arXiv subjects

Suzy Collin

Publications and source records attributed to Suzy Collin.

15 recordsLinked to original sources

arXiv2008

Are AGN the Best Factories for High Energy Particles and Photons?

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.

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arXiv2007

Star formation in accretion discs : from the Galactic Center to Active Galactic Nuclei

Keplerian accretion discs around massive black holes (MBHs) are gravitationally unstable beyond a few hundredths of parsec and should collapse to form stars. Indeed an accretion/star formation episode took place a few millions 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. We study the accretion/star formation process, with one aim in mind, to show that a spectrum similar to the observed AGN one can be produced by the disc. We compute models of stationary accretion discs, both continuous and clumpy. Continuous discs must be maintained in a state of marginal stability for the rate of star formation to remain modest, so they require additional heating and transport of angular momentum. Non-viscous heating can be provided by stellar illumination, but momentum transport by supernovae is insufficient to sustain 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 as its parameters are tightly constrained without any physical justification. 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 to luminous high redshift quasars neither to radio-loud quasars.

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arXiv2006

Uncertainties of the masses of black holes and Eddington ratios in AGN

Black hole masses in Active Galactic Nuclei have been determined in 35 objects through reverberation mapping of the emission line region. I mention some uncertainties of the method, such as the ``scale factor'' relating the Virial Product to the mass, which depends on the unknown structure and dynamics of the Broad Line Region. When the black hole masses are estimated indirectly using the empirical size-luminosity relation deduced from this method, the uncertainties can be larger, especially when the relation is extrapolated to high and low masses and/or luminosities. In particular they lead to Eddington ratios of the order of unity in samples of Narrow Line Seyfert 1. As the optical-UV luminosity is provided by the accretion disk, the accretion rates can be determined and are found to be much larger than the Eddington rates. So, accretion must be performed at a super-critical rate through a slim disk, resulting in rapid growth of the black holes. The alternative is that the mass determination is wrong at this limit.

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arXiv2006

Quasars and Galactic Nuclei,a Half-Century Agitated Story

I recall how the discovery of quasars occurred more than forty years ago, and the strong debates marking out their story. It led to the discovery of Massive Black Holes, which are now known to be present in almost all galaxies, and it opened on a coherent physical model and on a new vision of galaxy evolution.

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arXiv2006

Systematic Effects in Measurement of Black Hole Masses by Emission-Line Reverberation of Active Galactic Nuclei: Eddington Ratio and Inclination

We try to identify the sources and systematics of the uncertainty of the masses of the central black holes in active galactic nuclei (AGNs) which have been obtained by reverberation-mapping methods. We characterize the broad H$β$ emission-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 H$β$ lines with 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. 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 in a small subset of objects with the narrowest emission lines. Our principal conclusion is that the H$β$ profile is sensitive primarily to Eddington ratio, but that inclination effects play a role in some cases. (abridged)

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arXiv2005

The role of absorption and reflection in the soft X-ray excess of Active Galactic Nuclei : 1. Preliminary results

The 2-10 keV continuum of AGN is well represented by a single power law, generally attributed to a hot comptonizing medium, such as a corona above the accretion disk. At smaller energies the continuum displays an excess with respect to the extrapolation of this power law, called the ``soft X-ray excess". Until now it was attributed, either to reflection of the hard X-ray source by the accretion disk, or to the presence of an additional comptonizing medium. An alternative solution proposed by Gierliński & Done (2004) is that a single power law represents correctly both the soft and the hard X-ray emission, and the soft X-ray excess is an artefact due to the absorption of the primary power law by a relativistic wind. We examine the advantages and drawbacks of the reflection versus absorption models. We argue that in the absorption hypothesis, the absorbing medium should be in total pressure equilibrium, to constrain the spectral distribution which otherwise would be too strongly variable in time and from one object to the other, as compared to observations. We conclude that some X-ray spectra, in particular those with strong soft X-ray excesses, can be modelled by absorption in the 0.3-10 keV range. However, due to the lack of a complete grid of models and good data extending above 10 keV, we are not able to conclude presently that all objects can be accommodated with such models. These absorption models imply either strong relativistic outflowing winds with mass rates of the order of the Eddington value (or even larger), or quasi-spherical inhomogeneous accretion flows. Only weak excesses can be modelled by reflection, unless the primary continuum is not directly seen. Finally, a reflection model absorbed by a modest relativistic wind could be the best solution to the problem.

