Three Spectral States of the Disk X-Ray Emission of the Black-Hole Candidate 4U 1630−47

We studied a time history of X-ray spectral states of a black-hole candidate,4U 1630-47, utilizing data from a number of monitoring observations with theRossi X-Ray Timing Explorer over 1996--2004. These observations covered fiveoutbursts of 4U 1630-47, and most of the data recorded typical features of thehigh/soft states. We found that the spectra in the high/soft states can befurther classified into three states. The first spectral state is explained bya concept of the standard accretion disk picture. The second state appears inthe so-called very high state, where a dominant hard component is seen and thedisk radius apparently becomes too small. These phenomena are explained by theeffect of inverse Compton scattering of disk photons, as shown by Kubota,Makishima, & Ebisawa (2001, ApJ, 560, L147) for GRO J1655-40. The third stateis characterized in such a way that the disk luminosity varies in proportion to$T_{\rm in}^2$, rather than $T_{\rm in}^4$, where $T_{\rm in}$ is theinner-disk temperature. This state is suggested to be an optically-thick andadvection-dominated slim disk, as suggested by Kubota & Makishima (2004, ApJ,601, 428) for XTE J1550-564. The second and third states appear, with goodreproducibility, when $T_{\rm in}$ and the total X-ray luminosity are higherthan 1.2 keV and $\sim2.5\times10^{38}(D/10\quad{\rm kpc})^2\left[\cos{\theta}/(1/\sqrt{3})]^{-1}$ erg s$^{-1}$, respectively, where $D$ isthe distance to the object and $\theta$ is the inclination angle to the disk.The present results suggest that these three spectral states commonly appearamong black-hole binaries under high accretion rates.Comment: 24 pages, 9 figures, to appear in PASJ 57 p.62