Modeling the optical/UV polarization while flying around the tilted outflows of NGC 1068

Recent modeling of multi-waveband spectroscopic and maser observations suggests that the ionized outflows in the nuclear region of the archetypal Seyfert-2 galaxy NGC 1068 are inclined with respect to the vertical axis of the obscuring torus. Based on this suggestion, we build a complex reprocessing model of NGC 1068 for the optical/UV band. We apply the radiative transfer code STOKES to compute polarization spectra and images. The effects of electron and dust scattering and the radiative coupling occurring in the inner regions of the multi-component object are taken into account and evaluated at different polar and azimuthal viewing angles. The observed type-1/type-2 polarization dichotomy of active galactic nuclei is reproduced. At the assumed observer's inclination toward NGC 1068, the polarization is dominated by scattering in the polar outflows and therefore it indicates their tilting angle with respect to the torus axis. While a detailed analysis of our model results is still in progress, we briefly discuss how they relate to existing polarization observations of NGC 1068.Comment: To appear in the proceedings of the meeting "The Central Kiloparsec in Galactic Nuclei" held in Bad Honnef (Germany) from August 29th to September 2nd 201

Modeling the Polarization of Dusty Scattering Cones in Active Galactic Nuclei

We have used the STOKES radiative transfer code, to model polarization induced by dust scattering in the polar regions of Active Galactic Nuclei (AGN). We discuss the wavelength-dependence of the spectral intensity and polarization over the optical/UV range at different viewing angles for two different types of dust: a Galactic dust model, and a dust model inferred from extinction properties of AGN

Spectral and polarization properties of reflected X-ray emission from black-hole accretion discs for a distant observer: the lamp-post model

Rebirth of X-ray polarimetric instruments will have a significant impact on our knowledge of compact accreting sources. The properties of inner-accreting regions of active galactic nuclei (AGNs) or X-ray binary systems (XRBs), such as black-hole spin, their disc inclination and orientation, shape and size of their corona, can be polarimetrically studied, parallelly to the well-known X-ray spectroscopic and timing techniques. In this work, we provide a new spectropolarimetric numerical estimate of X-rays in the lamp-post coronal model for a distant observer, including a polarized reflected radiation from the accretion disc. The local disc reflection was simulated using the codes TITAN and STOKES and includes variable disc ionization as well as Monte Carlo treatment of Compton multiple scatterings. We introduce a relativistic code KYNSTOKES based on our well-tested KY package that accounts for all relativistic effects on radiation near a black hole, apart from the returning radiation, and adds a possibility of polarized coronal emission. We study the spectrum, polarization degree and polarization angle at spatial infinity for various global system parameters and we demonstrate the difference at infinity, if analytical local reflection computations are used. We newly predict that in the hard X-rays the reflected component can be 25% polarized and the total emission can be 9% polarized in the most favourable, yet realistic configurations of radio-quiet AGNs. Thus, the relativistic disc reflection remains important for the interpretation of X-ray polarimetric observations.Comment: 25 pages, 23 figures, 2 tables. Accepted for publication in MNRA

DISCOVERY OF POLARIZATION REVERBERATION IN NGC 4151

Observations of the optical polarization of NGC 4151 in 1997–2003 show variations of an order of magnitude in the polarized flux while the polarization position angle remains constant. The amplitude of variability of the polarized flux is comparable to the amplitude of variability of the total U-band flux, except that the polarized flux follows the total flux with a lag of 8±3 days. The time lag and the constancy of the position angle strongly favor a scattering origin for the variable polarization rather than a non-thermal synchrotron origin. The orientation of the position angle of the polarized flux (parallel to the radio axis) and the size of the lag imply that the polarization arises from electron scattering in a flattened region within the low-ionization component of the broad-line-region. Polarization from dust scattering in the equatorial torus is ruled out as the source of the lag in polarized flux because it would produce a larger lag and polarization perpendicular to the radio axis. We note a long-term change in the percentage polarization at similar total flux levels and we attribute this to a change in the number of scatterers on a timescale of years

Discovery of Polarization Reverberation in NGC 4151

Observations of the optical polarization of NGC 4151 in 1997-2003 show variations of an order of magnitude in the polarized flux while the polarization position angle remains constant. The amplitude of variability of the polarized flux is comparable to the amplitude of variability of the total U-band flux, except that the polarized flux follows the total flux with a lag of 8 +/- 3 days. The time lag and the constancy of the position angle strongly favor a scattering origin for the variable polarization rather than a non-thermal synchrotron origin. The orientation of the position angle of the polarized flux (parallel to the radio axis) and the size of the lag imply that the polarization arises from electron scattering in a flattened region within the low-ionization component of the broad-line-region. Polarization from dust scattering in the equatorial torus is ruled out as the source of the lag in polarized flux because it would produce a larger lag and, unless the half-opening angle of the torus is more than 53 degrees, the polarization would be perpendicular to the radio axis. We note a long-term change in the percentage of polarization at similar total flux levels and this could be due either to changing non-axisymmetry in the optical continuum emission, or a change in the number of scatterers on a timescale of years.Comment: 5 pages, 2 figures. Astrophysical Journal in press. Some clarifications, additional discussion and references adde

