Tracing the power-law component in the energy spectrum of black hole candidates as a function of the QPO frequency

We investigated the relation between the centroid frequency of the quasi-periodic oscillation observed in the power density spectra of a sample of galactic black-hole candidates with the power-law photon index obtained from spectral fits. Our aim is to avoid inner accretion disk radius determination directly from spectral fits, given the uncertainties of the absolute values obtained in that way, but to base our analysis on the likely association of QPO frequency to a characteristic radius. We used archival RXTE data of GRS 1915+105 and published parameters for GRO 1655-40, XTE J1550-564, XTE J1748-288 and 4U 1630-47. While for low values of the QPO frequency, the two parameters are clearly correlated for each source, there is evidence for a turnoff in the correlation above a characteristic frequency, different for different sources. We discuss the possible nature of this turnoff.Comment: 11 pages, 10 figures. Accepted for publication on Astronomy & Astrophysic

States and transitions in black-hole binaries

With the availability of the large database of black-hole transients from the Rossi X-Ray Timing Explorer, the observed phenomenology has become very complex. The original classification of the properties of these systems in a series of static states sorted by mass accretion rate proved not to be able to encompass the new picture. I outline here a summary of the current situation and show that a coherent picture emerges when simple properties such as X-ray spectral hardness and fractional variability are considered. In particular, fast transition in the properties of the fast time variability appear to be crucial to describe the evolution of black-hole transients. Based on this picture, I present a state-classification which takes into account the observed transitions. I show that, in addition to transients systems, other black-hole binaries and Active Galactic Nuclei can be interpreted within this framework. The association between these states and the physics of the accretion flow around black holes will be possible only through modeling of the full time evolution of galactic transient systems.Comment: 30 pages, 11 figures, To appear in Belloni, T. (ed.): The Jet Paradigm - From Microquasars to Quasars, Lect. Notes Phys. 794 (2009