Thermal instability in X-ray photoionized media in Active Galactic Nuclei: Influence on the gas structure and spectral features

A photoionized gas in thermal equilibrium can display a thermal instability, with 3 or more solutions in the multi-branch region of the S-shape curve giving the temperature versus the radiation-to-gas-pressure ratio. Many studies have been devoted to this curve and to its dependence on different parameters, always in the optically thin case. The purpose of our study is the thermal instability in optically thick, stratified media, in total pressure equilibrium. We have developped a new algorithm to select the hot/cold stable solution, and thereof to compute a fully consistent photoionization model. We have implemented it in the TITAN code and computed a set of models encompassing the range of conditions valid for the Warm Absorber in Active Galactic Nuclei. We have demonstrated that the thermal instability problem is quite different in thin or thick media. In thick media the spectral distribution changes as the radiation progresses inside the ionized gas. This has observational implications in the emitted/absorbed spectra, ionization states, and variability. However impossible to know what solution the plasma will adopt when attaining the multi-solutions regime, we expect the emitted/absorbed spectrum to be intermediate between those resulting from pure cold and hot models. Large spectral fluctuations corresponding to the onset of a cold/hot solution could be observed in timescales of the order of the dynamical time. A strong turbulence implying supersonic velocities should permanently exist in the multi-branch region of thick, stratified, pressure equilibrium media.Comment: LaTeX file: 18 pages, including 14 figures. Accepted for publication in Astronomy & Astrophysic

Reprocessing of X-rays in AGN. I. Plane parallel geometry -- test of pressure equilibrium

We present a model of the vertical stratification and the spectra of an irradiated medium under the assumption of constant pressure. Such a solution has properties intermediate between constant density models and hydrostatic equilibrium models, and it may represent a flattened configuration of gas clumps accreting onto the central black hole. Such a medium develops a hot skin, thicker than hydrostatic models, but thinner than constant density models, under comparable irradiation. The range of theoretical values of the alpha_ox index is comparable to those from hydrostatic models and both are close to the observed values for Seyfert galaxies but lower than in quasars. The amount of X-ray Compton reflection is consistent with the observed range. The characteristic property of the model is a frequently multicomponent iron K alpha line.Comment: accepted for publication in Astronomy and Astrophysic

X-ray He-like ions diagnostics: New Computations for Photoionized Plasmas: I. preliminary considerations

Using the new version of the photoionization code Titan designed for plane-parallel photoionized thick hot media, which is unprecedented from the point of view of line transfer, we have undertaken a study of the influence of different parameters on the He-like and H-like emission of a medium photoionized by an X-ray source. We explain why in modelling the emitting medium it is important to solve in a self-consistent way the thermal and ionization equilibria and to take into account the interconnection between the different ions. We give the equivalent widths of the sum of the He-like triplets and the triplet intensity ratios $G$ and $R$, for the most important He-like ions, for a range of density, column density, and ionization parameter, in the case of constant density media. We show that the line intensities from a given ion can be accounted for, either by small values of both the column density and of the ionization parameter, or by large values of both quantities, and it is necessary to take into account several ions to disentangle these possibilities. We show also that a "pure recombination spectrum" almost never exists in a photoionized medium: either it is thin, and resonance lines are formed by radiative excitation, or it is thick, and free-bound absorption destroys the resonance photons as they undergo resonant diffusion.Comment: 19 pages, 14 figures, accepted in A &

Constraints for the accretion disk evaporation rate in AGN from the existence of the Broad Line Region

