Rotating multistate axion boson stars

TL;DR

This paper explores rotating axion boson stars with excited and multistate configurations, analyzing their solution space, mass limits, and stability features, including potential superradiant instability in certain branches.

Contribution

It introduces new multistate and excited rotating axion boson star solutions, expanding understanding of their structure, mass limits, and stability characteristics.

Findings

Fundamental RABSs have higher maximum mass than excited states at low decay constants.

Multistate configurations can achieve higher mass than single-state configurations at the same frequency.

Second branches of RMABSs may be superradiantly unstable due to ergoregion emergence.

Abstract

We consider excited configuration and multistate configuration of rotating axion boson stars~(RABSs).RABSs are asymptotically flat, stationary, spinning, horizonless solutions of Einstein-Klein-Gordon theory in which the scalar potential depends on scalar field mass $\mu$ and axion decay constant $f_a$. The excited RABSs have two types of solutions, including $^2S$ state and $^2P$ state. The rotating multistate axion boson stars~(RMABSs) consist of coupled fundamental configuration and excited configuration, and also include two types of solutions: $^1S^2S$ state and $^1S^2P$ state. Some differences between RABSs models and rotating mini-boson stars models are discussed. We show the solution space of these models for different values of decay constant $f_a$. We found fundamental RABSs have a higher maximum mass than the excited state in the low decay constant region. Moreover, the multistate configuration allows a higher mass than both the fundamental configuration and the excited configuration at the same frequency. We also found the RMABSs have the second branch in which the ergo-region emerges. This means the second branch may be superradiant unstable.