CANONICAL TIME VARIATIONS OF X-RAYS FROM BLACK-HOLE CANDIDATES IN THE LOW-INTENSITY STATE

X-rays from black hole candidates, Cyg X-1, GX 339-4, and GS 2023 + 338 in their low-intensity state, consist of various shots or burst events, and shapes of the shots are similar among these sources. In their low-intensity states, the normalized power spectrum density functions of the X-rays except for the Fourier frequencies below about 0.2 Hz are the same not only in their shape but also in their absolute values, even if the X-ray energy range is different. Phase lags of time variations between different energy X-rays are also similar for these sources, in addition to the well-known similarity of their energy spectra of a power law. These similarities suggest that although time variations of X-rays from these sources seem to be chaotic, they are canonical, and their X-ray production process and dynamic behavior of accreting matter in the vicinity of the black hole are the same. During an individual X-ray shot or burst event, the X-ray energy spectrum becomes harder with time, and duration of the shot event is of the order of 0.1-1 s. This time scale is of the order of viscous time scale, on which accreting matter drifts through the X-ray-emitting part of the accretion disk. This coincidence suggests that the X-ray shots are produced by many rings of accreting matter, and the hardening of the energy spectrum of the X-ray shots is due to powering up of the rings of matter by gravitational energy released in the vicinity around black holes. On the other hand, in the very high intensity state of GX 339-4, the normalized power spectrum density functions are different from those in the low-intensity state as well as their energy spectra, which suggests that X-ray production process in the very high intensity state is different from those in the low-intensity state.