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

Abstract

Abstract We studied a time history of X-ray spectral states of a black-hole candidate, 4U 1630$-$47, utilizing data from a number of monitoring observations with the Rossi X-Ray Timing Explorer over 1996–2004. These observations covered five outbursts of 4U 1630$-$47, and recorded typical features of the high/soft states. The spectra in the high/soft states can be classified into three states. The first state is explained by a concept of the standard accretion disk picture. The second appears in the very high state, where a dominant hard component is seen and the disk radius apparently becomes too small. These phenomena are explained by the effect of inverse Compton scattering of disk photons, as shown by Kubota, Makishima, and Ebisawa (2001, ApJ, 560, L147) for GRO J1655$-$40. The third shows that the disk luminosity varies in proportion to $T_{\mathrm{in}}^2$, rather than $T_{\mathrm{in}}^4$, where $T_{\mathrm{in}}$ is the inner-disk temperature. This state suggests an optically-thick and advection-dominated slim disk, as given by Kubota and Makishima (2004, ApJ, 601, 428) for XTE J1550$-$564. The second and third states appear, with good reproducibility, when $T_{\mathrm{in}}$ and the total X-ray luminosity are higher than 1.2 keV and $\sim 2.5 \times 10^{38} (D/10 \,\mathrm{kpc})^2 [\cos \theta / (1/\sqrt{3})]^{-1} \,\mathrm{erg} \,\mathrm{s}^{-1}$, respectively, where $D$ is the distance to the object and $\theta$ is the inclination angle to the disk. The results suggest that these spectral states commonly appear among black-hole binaries under high accretion rates.