A Strategy to Search for an Inner Binary Black Hole from the Motion of the Tertiary Star

Abstract

There are several ongoing projects to search for stars orbiting around an invisible companion. A fraction of such candidates may be a triple, instead of a binary, consisting of an inner binary black hole (BBH) and an outer orbiting star. In this paper, we propose a methodology to search for a signature of such an inner BBH, possibly a progenitor of gravitational-wave sources discovered by LIGO, from the precise radial velocity (RV) follow-up of the outer star. We first describe a methodology using an existing approximate RV formula for coplanar circular triples. We apply this method and constrain the parameters of a possible inner binary object in 2M05215658+4359220, which consists of a red giant and an unseen companion. Next we consider coplanar but non-circular triples. We compute numerically the RV variation of a tertiary star orbiting around an inner BBH, generate mock RV curves, and examine the feasibility of detection of the BBH for our fiducial models. We conclude that short-cadence RV monitoring of a star–BH binary provides an interesting and realistic method to constrain and/or search for possible inner BBHs. Indeed a recent discovery of the star–BH binary system LB-1 may imply that there are a large number of such unknown objects in our Galaxy, which are ideal targets for the methodology proposed here.