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arXiv2004

X-ray He-like ion diagnostics: New computations for photoionized plasma. II: Influence of different parameters

In a previous article, with a new code designed for a photoionized thick hot medium using a full transfer treatment of both lines and continuum, we have studied the He-like emission of a medium photoionized by an X-ray source. We considered the influence of density, ionization parameter and column density. We stressed that photoionized media are not uniform in temperature and we showed that photoexcitation and large column densities should be considered to avoid misinterpretation of the properties of the medium. To complete the possible situations observed in photoionized plasmas, we consider here the influence of microturbulence due for instance to a velocity gradient, of different element abundance, and of different ionizing spectral distributions. All new situations are computed using the same code, but with an improved version of our He-like atomic model. We give values of the G and R ratios, the equivalent width, and the ionic column densities with a greater accuracy than previously achieved. A large microturbulence results in a decrease of the G ratios for large column densities, for which the He-triplet resonant lines are optically thick. On the contrary, the R ratio is almost independent of the dispersion velocity and the element abundances, as long as the intercombination lines are optically thin. Through two 'universal diagrams' not depending on any parameter, we also present a way to determine the ionization parameter and the total column density of the emitting regions from the temperature of the observed radiative recombination continuum of a given ion and its column density.

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arXiv2004

A new type of photoionized code required for the new era of X-ray spectroscopy

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 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, which is most reliable for line transfer. In all other photoionization codes the line intensities are computed with the so-called "escape probability formalism", used in its simplest approximation. In a previous paper (Dumont et al. 2003), it was shown that this approximation leads to errors which can exceed one order of magnitude in the case a Thomson thickness of the order of unity. We show here that it also happens in the case of a Thomson thickness of 0.001 to 0.1. The errors on the line fluxes and line ratios are of the order of 30% for a column density of 10^{20} cm^{-2}, and a factor five for a column density of 10^{23} cm^{-2}, in conditions appropriate for Seyfert 2 and for the Warm Absorber of Seyfert 1.

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arXiv2003

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.

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arXiv2001

Codes for optically thick and hot photoionized media - Radiative transfer and new developments

We describe a code designed for hot media {(T $\ge$} a few 10$^4$ K), optically thick to Compton scattering. It computes the structure of a plane-parallel slab of gas in thermal and ionization equilibrium, illuminated on one side or on both sides by a given spectrum. This code has been presented in a previous paper (Dumont, Abrassart & Collin 2000), where several aspects were already discussed. So we focus here mainly on the recent developments. Presently the code solves the transfer of the continuum with the Accelerated Lambda Iteration method (ALI) and that of the lines in a two stream Eddington approximation, without using the local escape probability formalism to approximate the line transfer. This transfer code is coupled with a Monte Carlo code which allows to take into account direct and inverse Compton diffusions, and to compute the spectrum emitted up to MeV energies, in any geometry. The influence of a few physical parameters is shown, and the importance of the density and pressure distribution (constant density, pressure equilibrium, or hydrostatic equilibrium) is stressed. Recent improvements in the treatment of the atomic data are described, and foreseen developments are mentioned.

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arXiv2001

Size-Mass-luminosity relations in AGN and the role of the accretion disc

We address the question of the relations between the black hole's mass, the accretion rate, the bolometric luminosity, the optical luminosity and the size of the Broad Line Region (BLR) in Active Galactic Nuclei, using recent observational data obtained from monitoring campaigns. We show that a standard accretion disc cannot account for the observed optical luminosity, unless it radiates at super-Eddington rates. This implies the existence of another, dominant emission mechanism in the optical range, or a non standard disc (non stationary, ADAF and/or strong outflows). Narrow Line Seyfert 1 galaxies (NLS1s) are most extreme in this context: they have larger bolometric to Eddington luminosity ratios than Broad Line Seyfert 1 (BLS1s), and most likely a larger "non disc" component in the optical range. From realistic simulations of self-gravitating $α$-discs, we have systematically localized the gravitationally unstable disc and shown that, given uncertainties on both the model and observations, it coincides quite well with the size of the BLR. We therefore suggest that the gravitationally unstable disc is the source which releases BLR clouds in the medium. However the influence of the ionization parameter is also required to explain the correlation found between the size of the BLR and the luminosity. In this picture the size of the BLR in NLS1s (relative to the black hole size) is larger (and the emission line width smaller) than in BLS1s simply because their Eddington ratio is larger.

