The discovery of active galaxies can be traced back to 1943, when Carl Seyfert published an article in which he studied six galaxies with a bright point-like nucleus, whose spectrum presented intense emission lines superimposed on a blue continuum. The division between radio and non-radio objects is therefore in fact a division between Active Galactic Nuclei (AGNs) with the jet and jetless AGNs. Observation and interpretation of AGN spectra has led to the publication of thousands of articles and the holding of dozens of conferences, and they continue to grow with the advent of large surveys. Magneto-rotational instability is probably the mechanism by which the turbulence necessary for accretion is produced in the disk. A major hurdle when unifying AGNs, especially with the generalization of large surveys, is that they are based on diagnostics that bias the samples.

We are approaching the 50th anniversary of the discovery of quasars. Those old enough to have been cognizant of astronomy in 1962–1963 can remember the sense of excitement connected with this finding. There was talk of a major new constituent of the universe. The excitement of the discovery was palpable even to one of us (the most senior of the editors) who was then a high school teenager.

How will quasar studies be pursued in the near future? Starting from a summary of past achievement, we ask questions on expected instrumental and computational advancements that can shape quasar research in a foreseeable way. There is excitement due to the planned large new-generation telescopes, ground-based and in space. New surveys will lead to an order-of-magnitude increase of known quasars.

Outflows in Active Galactic Nuclei (AGN) play an important role in the evolution of their host galaxy and on the enrichment of the intergalactic medium. In the X-ray range, the outflows appear as a warm absorber (WA) gas displaying high bulk velocities.The Absorption Measure Distribution (AMD) method provides a new handle on constraining the physics of the WA. Here, we report on the theoretical interpretation of an AMD analysis of the Seyfert galaxy NGC 3783. Computations in constant total pressure performed with the transfer-photoionization code TITAN allow us to reproduce the WA gas ionic column-densities by adjusting some important plasma parameters, e.g., the ionization parameter (xi), the total column density (N-H) or the amount of micro-turbulence.Results from both the AMD method and the TITAN code suggest the presence of a region where thermal instabilities are observed. This is compatible with the hypothesis of the WA being a clumpy, two-phase medium with colder, dense clouds embedded in a hotter, diffuse gas.

Results of a long-term monitoring ($\gtrsim 10$ years) of the broad line and continuum fluxes of three Active Galactic Nuclei (AGN), 3C 390.3, NGC 4151, and NGC 5548, are presented. We analyze the H$\alpha$ and H$\beta$ profile variations during the monitoring period and study different details (as bumps, absorption bands) which can indicate structural changes in the Broad Line Region (BLR). The BLR dimensions are estimated using the time lags between the continuum and the broad lines flux variations. We find that in the case of 3C 390.3 and NGC 5548 a disk geometry can explain both the broad line profiles and their flux variations, while the BLR of NGC 4151 seems more complex and is probably composed of two or three kinematically different regions.

For more than twenty years, strong FeII emission lines have been observed in Active Galactic Nuclei and in particular in Narrow Line Seyfert 1 galaxies. A quick overview of the observations and of the models proposed to interpret the Fe II spectrum is given. The influence of atomic data and of physical parameters are discussed, and it is shown that the strengths of the Fe II lines cannot be explained in the framework of photoionization models. A non-radiative heating, for instance due to shocks, with an overabundance of iron, can help to solve the problem. A comparison with other objects emitting intense Fe II lines favors also the presence of strong outflows and shocks. We suggest some issues in the context of AGN evolution.

Aims. Results of long-term spectral monitoring of the active galactic nucleus of NGC 4151 are presented (11 years, from 1996 to 2006). Methods. High quality spectra (S /N > 50 in the continuum near Hα and Hβ) were obtained in the spectral range ∼4000 to 7500 A, with a resolution between 5 and 15 A, using the 6-m and the 1-m SAO’s telescopes (Russia), the GHAO’s 2.1-m telescope (Cananea, Mexico), and the OAN-SPM’s 2.1-m telescope (San-Pedro, Mexico). The observed fluxes of the Hα ,H β ,H γ ,a nd HeIIλ4686 emission lines and of the continuum at the observed wavelength 5117 A were corrected for the position angle, the seeing, and the aperture effects. Results. We found that the continuum and line fluxes varied strongly (up to a factor 6) during the monitoring period. The emission was maximum in 1996−1998, and there were two minima in 2001 and in 2005. As a consequence, the spectral type of the nucleus changed from a Sy1.5 in the maximum activity state to a Sy1.8 in the minimum state. The Hα ,H γ ,a nd Heλ4686 fluxes correlated well with the Hβ flux. The line profiles were strongly variable, showing changes of the blue and red asymmetry. The flux ratios of the blue/red wings and of the blue (or red) wing/core of Hα and Hβ varied differently. We considered three characteristic periods during which the Hβ and Hα profiles were similar: 1996−1999, 2000−2001, and 2002−2006. The line-to-continuum flux ratios were different; in particular during the first period (1996–1999), the lines were not correlated with the continuum and saturated at high fluxes. In the second and third periods (2000–2006), where the continuum flux was small, the Hα and Hβ fluxes were well correlated to the continuum flux, meaning that the ionizing continuum was a good extrapolation of the optical continuum. The CCFs are often asymmetrical and the time lags between the lines and the continuum are badly defined, indicating the presence of a complex BLR, with dimensions from 1 to 50 light-days. Conclusions. We discuss the different responses of Hβ and Hα to the continuum during the monitoring period.

