Inventory of Black Hole Binaries

A small group of X-ray binaries currently provides the best evidence for the existence of stellar-mass black holes. These objects are interacting binary systems where the X-rays arise from accretion of material onto a compact object (i.e. an object with a radius of less than a few hundred km). In some favourable cases, optical studies of the companion star lead to dynamical mass estimates for both components. In 17 cases, the mass of the compact object an X-ray binary has been shown to exceed the maximum mass of a stable neutron star (about 3 M ), which leads to the conclusion that these objects are black holes. In this contribution I will review the basic properties of these black hole binaries. 1. Definition and Source Selection Black holes represent an extreme manifestation of Einstein’s theory of general relativity. As an observational astronomer, I will not consider any of the detailed theory of black holes but will instead consider only the practical question of how to find them. The usual route is by the process of elimination. In the current universe, black holes must form via the gravitational collapse of a dying star. Three outcomes are possible, depending on the mass of the degenerate core: a white dwarf, a neutron star, or a black hole. The mass of a white dwarf cannot exceed the well-known Chandrasekhar limit, and a neutron star has a somewhat analogous upper mass limit, generally thought to be on the order of 3M (Rhoades & Ruffini 1974; Kalogera & Baym 1996). Once the mass of the degenerate core exceeds about 3 M , no known force can halt the gravitational collapse, and a black hole must be formed. Given this, we have a relatively straightforward observational definition of a black hole: A black hole is a compact object with a mass greater than three solar masses, where “compact” in this context means an object with a radius smaller than about 100 km (i.e. much less than the radius of normal stars). Since black holes are dark, the only way one could hope to observe them is through their gravitational influence on surrounding matter. Early attempts to search catalogs of spectroscopic binaries to look for single-lined binaries with massive and undetected companions were not successful (Zel’dovich & Guseynov 1966; Trimble & Thorne 1969). A far more efficient approach for source selection has its roots in the 1960s when Zel’dovich and others realized that if a black hole accreted material (either from a nearby companion star or the interstellar medium), then it might shine brightly in X-rays and γ rays. Today we know of