4 research outputs found
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