We show that stellar black hole X-ray binaries have two novel states: the power-law hard state and the power-law soft state, which depend not only on X-ray luminosity but also on previous history of X-ray luminosity, i.e., the states show large hysteretic behavior which is dependent upon X-ray luminosity by a factor of about 100. This large hysteresis probably cannot be explained by already existing accretion disk models. The two states are represented by two (hard and soft) types of power-law spectral components. In the rising phase of the outburst of an X-ray flare or an X-ray nova, the black hole X-ray binary is in the power-law hard state and the power-law component has a photon index (-gamma) of about -1.7 (gamma = 1.5-1.9) in the X-ray range of 3-40 keV. This state continues until near or just before the maximum X-ray flux (2-100 keV). The state then changes to the power-law soft state, in which the photon index (-gamma) of the power-law component changes to about -2.7 (gamma = 2.2-3.0) in the X-ray range of 3-40 keV. Following this change, a large multi-temperature disk blackbody component (approximately represented by blackbody radiation of kT = 0.5-1 keV) appears. This state continues until the X-ray flux has decreased to about 1/100 of its maximum. The disk blackbody component then decreases rapidly and the photon index (-gamma) of the power-law component changes to about -1.7. This is the beginning of the power-law hard state. In the power-law hard state, slight traces of or no disk blackbody component is observed. The short-term variability of the power-law components and the size of the X-ray emitting regions are different in these two power-law states, implying completely different X-ray production processes. This result was derived from the combined observed results of GS 1124-683, GX 339-4, and Cyg X-1.
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Miyamoto et al. (1995) studied this question.