Toward a unified view of black-hole high-energy states

We present here a review of high-energy (>1 keV) observations of seven black-hole candidates, six of which have estimated masses. In this review we focus on two parameters of interest: the ratio of ''nonthermal'' to total luminosity as a function of the total luminosity divided by the Eddington luminosity, and the root-mean-square (rms) variability as a function of the nonthermal-to-total luminosity ratio. Below similar to 10% Eddington luminosity, the sources tend to be strictly nonthermal (the so called ''off'' and ''low'' states). Above this luminosity the sources become mostly thermal (the ''high'' state), with the nonthermal component increasing with luminosity (the ''very high'' and ''flare'' states). There are important exceptions to this behavior, however, and no steady - as opposed to transient - source has been observed over a wide range of parameter space. In addition, the rms variability is positively correlated with the ratio of nonthermal to total luminosity, although there may be a minimum level of variability associated with ''thermal'' states. We discuss these results in light of theoretical models and find that currently no single model describes the full range of black-hole high-energy behavior. In fact, the observations are exactly opposite from what one expects based upon simple notions of accretion disk instabilities.