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arXiv2001

Accretion and emission processes in AGN

The UV-X continuum, the X-ray spectral features, and the variability in these bands provide powerful tools for studying the innermost regions of AGNs from which we gain an insight into the accretion process. In this chapter the discussion focusses on luminous AGN, i.e. Seyfert galaxies and quasars. The standard accretion disk model (a stationary geometrically thin disk) is described, and vertically averaged solutions for the radial structure are given. The emission of the standard disk is discussed using different approximations, and it is compared to the observations. This leads to the conclusion that more complex models are required, such as the irradiated disk and the disk-corona models. The advantage of this last model is that it explains the overall UV-X spectral distribution. In the framework of these disk models, the profile, intensity, and variability properties of the X-ray iron line can be explained by reprocessing at the surface of the cold disk very close to the black hole (the "relativistic disk model"). An alternative possibility is discussed, where the UV-X continuum is produced by a quasi-spherical distribution of dense clouds surrounded by (or embedded in) a hot medium. In such a model the iron line profile could be due to Comptonization instead of relativistic effects.

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arXiv2001

Non LTE radiation processes: application to the solar corona

These lectures are intended to present a simple but relatively complete description of the theory needed to understand the formation of lines in non-local thermodynamical equilibrium (NLTE), without appealing to any previous knowledge except a few basics of physics and spectroscopy. After recalling elementary notions of radiation transfer, the chapter is focussed on the computation of the level populations, the source function, the ionization state, and finally the line intensity. An application is made to forbidden coronal lines which were observed during eclipses since decades.

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arXiv1999

Star formation and evolution in accretion disks around massive black holes

We develop a model for the outer gravitationally unstable regions of accretion disks around massive black holes, for primeval or solar abundances. First we study star formation and evolution in a purely gaseous marginally unstable disk, and we show that unstable fragments should collapse rapidly and give rise to protostars which accrete at a high rate, acquire a mass of a few tens of Mo, and explode as supernovae, producing strong enriched outflows. Second we assume that the whole mass transport is ensured by the supernovae which induce a transfer of angular momentum. For a star formation rate proportional to the growth rate of gravitational instability, we find solutions for the gas and stellar densities where the gas is close to gravitational instability, between 0.1 and 10 pc M(BH) =10**6 Mo, and between 1 and 100 pc for M(BH)=10**8 Mo or larger, whatever the abundances, but for relatively low accretion rates. For larger accretion rates the number of stars becomes so large that they inhibit further star formation, and/or the rate of SN is so high that they distroy the homogeneity and the marginal stability of the disk. We postpone the study of this case. Besides the fact that the mechanism solves the problem of mass transport in a region of where global instabilities do not work, there are several consequences of this model: it can explain the high velocity metal enriched outflows implied by Broad Absorption Lines in quasars, it can account for a pregalactic enrichment, if black holes formed early, finally it can trigger starbursts in the central regions of galaxies. A check of the model would be to detect a supernova exploding within a few parsecs from the center of an AGN, an observation which can be performed in the near future.

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arXiv1998

The Warm Absorber constrained by the coronal lines in Seyfert 1 galaxies

We present results of the photoionization code IRIS, which calculates the spectrum emitted by the Warm Absorber (WA) in Seyfert 1 galaxies for a large grid of parameters (density, column density, ionization parameter...). We show that in Seyfert 1s, coronal lines Fe X, Fe XI, Fe XIV...), whose emission region shares common characteristics with the WA, could be formed in the WA. Unlike the absorption edges, such as those of O VII and O VIII observed in soft X-rays which are produced by the WA, these lines strongly constrain the physical parameters of the WA, especially the hydrogen density. Indeed, in order to avoid producing coronal line equivalent widths larger than observed, a high density (nH \geq 10^{10} cm-3) is required for the WA in most cases. This result is obtained for the mean observed Seyfert 1 features, as well as for the case study of MCG-6-30-15. It implies that the distance of the WA from the incident radiation source is of the order of that of the Broad Line Region (BLR)

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