Context.Keplerian accretion discs around massive black holes (MBHs) are gravitationally unstable beyond a few hundredths of a parsec, and they should collapse to form stars.It has indeed been shown recently that an accretion/star formation episode took place a few million years ago in the Galactic center (GC).This raises the question of how the disc can survive in AGN and quasars and continue to transport matter towards the black hole.Aims.We study the accretion/star formation process in quasars and AGN with one aim in mind: to show that a spectrum similar to the observed one can be produced by the disc.Methods.We compute models of stationary accretion discs that are either continuous or clumpy.Continuous discs must be maintained in a state of marginal stability so that the rate of star formation remains modest and the disc is not immediately destroyed.The disc then requires additional heating and additional transport of angular momentum.In clumpy discs, the momentum transport is provided by cloud interactions.Results.Non-viscous heating can be provided by stellar illumination, but in the case of continuous discs, even momentum transport by supernovae is insufficient for sustaining a marginal state, except at the very periphery of the disc.In clumpy discs it is possible to account for the required accretion rate through interactions between clouds, but this model is unsatisfactory because its parameters are tightly constrained without any physical justification.Conclusions.Finally one must appeal to non-stationary discs with intermittent accretion episodes like those that occurred in the GC, but such a model is probably not applicable either to luminous high redshift quasars or to radio-loud quasars.

The main properties of AGN are reviewed, focussing on the accretion process and on the question of whether AGN are the best factories of ultra high energy particles and photons. I recall the large differences between the accretion/ejection flows in strong and weak accretors, and I conclude that, since low luminosity AGN and even "dormant" massive black holes in nuclei of galaxies are powering strong confined magnetized jets able to accelerate high energy particles, and are present in a large proportion of galaxies, they might be better potential sources of high energy particles and photons than luminous AGN and powerful radio galaxies.

Context. A photoionized gas in thermal equilibrium can display a thermal instability, with three or more solutions in the multi-branch region of the S-shape curve that gives 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.Aims. The subject of our study is the thermal instability in optically thick, stratified media in total pressure equilibrium. We are also interested in comparing photoionization models issued from the hot and cold stable solutions with the currently used models, which are computed with an approximate, intermediate solution.Methods. We developed a new algorithm that selects the hot/cold stable solution and there from computes a fully consistent photoionization model. We 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.Results. We demonstrate that the thermal instability problem is quite different in thin and thick media. Models computed with the hot/cold stable solution and with an intermediate solution differ throughout the gas slab, with the spectral distribution changing as the radiation progresses inside the ionized gas. These effects depend on the thickness of the medium and on its ionization.Conclusions. This has observational implications for the emitted/absorbed spectra, ionization states, and variability. However impossible it is to know what solution the plasma will adopt when attaining the multi-solution regime, we expect the emitted/absorbed spectrum to be intermediate between those resulting from pure cold and hot models; such a phase-mixed medium can be reproduced well by intermediate solution models. Large spectral fluctuations corresponding to the onset of a cold/hot solution could be observed in timescales on 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.

Context.A photoionized gas in thermal equilibrium can display a thermal instability, with three or more solutions in the multi-branch region of the S-shape curve that gives 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 "Warm Absorber" (WA) observed in Active Galactic Nuclei (AGN) displays zones of different density, temperature and ionization. Our approach to the study of the WA relies on the assumption of total pressure equilibrium, which results in the natural stratification of the medium and allows to explain the presence of lines from different ionization states in many AGN observed by XMM-Newton and Chandra. We have used the photoionization code TITAN, developed by our team, to calculate a grid of constant total pressure models dedicated to fit the WA in NGC 3783. Our study shows that the WA can be modelled in pressure equilibrium. Finally, this work provides a good example of the application of the TITAN code to the study of the WA in AGN, and opens perspectives for its use by a larger community, through a grid of constant total pressure models to be made available via XSPEC and/or via Virtual Observatory facilities.

