1995PASP..107.1207N

We present here a review of high energy (>1 keV) observations of seven black hole candidates, six of which have estimates masses. In this review we focus on two parameters of interest: the ratio of "non-thermal" 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 non-thermal to total luminosity ratio. Below ∼10% Eddington luminosity, the sources tend to be strictly non-thermal (the so called "off" and "low" states). Above this luminosity the sources become mostly thermal (the "high" state), with the non-thermal 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 non-thermal to total luminosity, although there may be a minimum level of varibility 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.