Constraining the Binarity of Black Hole Candidates: A Proof-of-concept Study of Gaia BH1 and Gaia BH2

Abstract

Nearly a hundred 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 the 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.

ESPRESSO Observations of Gaia BH1: High-precision Orbital Constraints and no Evidence for an Inner Binary

“…High-precision RVs can thus put the hierarchical triple model to the test. In parallel with our work, the feasibility of such a test was recently explored by Hayashi et al (2023) using idealized simulations. Here, we report new RV data and simulations matched to that data.…”

“…In that case, the two massive stars could prevent one another from expanding to red supergiant dimensions, and after two episodes of mass transfer, the Sunlike star could find itself orbiting two BHs without ever having interacted with either one. This and related scenarios have been discussed as a possible formation channel for Gaia BH1-like binaries by several works (e.g., E23; El-Badry et al 2023b;Chakrabarti et al 2023;Di Carlo et al 2023;Hayashi et al 2023). Besides the triple scenario, other solutions for the current orbit have been proposed (see Section 5.5).…”

“…In these cases, the trend is not a perfect power law, and the best-fit power law index is smaller than 7/3. This reflects the fact that the short-timescale and long-timescale perturbations have different scalings with P inner (Hayashi et al 2023). In these cases, numerical integrations are critical, since the behavior of the RV residuals is not easy to Figure 1.…”

“…In this work, we explore and test the possibility that Gaia BH1 contains an inner BH binary. If an inner binary exists, its orbital motion would introduce non-Keplerian perturbations to the radial velocity (RV) curve of the outer star, which could be detectable with high-precision RV measurements (e.g., Liu et al 2022;Hayashi et al 2023).…”