Testing a model of variability of X-ray reprocessing features in Active Galactic Nuclei

A number of recent results from X-ray observations of Active Galactic Nuclei involving the Fe K alpha line (reduction of line variability compared to the X-ray continuum variability, the X-ray ``Baldwin effect'') were attributed to a presence of a hot, ionized skin of an accretion disc, suppressing emission of the line. The ionized skin appears as a result of the thermal instability of X-ray irradiated plasma. We test this hypothesis by computing the Thomson thickness of the hot skin on top of the 'alpha P_tot' Shakura-Sunyaev disc, by simultaneously solving the vertical structure of both the hot skin and the disc. We then compute a number of relations between observable quantities, e.g. the hard X-ray flux, amplitude of the observed reprocessed component, relativistic smearing of the K alpha line, the r.m.s. variability of the hard X-rays. These relations can be compared to present and future observations. We point out that this mechanism is unlikely to explain the behaviour of the X-ray source in MCG-6-30-15, where there is a number of arguments against the existence of a thick hot skin, but it can work for some other Seyfert 1 galaxies.Comment: 12 pages. MNRAS, accepte

Modelling the energy dependencies of high-frequency QPO in black hole X-ray binaries

We model energy dependencies of the quasi periodic oscillations (QPO) in the model of disc epicyclic motions, with X-ray modulation caused by varying relativistic effects. The model was proposed to explain the high frequency QPO observed in X-ray binaries. We consider two specific scenarios for the geometry of accretion flow and spectral formation. Firstly, a standard cold accretion disc with an active X-ray emitting corona is assumed to oscillate. Secondly, only a hot X-ray emitting accretion flow oscillates, while the cold disc is absent at the QPO radius. We find that the QPO spectra are generally similar to the spectrum of radiation emitted at the QPO radius, and they are broadened by the relativistic effects. In particular, the QPO spectrum contains the disc component in the oscillating disc with a corona scenario. We also review the available data on energy dependencies of high frequency QPO, and we point out that they appear to lack the disc component in their energy spectra. This would suggest the hot flow geometry in the spectral states when high frequency QPO are observed.Comment: 8 pages, accepted for publication in MNRA

On the light-bending model of X-ray variability of MCG-6-30-15

We apply the light bending model of X-ray variability to Suzaku data of the Seyfert 1 galaxy MCG-6-30-15. We analyze the energy dependence of the root mean square (rms) variability, and discuss conditions necessary for the model to explain the characteristic decrease of the source variability around 5-8 keV. A model, where the X-ray source moves radially rather than vertically close to the disk surface, can indeed reproduce the reduced variability near the energy of the Fe Kalpha line, although the formal fit quality is poor. The model then predicts the energy spectra, which can be compared to observational data. The spectra are strongly reflection dominated, and do not provide a good fit to Suzaku spectral data of the source. The inconsistency of this result with some previous claims can be traced to our using data in a broader energy band, where effects of warm absorber in the spectrum cannot be neglected.Comment: 6 pages, PASJ, accepte

The role of advection in the accreting corona model for active galactic nuclei and Galactic black holes

We consider the role of advection in the two-temperature accreting corona with an underlying optically thick disc. The properties of coronal solutions depend significantly on the description of advection. Local parameterization of advection by a constant coeficient $\delta$ replacing the radial derivatives lead to complex topology of solutions, similar to some extent to other advection-dominated accretion flow solutions. One, radiatively cooled branch exists for low accertion rates. For higher accretion rates two solutions exist in a broad range of radii: one is radiatively cooled and the other one is advection-dominated. With further increase of accretion rate the radial extensions of the two solutions shrink and no solutions are found above certain critical value. However, these trends change if the local parameterization of advection is replaced by proper radial derivatives computed iteratively from the model. Only one, radiatively cooled solution remains, and it exists even for high accretion rates. The advection-dominated branch disappears during the iteration process which means that a self-consistently described advection-dominated flow cannot exist in the presence of an underlying cold disc.Comment: 12 pages, 8 figures, accepted to MNRA

Vertical Structure of Accretion Discs with Hot Coronae in AGN

We study vertical structure of radiation pressure dominated disc with a hot corona. We include all the relevant processes like bound--free opacity and convection. We show that the presence of the corona modifies considerably the density and the opacity of the disc surface layers which are important from the point of view of spectrum formation. The surface of the disc with a corona is much denser and less ionized than the surface of a bare disc. Such a disc is likely to produce a neutral reflection and a local spectrum close to a black body. This effect will help to reconcile the predictions of accretion disc models with the observational data since a neutral reflection and a lack of Lyman edge are generally seen in AGN.Comment: 10 pages, 9 figures, submitted to MNRA

Can the cosmic x ray and gamma ray background be due to reflection of a steep power law spectrum and Compton scattering by relativistic electrons?

We reconsider the recent model for the origin in the cosmic X-ray and gamma-ray background by Rogers and Field. The background in the model is due to an unresolved population of AGNs. An individual AGN spectrum contains three components: a power law with the energy index of alpha = 1.1, an enhanced reflection component, and a component from Compton scattering by relativistic electrons with a low energy cutoff at some minimum Lorentz factor, gamma(sub min) much greater than 1. The MeV bump seen in the gamma-ray background is then explained by inverse Compton emission by the electrons. We show that the model does not reproduce the shape of the observed X-ray and gamma-ray background below 10 MeV and that it overproduces the background at larger energies. Furthermore, we find the assumptions made for the Compton component to be physically inconsistent. Relaxing the inconsistent assumptions leads to model spectra even more different from that of the observed cosmic background. Thus, we can reject the hypothesis that the high-energy cosmic background is due to the described model