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A. Kubota, K. Makishima (Univ. of Tokyo), K. Ebisawa (NASA)
To critically examine the validity of the standard Shakura-Sunyaev disk picture, we extensively studied ASCA and RXTE spectra of eight soft-state black hole binaries (BHBs). The behavior of some of them (e.g., LMC X-3 is typical of them) has been explained perfectly by the standard-disk picture, even when the disk luminosity Ldisk gets close to the Eddington limit LE. However, in other BHBs (e.g., GRO J1655-40, XTE J1550-564, and LMC X-1), we have found that the disks make a transition into an anomalous state when Ldisk reach a certain critical upper-limit luminosity, Lc, which is considerably lower than LE. When Ldisk hits Lc in the latter type of objects, Ldisk is moderately saturated, even though the spectral shape resembles those of standard disks. The radial temperature gradient in the disk become flatter than in the standard disk. These properties support the picture of the optically-thick ADAF (or slim disk; e.g., Abramowicz et al. 1989, Watarai et al. 2000). At an intermediate case (Ldisk~Lc), the spectral hard tail dominates, and the apparent inner-disk radius becomes no longer constant. We have discovered these effects are due to a sudden increase in the disk Comptonization, while the underlying disk itself remains in the standard state.