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

Accretion and ejection in black-hole X-ray transients

Aims: We summarize the current observational picture of the outbursts of black-hole X-ray transients (BHTs), based on the evolution traced in a hardness-luminosity diagram (HLD), and we offer a physical interpretation. Methods: The basic ingredient in our interpretation is the Poynting-Robertson Cosmic Battery (PRCB, Contopoulos & Kazanas 1998), which provides locally the poloidal magnetic field needed for the ejection of the jet. In addition, we make two assumptions, easily justifiable. The first is that the mass-accretion rate to the black hole in a BHT outburst has a generic bell-shaped form. This is guaranteed by the observational fact that all BHTs start their outburst and end it at the quiescent state. The second assumption is that at low accretion rates the accretion flow is geometrically thick, ADAF-like, while at high accretion rates it is geometrically thin. Results: Both, at the beginning and the end of an outburst, the PRCB establishes a strong poloidal magnetic field in the ADAF-like part of the accretion flow, and this explains naturally why a jet is always present in the right part of the HLD. In the left part of the HLD, the accretion flow is in the form of a thin disk, and such a disk cannot sustain a strong poloidal magnetic filed. Thus, no jet is expected in this part of the HLD. The counterclockwise traversal of the HLD is explained as follows: the poloidal magnetic field in the ADAF forces the flow to remain ADAF and the source to move upwards in the HLD rather than to turn left. Thus, the history of the system determines the counterclockwise traversal of the HLD. As a result, no BHT is expected to ever traverse the entire HLD curve in the clockwise direction. Conclusions: We offer a physical interpretation of accretion and ejection in BHTs with only one parameter, the mass transfer rate.Comment: Accepted for publication in A&

Modelling spectral and timing properties of accreting black holes: the hybrid hot flow paradigm

The general picture that emerged by the end of 1990s from a large set of optical and X-ray, spectral and timing data was that the X-rays are produced in the innermost hot part of the accretion flow, while the optical/infrared (OIR) emission is mainly produced by the irradiated outer thin accretion disc. Recent multiwavelength observations of Galactic black hole transients show that the situation is not so simple. Fast variability in the OIR band, OIR excesses above the thermal emission and a complicated interplay between the X-ray and the OIR light curves imply that the OIR emitting region is much more compact. One of the popular hypotheses is that the jet contributes to the OIR emission and even is responsible for the bulk of the X-rays. However, this scenario is largely ad hoc and is in contradiction with many previously established facts. Alternatively, the hot accretion flow, known to be consistent with the X-ray spectral and timing data, is also a viable candidate to produce the OIR radiation. The hot-flow scenario naturally explains the power-law like OIR spectra, fast OIR variability and its complex relation to the X-rays if the hot flow contains non-thermal electrons (even in energetically negligible quantities), which are required by the presence of the MeV tail in Cyg X-1. The presence of non-thermal electrons also lowers the equilibrium electron temperature in the hot flow model to <100 keV, making it more consistent with observations. Here we argue that any viable model should simultaneously explain a large set of spectral and timing data and show that the hybrid (thermal/non-thermal) hot flow model satisfies most of the constraints.Comment: 26 pages, 13 figures. To be published in the Space Science Reviews and as hard cover in the Space Sciences Series of ISSI - The Physics of Accretion on to Black Holes (Springer Publisher

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

What is the origin of the soft excess in active galactic nuclei?

We investigate the nature of the soft excess below 1 keV observed in active galactic nuclei. We use the XMM–Newton data of the low-redshift, optically bright quasar, PG 1211+143, and we compare it with the narrow-line Seyfert 1 galaxy, 1H 0707−495, which has one of the strongest soft excesses seen. We test various ideas for the origin of the soft X-ray excess, including a separate spectral component (for example, low-temperature Comptonized emission), a reflection-dominated model, or a complex absorption model. All three can give good fits to the data, and χ2-fitting criteria are not sufficient to discriminate among them. Instead, we favour the complex absorption model on the grounds that it requires less extreme parameters. In particular, the geometry appears to be more physically plausible as the reflected component in the smeared absorption model is no longer dominant, and relativistic distortions, while still clearly present, are not tremendously larger than expected for a disc around a Schwarzchild black hole

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

High-redshift radio-quiet quasars: Exploring the parameter space of accretion models. I. Hot semispherical flow

Two families of models are currently considered to describe an accretion flow onto black holes and production of the observed X-ray radiation: (1) a standard cold accretion disk with a hot corona above it and (2) an outer truncated accretion disk with a hot semispherical inner flow. We compute spectra in the scenario with a hot inner flow surrounded by a truncated accretion disk covered by a hot corona and test the results on a sample of high-redshift (z > 4) quasars observed with Chandra. We find that in order to reproduce the ratio of optical to X-ray fluxes (the ox parameter), the optical depth of the Comptonizing plasma has to be rather low ( = 0.020.25 in the corona above the disk, and = 0.100.70 in the hot inner flow). This, together with the observed X-ray photon indices, implies either a high temperature in a thermal plasma (kTe = 90500 keV) or a nonthermal electron distribution in the plasma. We put an upper limit on the disk truncation radius, rtr 40RS. The modeled accretion rate is high, > 0.2Edd, which may suggest that high-z radio-quiet quasars are analogs of X-ray binaries in their high or very high state

High-redshift radio-quiet quasars: Exploring the parameter space of accretion models. II. Patchy corona model

We modeled the spectral energy distribution of high-redshift radio-quiet quasars (high-z RQQs). We computed spectra in a patchy corona geometry in which an accretion disk extends to the last stable orbit and the Comptonizing active regions (hot clouds) are distributed above the disk. We explored the model parameter space to find theoretical parameters that give spectra with an optical/UV luminosity, X-ray loudness, and X-ray photon index compatible with those of high-z RQQs observed with Chandra. We found that a range of solutions is possible, from high-kTe low- to low-kTe high- models. The solutions require a low level of energy dissipation in the hot clouds and a low disk covering factor. The modeled mass is on the order of 1010 M, and the accretion rate is ˙M 0:2 ˙MEdd. We compare our results with those obtained previously for a hot inner flow geometry

Evidence for a change in the X-ray radiation mechanism in the hard state of Galactic black holes

We present results on spectral variability of two Galactic black hole X-ray binaries, GRO J1655-40 and GX 339-4, in the hard state. We confirm a transition in behaviour of the photon index with luminosity, such that the well known decrease in X-ray photon index with decreasing luminosity only continues down to L_bol ~ 0.01 L_Edd . Below this point the photon index increases again. For Comptonisation models, this implies that the ratio of the Compton luminosity to seed photon luminosity, lh/ls, changes with bolometric luminosity, consistent with a scenario where seed photons change from cyclo-synchrotron at the lowest luminosities to those from a truncated disc. Alternatively, the transition could mark the point below which the non-thermal jet starts to dominate, or where reprocessed photons replace the viscous ones in an outflowing corona model.Comment: 10 pages, 5 figures, MNRAS, in pres

Stochastic Modeling of the Fermi/LAT gamma-ray Blazar Variability

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