Line Shifts, Broad-Line Region Inflow, and the Feeding of AGNs

Velocity-resolved reverberation mapping suggests that the broad-line regions (BLRs) of AGNs can have significant net inflow. We use the STOKES radiative transfer code to show that electron and Rayleigh scattering off the BLR and torus naturally explains the blueshifted profiles of high-ionization lines and the ionization dependence of the blueshifts. This result is insensitive to the geometry of the scattering region. If correct, this model resolves the long-standing conflict between the absence of outflow implied by velocity-resolved reverberation mapping and the need for outflow if the blueshifting is the result of obscuration. The accretion rate implied by the inflow is sufficient to power the AGN. We suggest that the BLR is part of the outer accretion disk and that similar MHD processes are operating. In the scattering model the blueshifting is proportional to the accretion rate so high-accretion-rate AGNs will show greater high-ionization line blueshifts as is observed. Scattering can lead to systematically too high black hole mass estimates from the C IV line. We note many similarities between narrow-line region (NLR) and BLR blueshiftings, and suggest that NLR blueshiftings have a similar explanation. Our model explains the higher blueshifts of broad absorption line QSOs if they are more highly inclined. Rayleigh scattering from the BLR and torus could be more important in the UV than electron scattering for predominantly neutral material around AGNs. The importance of Rayleigh scattering versus electron scattering can be assessed by comparing line profiles at different wavelengths arising from the same emission-line region.Comment: 10 pages, 7 figures, Astrophysical Journal in press. The only changes from the previous version are to include some additional discussion of the plausibility of supersonic inflow velocities (see section 5.2) and some additional reference

L'accrétion et l'émission à proximité des trous noirs supermassifs dans les quasars et les NAG: Modélisation du spectre UV-X

