Agata Rozanska

Universal spectral shape of high accretion rate AGN

The spectra of quasars and NLS1 galaxies show surprising similarity in their spectral shape. They seem to scale only with the accretion rate. This is in contradiction with the simple expectations from the standard disk model which predicts lower disk temperature for higher black hole mass. Here we consider two mechanisms modifying the disk spectrum: the irradiation of the outer disk due to the scattering of the flux by the extended ionized medium (warm absorber) and the development of the warm Comptonizing disk skin under the eect of the radiation pressure instability. Those two mechanisms seem to lead to a spectrum which indeed roughly scales, as observed, only with the accretion rate. The scenario applies only to objects with relatively high Eddington ratio for which disk evaporation is inecient.

MODEL ATMOSPHERES AND X-RAY SPECTRA OF BURSTING NEUTRON STARS: HYDROGEN-HELIUM COMPTONIZED SPECTRA

Compton scattering plays a crucial role in determining the structure of the atmosphere of an X-ray burster and its theoretical spectrum. Our paper presents a description of the plane-parallel model atmosphere of a very hot neutron star and its theoretical flux spectrum of outgoing radiation. Our model equations take into account all bound-free and free-free monochromatic opacities relevant to hydrogen-helium chemical composition and take into account the effects of Compton scattering of radiation in thermal plasma with fully relativistic thermal velocities. We use Compton scattering terms in the equation of transfer, which precisely describe photon-electron energy and momentum exchange for photons with initial energies exceeding the electron rest mass of 511 keV. Model atmosphere equations are solved with the variable Eddington factors technique. The grid of H-He model atmospheres and flux spectra is computed on a dense mesh of 107 K ≤ Teff ≤ 3 × 107 K and a surface gravity of log g. In many cases, the assumed log g approached the critical gravity log gcr, i.e., the Eddington limit. We confirm that H-He spectra of X-ray bursters deviate from blackbody spectra and discuss their shapes. The table of color to effective temperature ratios shows that theoretical values of Tc/Teff do not exceed 1.9 in H-He atmospheres in hydrostatic and radiative equilibrium.

Vertical structure of accretion discs with hot coronae in active galactic nuclei

ABSTRA C T We study the vertical structure of a 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 blackbody. 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 active galactic nuclei.

Relativistic slim disks with vertical structure

We report on a scheme for incorporating vertical radiative energy transport into a fully relativistic, Kerr-metric model of optically thick, advective, transonic alpha disks. Our code couples the radial and vertical equations of the accretion disk. The flux was computed in the diffusion approximation, and convection is included in the mixing-length approximation. We present the detailed structure of this “two-dimensional” slim-disk model for α = 0.01. We then calculated the emergent spectra integrated over the disk surface. The values of surface density, radial velocity, and the photospheric height for these models differ by 20%‐30% from those obtained in the polytropic, height-averaged slim disk model considered previously. However, the emission profiles and the resulting spectra are quite similar for both types of models. The effective optical depth of the slim disk becomes lower than unity for high values of the alpha parameter and for high accretion rates.

Leaving the innermost stable circular orbit: the inner edge of a black-hole accretion disk at various luminosities

The “radiation inner edge” of an accretion disk is defined as the inner boundary of the region from which most of the luminosity emerges. Similarly, the “reflection edge” is the smallest radius capable of producing a significant X-ray reflection of the fluorescent iron line. For black hole accretion disks with very sub-Eddington luminosities these and all other “inner edges” coexist at the innermost stable circular orbit (ISCO). Thus, in this case, one may rightly consider ISCO as the unique inner edge of the black hole accretion disk. However, even at moderate luminosities, there is no such unique inner edge because differently defined edges are located at different places. Several of them are significantly closer to the black hole than ISCO. These differences grow with the increasing luminosity. For nearly Eddington luminosities, they are so huge that the notion of the inner edge loses all practical significance.

The structure and radiation spectra of illuminated accretion disks in AGN - II. Flare/spot model of X-ray variability

We discuss a model of the X-ray variability of active galactic nuclei (AGN). We consider multiple spots that originate on the surface of an accretion disk following intense irradiation by coronal flares. The spots move with the disk around the central black hole and eventually decay while new spots continuously emerge. We construct time sequences of the spectra of the spotted disk and compute the corresponding energy-dependent fractional variability amplitude. We explore the dependence on the disk inclination and other model parameters. AGN seen at higher inclination with respect to the observer, such as Seyfert 2 galaxies, are expected to have a fractional variability amplitude of the direct emission that is by a factor of a few higher than objects seen face on, such as Seyfert 1s.

