Fast Timing Analysis of Cygnus X-1 using SPI on board INTEGRAL

For the very first time, we report the high frequency analysis of Cyg X-1 up to hard X-ray using SPI on-board INTEGRAL. After analyzing the possible contribution fromthe background, and using INTEGRAL archive from March 2005 to May 2008, Power Density Spectra (PDS) were obtained until 130 keV. First, we show that their overall shape is very similar to that observed at lower energies, as they are well described by sets of Lorentzians. The strength of this fast variability (up to 40 Hz) does not drop at high energy since we show that it remains at ~25% rms, even in the highest energy bands. Second, the hard X-ray variability patterns of Cyg X-1 are state dependent: the softer the spectrum (or the lower the hardness ratio), the lower the total fractional variability and the higher the typical frequencies observed. The strength of total variability as a function of energy and state is then investigated. By comparison with simultaneous and published RXTE/PCA data, we showed that in the hard state, it remains quite constant in the 2-130 keV energy range. In our softer state, it is also flat until 50 keV and may increase at higher energy. The implications of this behavior on the models are then discussed.

On the variation of black hole accretion disc radii as a function of state and accretion rate

Black hole binary transients undergo dramatic evolution in their X-ray timing and spectral behaviour during outbursts. In recent years a paradigm has arisen in which soft X-ray states are associated with an inner disc radius at, or very close to, the innermost stable circular orbit (ISCO) around the black hole, while in hard X-ray states the inner edge of the disc is further from the black hole. Models of advective flows suggest that as the X-ray luminosity drops in hard states, the inner disc progressively recedes, from a few to hundreds gravitational radii. Recent observations which show broad iron line detections and estimates of the disc component strength suggest that a non-recessed disc could still be present in bright hard states. In this study we present a comprehensive analysis of the spectral components associated with the inner disc, utilising data from instruments with sensitive low-energy responses and including reanalyses of previously published results. A key component of the study is to fully estimate systematic uncertainties by e.g. investigating in detail the effect of having a hydrogen column density that is fixed or free to vary. We conclude that for L_x > 0.01 of the Eddington limit, spectral fits allow us to constrain the disc to be < 10R_g. There is, however, clear evidence that when L_x is between 10^-2-- 10^-3 Eddington, the disc does begin to recede. We include measurements of disc radii in two quiescent black hole binaries, and present the inferred evolution of accretion parameters in the entire range of bolometric luminosities 10^-8 -- 1 Eddington. We compare our results with theoretical models and note that the implied rate of disc recession with luminosity is consistent with recent empirical results on the X-ray timing behaviour of black holes of all masses.