With the last generation of X-ray satellites, such as XMM-Newton and Chandra, our knowledge about the X-ray properties of Active Galactic Nuclei (AGN) has greatly improved. Detailed spectroscopy and long-term observations of hundreds of kilo-seconds have provided new insights into the mechanisms of X-ray production, its modification by intervening matter, and X-ray variability.In this thesis I present radiative transfer modeling of AGN in the Far-UV and X-ray range. The modeling considers several aspects of the observed X-ray properties: the Compton reflection/reprocessed component, X-ray variability and flares, and the effects of the warm absorber.For the X-ray reprocessing, I investigate in detail the magnetic flare model assuming solar-like magnetic reconnections above the accretion disk. Such events should produce bright, compact sources of hard X-ray radiation creating a hot spot on the disk surface underneath. I evaluate the physical properties of the disk medium across such a spot and compute spectra of the reprocessed radiation as a function of the position in the spot and of the local viewing direction. Spots at several distances of the central black hole and for different orbital phases are considered and I vary the black hole mass, its accretion rate, and its spin-parameter. Spectra are also computed for a distant observer including a full relativistic treatment, which is based on a ray-tracing technique. I provide time-dependent simulations of single flare events taking into account the time-lags induced by the distance between the compact flare source and different positions of the hot spot. The flare modeling is conducted assuming an underlying accretion disk in hydrostatic equilibrium. The duration of the flare is supposed to be significantly smaller than the dynamical time-scale, so that the vertical density structure of the disk remains constant over the whole flare-period.Recent observations of the Seyfert-1 galaxy MCG -6-30-15 with XMM-Newton revealed a light-curve with a bright symmetric flare over 2000 s. For this flare, Ponti et al. (2004) presented a cross-correlation analysis deriving time-lags in several energy bands. I present a simple toy model describing such time-lags by assuming that the observer detects the primary radiation and the reprocessed component as two consecutive pulses. The toy model qualitatively reproduces the observed time-lags of MCG -6-30-15 and allows an estimation of the distance between the flare source and the disk.Using the results of the single-flare modeling, Monte-Carlo simulations for distributions of orbiting flare-spots across an accretion disk are conducted. The resulting fractional variability spectrum is constructed for different radial luminosity distributions and values of the black hole spin. These parameters are constrained for MCG -6-30-15 by applying the model to its observed fractional variability spectrum.Finally, the modification of the X-ray radiation in regions farther away from the central object is considered by modeling the warm absorber. A grid of models is computed for a warm absorber in pressure equilibrium as recently suggested by the observation of the Seyfert-1 galaxy NGC 3783. General tendencies for the stratification of the medium and for the resulting absorption spectra are shown by varying the slope of the incident spectrum, the ionization parameter, and the column density of the warm absorber._______________Reference: Ponti, G., Cappi, M., Dadina, M., & Malaguti, G. 2004, A&A, 417, 451La dernière génération de satellites X, comme XMM-Newton ou Chandra, a beaucoup enrichi notre savoir sur les propriétés du rayonnement X des Noyaux Actifs de Galaxies (NAG). La spectroscopie détaillée et les observations longues de plusieurs centaines de milliers de secondes ont ouvert de nouvelles perspectives sur les mécanismes de la production du rayonnement X, de sa modification par la matière située sur son chemin, et sur sa variabilité.Dans ma thèse, je présente des modèles de transfert de rayonnement thermique pour les NAG dans les domaines spectraux de l'extrême UV et des X. Les modèles prennent en considération plusieurs aspects des propriétés X observées: la composante du 'reprocessing', la variabilité et les flares X, ainsi que les effets du 'warm absorber'. Concernant le reprocessing, je modélise en détail des flares X en supposant des reconnexions magnétiques au-dessus du disque d'accrétion. Ces événements sont supposés être similaires aux flares solaires. Ils produisent des sources lumineuses et compactes émettant du rayonnement X dur et créant des taches chaudes sur le disque sous-jacent. J'évalue les propriétés physiques du milieu du disque à travers la tache et je calcule des spectres pour le rayonnement ré-émis en fonction de la position dans la tache et de la ligne de visée locale. Je fais varier la masse du trou noir, son taux d'accrétion, et son paramètre de rotation en évaluant des taches à plusieurs distances du trou noir et pour des phases orbitales différentes. Je calcule aussi des spectres vus par un observateur lointain en appliquant un traitement complet de relativité qui est basé sur une technique du tracés de rayons. Je fournis des simulations de l'évolution spectrale à laquelle on s'attend pour des flares particuliers, en tenant compte des délais causés par la distance entre la source compacte et différents endroits de la tache chaude. Les modèles du flare sont effectués en supposant un disque d'accrétion sous-jacent qui est à l'équilibre hydrostatique. La durée du flare est supposée être par un facteur significatif moins longue que l'échelle de temps dynamique du disque, afin que sa structure verticale reste constante pendant toute la période du flare.Des observations récentes de la galaxie Seyfert-1 MCG -6-30-15 avec XMM-Newton ont montré une courbe du lumière qui contient un flare lumineux et symétrique pendant environ 2000 secondes. Pour ce flare, Ponti et al. (2004) présentent une analyse temporelle à l'aide des fonctions d'auto-corrélation en dérivant des délais entre des bandes d'énergie différentes. Je suggère un modèle simple qui décrit ces délais en supposant que l'observateur détecte le rayonnement primaire et le reprocessing comme pulses consécutifs. Ce modèle reproduit les délais observés dans MCG -6-30-15 d'une manière qualitativement correcte, et il permet une estimation de la distance entre la source compacte du flare et la surface du disque.En utilisant les résultats de la modélisation d'un flare particulier, on effectue des simulations Monte-Carlo pour des distributions de flares répartis sur le disque. Le spectre de variabilité rms calculé est construit sur la base des ces simulations pour différentes distributions radiales de la luminosité du disque et pour différents paramètres de rotation du trou noir. En appliquant notre modèle au spectre rms observé, nous apportons à ces paramètres des contraintes pour le cas de MCG -6-30-15.Enfin, nous étudions la modification du rayonnement X dans des régions plus lointaines de l'objet central grâce à une modélisation du warm absorber. Une grille de modèles est calculée pour un warm absorber en équilibre de pression totale, comme l'a suggéré récemment l'observation de la galaxie Seyfert-1 NGC 3783. Nous montrons les tendances générales de la stratification du milieu et du spectre absorbé qui en résulte en faisant varier la pente du spectre incident, le paramètre d'ionisation, et la densité de colonne du warm absorber._______________Référence: Ponti, G., Cappi, M., Dadina, M., & Malaguti, G. 2004, A&A, 417, 45

Modelování dichotomie polarizace záření aktivních galaktických jader

I present polarization modelling of Active Galactic Nuclei in the optical/UV range. The modelling is conducted using the Monte-Carlo radiative transfer code Stokes, which self-consistently models the polarization signature of a complex model arrangement for an active nucleus

Modeling the UV-X-spetrum

With the last generation of X-ray satellites, such as XMM-Newton and Chandra, our knowledge about the X-ray properties of Active Galactic Nuclei (AGN) has greatly improved. Detailed spectroscopy and long-term observations of hundreds of kilo-seconds have provided new insights into the mechanisms of X-ray production, its modification by intervening matter, and X-ray variability. In this thesis I present radiative transfer modeling of AGN in the Far-UV and X-ray range. The modeling considers several aspects of the observed X-ray properties: the Compton reflection/reprocessed component, X-ray variability and flares, and the effects of the warm absorber. For the X-ray reprocessing, I investigate in detail the magnetic flare model assuming solar-like magnetic reconnections above the accretion diskthesi