Constraining Black Holes with Light Boson Hair and Boson Stars using Epicyclic Frequencies and QPOs

TL;DR

This paper explores how quasi-periodic oscillations (QPOs) can be used to detect or constrain light bosonic fields and boson stars around black holes, testing deviations from Kerr geometry and the no-hair theorem.

Contribution

It demonstrates that current and future X-ray observations can effectively constrain or rule out models involving light bosonic fields and boson stars near black holes.

Findings

Current QPO data can already constrain bosonic hair models.

Future missions like eXTP can set more stringent limits.

Boson stars are unlikely to explain certain X-ray sources.

Abstract

Light bosonic fields are ubiquitous in extensions of the Standard Model. Even when minimally coupled to gravity, these fields might evade the assumptions of the black-hole no-hair theorems and give rise to spinning black holes which can be drastically different from the Kerr metric. Furthermore, they allow for self-gravitating compact solitons, known as (scalar or Proca) boson stars. The quasi-periodic oscillations (QPOs) observed in the X-ray flux emitted by accreting compact objects carry information about the strong-field region, thus providing a powerful tool to constrain deviations from Kerr's geometry and to search for exotic compact objects. By using the relativistic precession model as a proxy to interpret the QPOs in terms of geodesic frequencies, we investigate how the QPO frequencies could be used to test the no-hair theorem and the existence of light bosonic fields near accreting compact objects. We show that a detection of two QPO triplets with current sensitivity can already constrain these models, and that the future eXTP mission or a LOFT-like mission can set very stringent constraints on black holes with bosonic hair and on (scalar or Proca) boson stars. The peculiar geodesic structure of compact scalar/Proca boson stars implies that these objects can easily be ruled out as alternative models for X-ray source GRO J1655-40.