Stellar mass black hole systems and X-ray binaries

Now that we have introduced the basic theory of black holes, we present some of the phenomenology of real-life astrophysical black holes. In fact, the scientific communities studying stellar mass and supermassive black holes have remained rather distinct from each other, leading to the development of rather different languages and phenomenological descriptions. We begin by discussing the stellar mass black holes that we believe arise from the collapse of a massive star.

Although most observed stellar mass black hole candidates reside in binary star systems, it is expected that the vast majority of such black holes occur as isolated systems. They remain undetected due to their lack of any significant radiation. Current estimates indicate that there are between 0.1-1 billion such systems within our Galaxy, a very small fraction of which, $< 0.001\%$, might be detectable in a deep X-ray survey of the Galactic plane [31]. Even a so-called isolated black hole will accrete at some level from the surrounding gaseous interstellar medium (ISM) -- this mode of accretion is called Bondi-Hoyle accretion after Herman Bondi and Fred Hoyle who calculated the details of this process [32]. Low-level X-ray emission is expected due to Bondi-Hoyle accretion, although there are large uncertainties regarding both the mass flux and radiative efficiency of the accretion flow [31]. Currently, the best prospects of detecting such isolated black holes is via their gravitational effects, specifically via gravitational micro-lensing -- the amplification of the observed light from a background star due the gravitational focusing (i.e., bending) of that light in the potential of a foreground compact object. The amplification is expected to be time-symmetric, achromatic (i.e., independent of wavelength), and of a duration from weeks to months for expected stellar distances, velocities, and compact object masses. A number of surveys for such micro-lensing events are currently being undertaken, and there are claims of candidate detections of stellar mass black holes [33,34].