Correlated X-ray spectral and timing behavior of the black hole candidate XTE J1550-564: A new interpretation of black hole states

We present an analysis of data of the black hole candidate and X-ray transi ent XTE J1550-564, taken with the Rossi X-Ray Timing Explorer between 1998 November 22 and 1999 May 20. During this period the source went through sev eral different states, which could be divided into soft and hard states bas ed on the relative strength of the high-energy spectral component. These st ates showed up as distinct branches in the color-color and hardness-intensi ty diagrams, connecting to form a structure with a comblike topology, the b ranch corresponding to the soft state forming the spine and the branches co rresponding to the various hard states forming the teeth of the comb. The p ower spectral properties of the source were strongly correlated with its po sition on the branches. The broadband noise became stronger and changed fro m power law-like to band-limited, as the spectrum became harder. Three type s of quasi-periodic oscillations (QPOs) were found: 1-18 Hz and 102-284 Hz QPOs on the hard branches, and 16-18 Hz QPOs on and near the soft branch. T he 1-18 Hz QPOs on the hard branches could be divided into three subtypes. The frequencies of the high- and low-frequency QPOs on the hard branches we re correlated with each other and were anticorrelated with spectral hardnes s. The changes in QPO frequency suggest that the inner disk radius only inc reases by a factor of 3-4 as the source changes from a soft to a hard state . Our results on XTE J1550-564 strongly favor a two-dimensional description of black hole behavior, where the regions near the spine of the comb in th e color-color diagram can be identified with the high state, and the teeth with transitions from the high state, via the intermediate state (which inc ludes the very high state) to the low state, and back. The two physical par ameters underlying this two-dimensional behavior vary to a large extent ind ependently and could for example be the accretion rate through the disk and the size of the Comptonizing region causing the hard tail. The difference between the various teeth is then associated with the mass accretion rate t hrough the disk, suggesting that high state <----> low state transitions ca n occur at any disk mass accretion rate and that these transitions are prim arily caused by another, independent parameter. We discuss how this picture could tie in with the canonical, one-dimensional behavior of black hole ca ndidates that has usually been observed.