Constraining the binarity of black hole candidates: a proof-of-concept study of Gaia BH1 and Gaia BH2

TL;DR

This study explores the potential to detect hidden inner binary black holes within star-BH systems like Gaia BH1 and Gaia BH2 by analyzing radial velocity modulations, offering a new method for identifying undetected BBHs.

Contribution

It presents a detailed feasibility analysis of using radial velocity signals to constrain the binarity of star-BH systems, applicable to current and future candidates.

Findings

Gaia BH1 may show observable signatures of an inner BBH.

Methodology is applicable to other star-BH binary candidates.

Three types of RV modulations can indicate inner BBHs.

Abstract

Nearly a hundred of binary black holes (BBHs) have been discovered with gravitational-wave signals emitted at their merging events. Thus, it is quite natural to expect that significantly more abundant BBHs with wider separations remain undetected in the universe, or even in our Galaxy. We consider a possibility that star-BH binary candidates may indeed host an inner BBH, instead of a single BH. We present a detailed feasibility study of constraining the binarity of the currently available two targets, Gaia BH1 and Gaia BH2. Specifically, we examine three types of radial velocity (RV) modulations of a tertiary star in star-BBH triple systems; short-term RV modulations induced by the inner BBH, long-term RV modulations induced by the nodal precession, and long-term RV modulations induced by the von Zeipel-Kozai-Lidov oscillations. Direct three-body simulations combined with approximate analytic models reveal that Gaia BH1 system may exhibit observable signatures of the hidden inner BBH if it exists at all. The methodology that we examine here is quite generic, and is expected to be readily applicable to future star-BH binary candidates in a straightforward manner.