Toward a Uniied 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 \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 \oo" 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 \\are" 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 variability associated with \thermal" states. We discuss these results in light of theoretical models and nd 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. Galactic X-ray sources are typically identiied as black hole candidates (BHC) if they have measured mass functions indicating a compact object with M > 3 M , or if their high energy spectra (1 keV 10 MeV) and temporal variability (10 3 10 2 Hz) are similar to other BHC. A review of the general observations, a number of theoretical models, plus individual descriptions of most of the approximately twenty galactic BHC can be found in Tanaka & Lewin (1995). A review of the timing analyses can be found in van der Klis (1994, 1995) and Miyamoto (1994). These latter reviews attempt to link the energy spectra and timing observations, and they draw analogies to similar observations of neutron stars in low mass X-ray binaries. The purpose of this work is is not to summarize or reanalyze these reviews, but rather to look at a subset of BHC where we have some indication of the compact object's mass and its variability properties. (We make one exception to this list, for reasons which we elaborate below, and include GX 339{4 which has no reliable mass estimate.) We …