The Three Spectral Regimes Found in the Stellar Black Hole XTE J1550--564 in its High/Soft State

Abstract

The present paper describes the analysis of multiple RXTE/PCA data of the black hole binary with superluminal jet, XTE J1550-564, acquired during its 1999--2000 outburst. The X-ray spectra show features typical of the high/soft spectral state, and can approximately be described by an optically thick disk spectrum plus a power-law tail. Three distinct spectral regimes, named standard regime, anomalous regime, and apparently standard regime, have been found from the entire set of the observed spectra. When the X-ray luminosity is well below 6E38 erg/s (assuming a distance of 5 kpc), XTE J1550-564 resides in the standard regime, where the soft spectral component dominates the power-law component and the observed disk inner radius is kept constant. When the luminosity exceeds the critical luminosity, the apparently standard regime is realized, where luminosity of the optically thick disk rises less steeply with the temperature, and the spectral shape is moderately distorted from that of the standard accretion disk. In this regime, radial temperature gradient of the disk has been found to be flatter than that of the standard accretion disk. The results of the apparently standard regime are suggestive of a slim disk (e.g., Abramowicz et al. 1988, Watarai et al. 2000) which is a solution predicted under high mass accretion rate. In the intermediate anomalous regime, the spectrum becomes much harder, and the disk inner radius derived using a simple disk model spectrum apparently varies significantly with time. These properties can be explained as a result of significant thermal inverse Comptonization of the disk photons, as was found from GRO J1655-40 in its anomalous regime by Kubota, Makishima and Ebisawa (2001).