion-like potential. They are regular everywhere on and outside the event horizon. The potential is characterised by two parameters: the mass of the axion-like particle, $m_a$ and the decay constant $f_a$. The limit $f_a \rightarrow \infty$ recovers the original example of Kerr BHs with synchronised scalar hair arXiv:1403.2757. The effects of the non-linearities in the potential become important for $f_a \lesssim 1$. We present an overview of the parameter space of the solutions together with a study of their basic geometric and phenomenological properties, for an illustrative value of the coupling that yields a non-negligible impact of the self-interactions.