Detectability of Black Hole Binaries with Gaia: Dependence on Binary Evolution Models

Abstract

The astrometric satellite Gaia is expected to observe noninteracting black hole (BH) binaries with luminous companions (LCs; hereafter BH-LC binaries), a different population from BH X-ray binaries previously discovered. The detectability of BH-LC binaries with Gaia might be dependent on binary evolution models. We investigated the Gaia's detectability of BH-LC binaries formed through isolated binary evolution by means of the binary population synthesis technique and examined its dependence on single and binary star models: supernova models, common envelope (CE) ejection efficiency α , and BH natal kick models. We estimated that 1.1–46 BH-LC binaries can be detected within the five-year observation, and found that α has the largest impact on the detectable number. In each model, observable and intrinsic BH-LC binaries have similar distributions. Therefore, we found three important implications: (1) if the lower BH mass gap is not intrinsic (i.e., 3–5 M ⊙ BHs exist), Gaia will observe ≤5 M ⊙ BHs; (2) we may observe short orbital period binaries with light LCs if CE efficiency is significantly high; and (3) we may be able to identify the existence of natal kick from eccentricity distribution.