Abstract. In a previous paper, Collin & Huré (2001), using a sample of Active Galactic Nuclei (AGN) where the mass has been determined by reverberation studies (the Kaspi et al. 2000 sample), have shown that if the optical luminosity is emitted by a steady accretion disc, it implies that about half of the objects of the sample are accreting close to the Eddington rate or at super-Eddington rates. We discuss here this problem in more detail, evaluating different uncertainties, and we conclude that this result is unavoidable, unless the masses are strongly underestimated by reverberation studies. This can occur if the broad line region is a flat thin rotating structure with the same axis as the accretion disc, close to the line of sight. However the masses deduced from reverberation mapping in AGN follow the same correlation between the black hole mass and the bulge mass as normal galaxies (Laor 2001), suggesting that they are correct within a factor of a few. There are then three issues to the problem: 1. accretion proceeds at Eddington or super-Eddington rates in these objects through slim or thick discs; 2. the optical luminosity is not produced directly by the gravitational release of energy, but by another mechanism, so super-Eddington rates are not required; 3. accretion discs are completely "non standard". Presently neither the predictions of models nor the observed spectral distributions are sufficient to help choose between these solutions. In particular, even for the super-Eddington model, the observed optical to bolometric luminosity ratio would be of the order of the observed one. In the super-Eddington solution, there is a strong anti-correlation between the observed velocity widths of the lines and the computed Eddington ratios (i.e. the accretion rate to the Eddington rate ratios), the largest ratios corresponding to the narrowest lines, actually to "Narrow Line Seyfert 1" nuclei. For the considered sample, the Eddington ratio decreases with an increasing black hole mass, while the opposite is found if the accretion rate is assumed to be proportional to the optical-UV luminosity, as is usually done. If these results are extrapolated to all quasars, it implies that the amount of mass locked in massive black holes should be larger than presently thought. If the Eddington ratio is assumed to be smaller than unity, the optical luminosity has to be produced by an additional non-gravitational mechanism. It has to be emitted by a dense and thick medium located at large distances from the center (10 3 to 10 4 gravitational radii). It can be due to reprocessing of the X-ray photons from the central source in a geometrically thin warped disc, or in dense "blobs" forming a geometrically thick system, which can be a part of the accretion flow or can constitute the basis of an outflow. The third possibility is not explored here, as it requires completely new models of accretion discs which are still to be developed.

Abstract. In the era of XMM-Newton and Chandra missions, it is crucial to use codes able to compute correctly the line spectrum of X-ray irradiated thick media (Thomson thickness of the order of unity), in order to build models for the structure and the emission of the central regions of Active Galactic Nuclei (AGN), or of X-ray binaries. In all photoionized codes except in our code Titan, the line intensities are computed with the so-called "escape probability approximation". In its last version, Titan solves the transfer of a thousand lines and of the continuum with the "Accelerated Lambda Iteration" method, which is one of the most efficient and at the same time the most secure for line transfer. We first review the escape probability formalism and mention various reasons why it should lead to wrong results concerning the line fluxes. Then we check several approximations commonly used instead of line transfer in photoionization codes, by comparing them to the full transfer computation. We find that for conditions typical of the AGN or X-ray binary emission medium, all approximations lead to an overestimation of the emitted X-ray line spectrum, which can reach more than one order of magnitude. We show that it is due mainly to the local treatment of line photons, implying a delicate balance between excitations of X-ray transitions by the very intense underlying diffuse X-ray continuum (which are not taken properly into account in escape probability approximations) and the net rate of excitations by the diffuse line flux. The most affected lines are those in the soft X-ray range. Such processes are much less important in cooler and thinner media (like the Broad Line Region of AGN), as the most intense lines lie in the optical and near ultraviolet range where the diffuse continuum is small. We conclude that it is very important to treat correctly the transfer of the continuum to get the best results for the line spectrum. On the other hand the approximations used for the escape probabilities have a relatively small influence on the computed thermal and ionization structure of the surface layers, but in the deep layers, they lead to an overestimation of the ionization state. As a consequence the computed continuum emitted by the back (non-irradiated) side is not correct, and might be strongly overestimated in the EUV range.

Abstract. Nayakshin & Kazanas (2002) have considered the time-dependent illumination of an accretion disc in Active Galactic Nuclei, in the lamppost model, where it is assumed that an X-ray source illuminates the whole inner-disc region in a relatively steady way. 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 to the lamppost model is that the region of the disc below the flare is not illuminated before the onset of the flare. After the onset, the temperature and the ionization state of the irradiated skin respond immediately to the increase of the continuum, but pressure equilibrium is achieved later. A few typical test models show that the reflected spectrum that follows immediately the increase in continuum flux should always 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 however different in the case of a "moderate" and a "strong" flare: for a moderate flare, the spectrum displays a neutral component of the Fe Kα 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 are very intense, with an equivalent width of several hundreds eV. A strong flare is also characterized by a steep soft X-ray spectrum. The variation timescale in the flare model is likely smaller than in the lamppost model, due to the smaller dimension of the emission region, so the timescale for pressure equilibrium is long compared to the duration of a flare. It is therefore highly probable that several flares contribute at the same time to the luminosity. We find that the observed correlations between R, Γ, and the X-ray flux are well accounted for by a combination of flares having not achieved pressure equilibrium, also strongly suggesting that the observed spectrum is always 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) and Doppler shifted, as stressed by . All these features should constitute specific variable signatures of the flare model, distinguishing it from the lamppost model. It is however difficult, on the basis of the present observations and models, to conclude in favor of one of the hypothese.

Abstract. Using the VLT together with the near infrared instrument ISAAC, we have obtained medium spectral and high spatial resolution observations of a sample of nearby Seyfert galaxies in the H-band. This band is particularly suited for stellar population studies since the stellar component dominates over the AGN nucleus. The H-band also includes the peak contribution from cool stars. The AGN spectra are very rich in strong metallic lines which are sensitive to stellar luminosity class. For 4 out of 5 galaxies the central velocity dispersions are found to be significantly lower than reported in previous studies. Gradients in the stellar population within the central regions were searched for, together with evidence for dilution of the stellar spectral features within the nucleus.