Modelling the energy dependencies of X-ray quasi-periodic oscillations in accreting compact objects

We have constructed models of the quasi-periodic variability of X-ray emission from accreting compact objects. Assuming a general scenario of a propagation model of variability, with inverse Compton upscattering as the emission mechanism, we have considered a number of cases for the periodic modulation: modulation of the plasma heating rate, cooling rate by external soft photons and the amplitude of the reprocessed component. We have computed various observational characteristics which can be compared to good quality data. These include Fourier-frequency resolved spectra and the results of cross-correlation analysis between light curves at different energies. Each model of modulation predicts specific observational signatures, which help in identifying the physical processes driving quasi-periodic oscillations emission in accreting sources

Spectral and Fourier analyses of X-ray quasi-periodic oscillations in accreting black holes

We study energy dependencies of quasi-periodic oscillations (QPOs) from a number of black hole X-ray binaries. The selected sources were observed by RXTE at time periods close to state transitions and showed QPOs in the 1–10 Hz range. We have constructed QPO rms energy spectra, which provide information about underlying physical process leading to QPO generation. These spectra show an interesting anticorrelation with the time-averaged spectra. The QPO rms spectra are harder than the time-averaged spectra when the latter are soft, while they are softer than the time-averaged spectra when the latter are hard. We then discuss these observational results in the context of simple spectral variability models. Hard QPO spectra can be produced by quasi-periodic modulations of the heating rate of the Comptonizing plasma, while soft QPO spectra result from modulations of the cooling rate by soft photons

Relativistic distortions in the X-ray spectrum of CYG X-1

We present the first significant detection of relativistic smearing of the X-ray reflection spectrum from the putative accretion disk in the low/hard state of Cyg X-1. The ionization state, and amount of relativistic smearing are simultaneously constrained by the X-ray spectra, and we conclude that the disk is not strongly ionized, does not generally extend down to the last stable orbit at 3 Schwarzschild radii and covers rather less than half the sky as seen from the X-ray source. These results are consistent with a geometry where the optically thick disk truncates at a few tens of Schwarzschild radii, with the inner region occupied by the X-ray hot, optically thin(ish) plasma. Such a geometry is also inferred from previous studies of the reflected spectrum in Galactic Black Hole transient sources, and from detailed considerations of the overall continuum spectral shape, suggesting that this is a robust feature of low/hard state accretion onto Galactic Black Holes.Comment: MNRAS, accepted by refere

Spectral modeling of the three spectral states of the galactic microquasar GRS 1915+105

We have analyzed Rossi X-ray Timing Explorer spectral data of the Galactic micro-quasar GRS 1915+105 in its various spectral states, as defined by Belloni et al. (2000). In states A and B the spectra are dominated by a strong soft thermal component, accompanied by a weak harder tail. The soft component is rather complex and cannot be described as a simple accretion disk emission. Relativistic effects in Kerr metric contribute to the complexity of the soft component but are not sufficient to fully account for it. As found previously, state C spectra are dominated by a Comptonized component, with small contribution from disk photons. The X-ray reprocessed component is highly significant in those spectra and, in contrast to the usual hard state spectra from accreting black holes, it is highly ionized

Geometry of Accretion in Soft X-Ray Transients

We present results of modelling Ginga data of a number of Soft X-ray Transient sources performed with the aim of constraining the geometry of accretion during various stages of the sources' evolution. Assuming a generic geometry of a central, extended X-ray source with an external, optically thick accretion disk, we use consistent models of X-ray reprocessing to constrain the inner radius of the accretion disk and its ionization state. We show that the evolution of GS 1124-68 (Nova Muscae 1991) and GS 2000+25 agree qualitatively with the recent ideas linking the high/low state transition with changing the inner radius of the optically thick disk, but the case of GS 2023+33 (V404 Cyg) poses a problem for any model.Comment: Aipproc LaTeX (4 pages, 4 figures), to appear in Proc. of the 8th October Astrophysics Conference in Maryland, "Accretion Processes in Astrophysical Systems", S. S. Holt & T. Kallman (eds.