Model atmospheres and X-ray spectra of bursting neutron stars II. Iron rich Comptonized Spectra

This paper presents the set of plane-parallel model atmosphere equations for a very hot neutron star (X-ray burst source). The model equations assume both hydrostatic and radiative equilibrium, and the equation of state of an ideal ga s in local thermodynamic equilibrium (LTE). The equation of radiative transfer includes terms describing Compton scattering of photons on free electrons in fully relativistic thermal motion, for photon energies approaching me c 2 . Model equations take into account many bound-free and free-free energy-dependent opacities of hydrogen, helium, and the iron ions, and also a dozen bound- bound opacities for the highest ions of iron. We solve model equations by partial linearisation and the technique of vari able Eddington factors. Large grid of H-He-Fe model atmospheres of X-ray burst sources has been computed for 10 7 ≤ Tef f ≤ 3×10 7 K, a wide range of surface gravity, and various iron abundances. We demonstrate that the spectra of X-ray bursters with iron present in the accreting matter differ significantly from pure H-He spectra (published in an earl ier paper), and also from blackbody spectra. Comptonized spectra with significant ir on abundance are generally closer to blackbody spectra than spectra of H-He atmospheres. The ratio of color to effective temperatures in our grid always remains in the range 1.2< Tc/Tef f < 1.85. The present grid of model atmospheres and theoretical X-ray spectra will be used to determine the effective temperatures, radii and M/R ratios of bursting neutron stars from observational data.

THE ORIGIN OF EMISSION AND ABSORPTION FEATURES IN TON S180 CHANDRA OBSERVATIONS

We present a new interpretation of the Ton S180 spectrum obtained by the Chandra Spectrometer (Low Energy Transmission Grating). Several narrow absorption lines and a few emission disk lines have been successfully fitted to the data. We have not found any significant edges accompanying line emission. We propose an interpretation of narrow lines consistent with that of the recent paper by Krolik, in which a warm absorber is strongly inhomogeneous. Such a situation is possible in the so-called multiphase medium, where regions with different ionization states, densities, and temperatures may coexist in thermal equilibrium under constant pressure. We illustrate this scenario with theoretical spectra of radiation transferred through a stratified cloud with constant pressure (instead of constant density) computed by the TITAN code in plane-parallel approximation. Detected spectral features are faint, and their presence does not alter the broadband continuum. We model the broadband continuum of Ton S180 assuming an irradiated accretion disk with a dissipative warm skin. The set of parameters appropriate for the data cannot be determined uniquely, but models with low values of the black hole mass have too hot and radially extended a warm skin to explain the formation of soft X-ray disk lines seen in the data.

A new model for the Warm Absorber in NGC 3783 : a single medium in total pressure equilibrium

Context. Many active galactic nuclei exhibit X-ray features typical of the highly ionized gas called “Warm Absorber” (WA). Such a material appears to be stratified, displaying zones of di fferent density, temperature, and ionization. In this paper, we investigate the possibility of modelling the WA gas in NGC 3783 as a single medium in total pressure equilibrium. Aims. Our goal is to demonstrate that the WA can be well modelled assuming constant total pressure, in contrast to the current de scriptions that are based on the presence of multiple regions, each in constant density. The assumption of total pressure equilibrium yields a more physical description of the WA, resulting in the natural stratificati on of the ionized gas, and providing an explanation for the presence of lines from different ionization states, as observed in WA spectra. Methods. We have used the photoionization code TITAN, developed by our team, to compute a grid of constant total pressure models with the purpose of fitting the WA in NGC 3783. We have compared our models to the 900 ks Chandra spectrum of NGC 3783 and to previous studies where the WA was described by multiple zones of constant density. Results. In the case of NGC 3783, the WA features can be well reproduced by a clumpy, ionized gas with cosmic abondances, ionization parameter� = 2500 erg cm s −1 , column density NH = 4 10 22 cm −2 , and constant total pressure. Conclusions. We have shown that the WA in NGC 3783 can be modelled by a single medium in total pressure equilibrium; this is probably the case for other WAs currently described by multi-zone, constant density models. In addition, our work demonstrates that the TITAN code is well adapted to the study of the WA in active galactic nuclei, opening new prospects for the use of TITAN by a larger community.

Warm and optically thick dissipative coronae above accretion disks

In the past years, several observations of AGN and X-ray binaries have suggested the existence of a warm T around 0.5-1 keV and optically thick, \tau ~ 10-20, corona covering the inner parts of the accretion disk. These properties are directly derived from spectral fitting in UV to soft-X-rays using Comptonization models. However, whether such a medium can be both in radiative and hydrostatic equilibrium with an accretion disk is still uncertain. We investigate the properties of such warm, optically thick coronae and put constraints on their existence. We solve the radiative transfer equation for grey atmosphere analytically in a pure scattering medium, including local dissipation as an additional heating term in the warm corona. The temperature profile of the warm corona is calculated assuming it is cooled by Compton scattering, with the underlying dissipative disk providing photons to the corona. Our analytic calculations show that a dissipative thick, (\tau_{cor} ~ 10-12) corona on the top of a standard accretion disk can reach temperatures of the order of 0.5-1 keV in its upper layers provided that the disk is passive. But, in absence of strong magnetic fields, the requirement of a Compton cooled corona in hydrostatic equilibrium in the vertical direction sets an upper limit on the Thomson optical depth \tau_{cor} < 5 . We show this value cannot be exceeded independently of the accretion disk parameters. However, magnetic pressure can extend this result to larger optical depths. Namely, a dissipative corona might have an optical depth up to ~ 20 when the magnetic pressure is 100 times higher that the gas pressure. The observation of warm coronae with Thomson depth larger than ~ 5 puts tights constraints on the physics of the accretion disk/corona systems and requires either strong magnetic fields or vertical outflows to stabilize the system.