We analyze the consequences of the hypothesis that the formation of the Broad Line Region is intrinsically connected with the existence of the cold accretion disk. We assume that the Broad Line Region radius is well estimated by the formula of Kaspi et al. (2000). We consider three models of the disappearance of the inner disk which limit the existence of the Broad Line Region: (i) classical ADAF approach, i.e. the inner hot flow develops whenever it can exist (ii) disk evaporation model of Meyer & Meyer-Hofmeister (2002) (iii) generalized disk evaporation model of Rozanska & Czerny (2000b). For each of the models, we determine the minimum value of the Eddington ratio and the maximum value of the broad line widths as functions of the viscosity parameter alpha and the magnetic field parameter beta. We compare the predicted parameter space with observations of several AGN. Weak dependence of the maximum value of the FWHM and minimum value of the Eddington ratio on the black hole mass in our sample is noticeable. It seems to favor the description of the cold disk/hot inner flow transition as in the classical ADAF approach than with the model of disk evaporation due to conduction between the disk and accreting corona.Comment: A&A, 428, 39 (2004

Evolution of the X-ray spectrum in the flare model of Active Galactic Nuclei

Nayakshin & Kazanas (2002) have considered the time-dependent illumination of an accretion disc in Active Galactic Nuclei, in the lamppost model. We extend their study to the flare model, which postulates the release of a large X-ray flux above a small region of the accretion disc. A fundamental difference with the lamppost model is that the region of the disc below the flare is not illuminated before the onset of the flare. A few test models show that the spectrum which follows immediately the increase in continuum flux should display the characteristics of a highly illuminated but dense gas, i.e. very intense X-ray emission lines and ionization edges in the soft X-ray range. The behaviour of the iron line is different in the case of a "moderate" and a ``strong'' flare: for a moderate flare, the spectrum displays a neutral component of the Fe K$\alpha$ line at 6.4 keV, gradually leading to more highly ionized lines. For a strong flare, the lines are already emitted by FeXXV (around 6.7 keV) after the onset, and have an equivalent width of several hundreds of eV. We find that the observed correlations between $R$, $\Gamma$, and the X-ray flux, are well accounted by a combination of flares having not achieved pressure equilibrium, strongly suggesting that the observed spectrum is dominated by regions in non-pressure equilibrium, typical of the onset of the flares. Finally a flare being confined to a small region of the disc, the spectral lines should be narrow (except for a weak Compton broadening), Doppler shifted, and moving.Comment: 14 pages, 13 figures, accepted in A & A, english corrected versio

Leaving the ISCO: 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" locate at ISCO. Thus, in this case, one may rightly consider ISCO as the unique inner edge of the black hole accretion disk. However, even for moderate luminosities, there is no such unique inner edge as differently defined edges locate at different places. Several of them are significantly closer to the black hole than ISCO. The differences grow with the increasing luminosity. For nearly Eddington luminosities, they are so huge that the notion of the inner edge losses all practical significance.Comment: 12 pages, 15 figures, submitted to A&

The LOFT perspective on neutron star thermonuclear bursts

This is a White Paper in support of the mission concept of the Large Observatory for X-ray Timing (LOFT), proposed as a medium-sized ESA mission. We discuss the potential of LOFT for the study of thermonuclear X-ray bursts on accreting neutron stars. For a summary, we refer to the paper.Comment: White Paper in Support of the Mission Concept of the Large Observatory for X-ray Timin

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

We discuss a model of X-ray variability of active galactic nuclei (AGN). We consider multiple spots which 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 fractional variability amplitude of the direct emission by a factor of a few higher than objects seen face on, such as the Seyfert 1s.Comment: Astronomy and Astrophysics (in press

Modeling the UBVRI time delays in Mrk 335

We develop a model of time delays between the continuum bands in the Narrow Line Seyfert 1 galaxy Mrk 335 to explain the observed delays measured in this source. We consider two geometries: an accretion disk with fully ionized warm absorber of considerable optical depth, located close to the symmetry axis, and an accretion disk with a hot corona. Both media lead to significant disk irradiation but the disk/corona geometry gives lower values of the time delays. Only the disk/corona models give results consistent with measurements of Sergeev et al., and a low value of the disk inclination is favored. The presence of an optically thick, fully ionized outflow is ruled out at the 2-sigma level.Comment: MNRAS (in press