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 transient 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 several different states, which could be divided into soft and hard states based on the relative strength of the high energy spectral component. These states showed up as distinct branches in the colorcolor and hardness-intensity diagrams, connecting to form a structure with a comb-like topology, the branch corresponding to the soft state forming the spine and the branches corresponding to the various hard states forming the teeth of the comb. The power spectral properties of the source were strongly correlated with its position on the branches. The broad band noise became stronger, and changed from power law like to band limited, as the spectrum became harder. Three types 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. The 1–18 Hz QPOs on the hard branches could be divided in three sub-types. The frequencies of the high and low frequency QPOs on the hard branches were correlated with each other, and anti-correlated with spectral hardness. The changes in QPO frequency suggest that the inner disc radius only increases 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 the color-color diagram can be identified with the high state, and the teeth with transitions from the high state, via the intermediate state (which includes the very high state) to the low state, and back. The two physical parameters underlying this two-dimensional behavior vary to a large extent independently and could for example be the accretion rate through the Astronomical Institute ’Anton Pannekoek’, University of Amsterdam, and Center for High Energy Astrophysics, Kruislaan 403, 1098 SJ, Amsterdam, The Netherlands MIT, Center for Space Research, Cambridge, MA 02139 – Chandra Fellow Osservatorio Astronomico di Brera, Via E. Bianchi 46, I-23807, Merate (LC), Italy University of Alabama in Huntsville, Department of Physics, Hunstville, AL 35899 Facultad de Ciencias Astronómicas y Geof́ısicas, Universidad Nacional de La Plata, Paseo del Bosquey S/N, 1900 La Plata, Argentina