Context. Scatter around the relationship between central black hole masses in active galactic nuclei (AGNs) obtained by reverberation-mapping methods and host-galaxy bulge velocity dispersion indicates that the masses are uncertain typically by a factor of about three. Aims. In this paper, we try to identify the sources and systematics of this uncertainty. Methods. We characterize the broad Hemission-line profiles by the ratio of their full-width at half maximum (FWHM) to their line dispersion, i.e., the second moment of the line profile. We use this parameter to separate the reverberation-mapped AGNs into two populations, the first with narrower Hlines that tend to have relatively extended wings, and the second with broader lines that are relatively flat-topped. The first population is characterized by higher Eddington ratios than the second. Within each population, we calibrate the black-hole mass scale by comparison of the reverberation-based mass with that predicted by the bulge velocity dispersion. We also use the distribution of ratios of the reverberation-based mass to the velocity-dispersion mass prediction in a comparison with a "generalized thick disk" model in order to see if inclination can plausibly account for the observed distribution. Results. We find that the line dispersion is a less biased parameter in general than FWHM for black hole mass estimation, although we show that it is possible to empirically correct for the bias introduced by using FWHM to characterize the emission- line width. We also argue that inclination effects are apparent only in some small subset of the reverberation-based mass measurements; it is primarily the objects with the narrowest emission lines that seem to be most strongly affected. Conclusions. Our principal conclusion is that the Hprofile is sensitive primarily to Eddington ratio, but that inclination effects play a role in some cases.

In the 2-10 keV range, the AGN continuum is generally well represented by a single power law but at lower energies it displays an excess with respect to the extrapolation of this power law, called the "soft X-ray excess"; the nature of this component is still under discussion. Until now the soft X-ray excess was attributed either to the reflection of the hard X-rays on the accretion disk, or to the presence of an additional Comptonizing medium. This feature could also be due to the absorption of an intrinsically steep power law source (whose origin is not clear) by a medium with a very large dispersion velocity (as a relativistic wind). Understanding the nature of the soft X-ray excess is essential for our knowledge of the Warm Absorber, the primary spectrum, and the accretion flow process. We have therefore examined the pros and cons of the reflection and absorption models. The observed soft X-ray spectra may probably be modeled by an "hybrid" model: absorption and reflection.

We present the results of spectral monitoring of NGC 5548 with the 6-m and 1-m telescopes of SAO (Russia) and at INAOE's 2.1-m telescope of the GHO at Cananea (Mexico). The mean and rms profiles of Ha. and H beta present a double-peaked structure. The relative brightness of the peaks varies. The radial velocity of a red peak decreased from - 2500 - 2600 km s(-1) in 20002001 to similar to 2100 km s(-1) in 2002-2003. There was no apparent correlation between the variations of the radial velocities of this peak and the average continuum flux. The fluxes of the various parts of the line profiles are well correlated with each other and also with the continuum flux. This indicates that the flux variability in different parts of the line profiles on short time scales is caused mainly by the reverberation effect. Our results favor the formation of the broad Balmer lines of NGC 5548 in a turbulent accretion disk with large and moving optically thick inhomogeneities, capable of reprocessing radiation from the central continuum source.

With the advent of the present and future spatial X-ray missions, it becomes crucial to model correctly the line spectrum of X-ray emitting/absorbing media. We have built a photoionization code, TITAN, solving the transfer of a thousand lines and of the continuum with the “Accelerated Lambda Iteration” method (ALI), which is most reliable for line transfer. We give some details about this method and a justification for its use as a complement to usual approximations (e.g. , escape probability or two-stream) made at present in other codes. We show that the escape probability approximation leads to a wrong estimation of the emitted X-ray line intensities, especially in the soft X-ray range. The errors can exceed one order of magnitude in the case of thick media (Thomson thickness of the order of unity). It also happens, but for different reasons, in the case of moderately thin media (Thomson thickness of 0.001 to 0.1), characteristic of the Warm Absorber in Seyfert 1 or of the X-ray emitting medium in Seyfert 2. Using TITAN, new diagnostics based on He-like ion lines of the n=2 complex are proposed and some examples are presented here, concerning in particular the influence of the transfer method on the G ratio. We show an example of the influence of the direction on the emission/absorption spectrum. We also give insights about the influence of numerical methods for the computation of radiative transfer, and how to handle the ALI method in such problems where important gradients occur.