L. Č. Popović

3D radiative transfer modelling of the dusty tori around active galactic nuclei as a clumpy two-phase medium

We investigate the emission of active galactic nuclei (AGN) dusty tori in the infrared domain. Following theoretical predictions coming from hydrodynamical simulations, we model the dusty torus as a 3D two-phase medium with high-density clumps and low-density medium filling the space between the clumps. Spectral energy distributions (SED) and images of the torus at different wavelengths are obtained using 3D Monte Carlo radiative transfer code SKIRT. Our approach of generating clumpy structure allows us to model tori with single clumps, complex structures of merged clumps or interconnected sponge-like structure. A corresponding set of clumps-only models and models with smooth dust distribution is calculated for comparison. We found that dust distribution, optical depth, clump size and their actual arrangement in the innermost region, all have an impact on the shape of near- and mid-infrared SED. The 10 µm silicate feature can be suppressed for some parameters, but models with smooth dust distribution are also able to produce a wide range of the sili cate feature strength. Finally, we find that having the dust distributed in a two-phase medium, m ight offer a natural solution to the lack of emission in the near-infrared, compared to observed data, which affects clumpy models currently available in the literature.

The Influence of Gravitational Microlensing on the Broad Emission Lines of Quasars

We discuss the effects of microlensing on the broad emission lines (BELs) of QSOs in the light of recent determinations of the size of the broad-line region (BLR) and its scaling with QSO luminosity. Microlensing by star-sized objects can produce significant amplifications in the BEL of some multiple-imaged QSOs, and could be very relevant for high-ionization lines. We have identified a group of 10 gravitational lens systems (~30% of the selected sample) in which microlensing could be observed. Using standard kinematic models for active galactic nuclei, we have studied the changes induced in the line profile by a microlens located at different positions with respect to the center of the BLR. We found that microlensing could produce important effects such as the relative enhancement of different parts of the line profile or the displacement of the peak of the line. The study of BEL profiles of different ionization in a microlensed QSO image could be an alternative method for probing the BLR structure and size.

Contribution of the disk emission to the broad emission lines in AGNs: Two-component model

We present an investigation of the structure of the emission line region in a sample of 12 single-peaked Active Galactic Nuclei (AGNs). Using the high resolution Hβ and Hα line profiles observed with the Isaac Newton Telescope (La Palma) we study the substructure in the lines (such as shoulders or bumps) which can indicate a disk or disk-like emission in Broad Line Regions (BLRs). Applying Gaussian analysis we found that both kinds of emission regions, BLR and NLR, are complex. In this sample the narrow (OIII) lines are composites of two components; NLR1 which have random velocities from ∼200 to 500 km s −1 and systematic velocities toward the blue from 20 to 350 km s −1 , and NLR2 with smaller random velocities (∼100−200 km s −1 ) and a redshift corresponding to the cosmological one. The BLR also have complex structure and we apply a two-component model assuming that the line wings originate in a very broad line region (VBLR) and the line core in an intermediate line region (ILR). The VBLR is assumed to be an accretion disk and the ILR a spherical emission region. The model fits very well the Hα and Hβ line profiles of the AGNs.

Super-massive binary black holes and emission lines in active galactic nuclei

Abstract It is now agreed that mergers play an essential role in the evolution of galaxies and therefore that mergers of supermassive black holes (SMBHs) must have been common. We see the consequences of past supermassive binary black holes (SMBs) in the light profiles of so-called ‘core ellipticals’ and a small number of SMBs have been detected. However, the evolution of SMBs is poorly understood. Theory predicts that SMBs should spend a substantial amount of time orbiting at velocities of a few thousand kilometers per second. If the SMBs are surrounded by gas observational effects might be expected from accretion onto one or both of the SMBHs. This could result in a binary Active Galactic Nucleus (AGN) system. Like a single AGN, such a system would emit a broad band electromagnetic spectrum and broad and narrow emission lines. The broad emission spectral lines emitted from AGNs are our main probe of the geometry and physics of the broad line region (BLR) close to the SMBH. There is a group of AGNs that emit very broad and complex line profiles, showing two displaced peaks, one blueshifted and one redshifted from the systemic velocity defined by the narrow lines, or a single such peak. It has been proposed that such line shapes could indicate an SMB system. We discuss here how the presence of an SMB will affect the BLRs of AGNs and what the observational consequences might be. We review previous claims of SMBs based on broad line profiles and find that they may have non-SMB explanations as a consequence of a complex BLR structure. Because of these effects it is very hard to put limits on the number of SMBs from broad line profiles. It is still possible, however, that unusual broad line profiles in combination with other observational effects (line ratios, quasi-periodical oscillations, spectropolarimetry, etc.) could be used for SMBs detection. Some narrow lines (e.g., [O III]) in some AGNs show a double-peaked profile. Such profiles can be caused by streams in the Narrow Line Region (NLR), but may also indicate the presence of a kilo-parsec scale mergers. A few objects indicated as double-peaked narrow line emitters are confirmed as kpc-scale margers, but double-peaked narrow line profiles are mostly caused by the complex NLR geometry. We briefly discuss the expected line profile of broad Fe Kα that probably originated in the accretion disk(s) around SMBs. This line may also be very complex and indicate the complex disk geometry or/and an SMB presence. Finally we consider rare configurations where a SMB system might be gravitationally lensed by a foreground galaxy, and discuss the expected line profiles in these systems.

VARIABILITY OF THE Hβ LINE PROFILES AS AN INDICATOR OF ORBITING BRIGHT SPOTS IN ACCRETION DISKS OF QUASARS: A CASE STUDY OF 3C 390.3

Here we show that in the case when double-peaked emission lines originate from outer parts of the accretion disk, their variability could be caused by perturbations in the disk emissivity. In order to test this hypothesis, we introduced a model of the disk perturbing region in the form of a single bright spot (or flare) by a modification of the power-law disk emissivity in an appropriate way. The disk emission was then analyzed using numerical simulations based on the ray-tracing method in the Kerr metric and the corresponding simulated line profiles were obtained. We applied this model to the observed H{beta} line profiles of 3C 390.3 (observed in the period 1995-1999) and estimated the parameters of both the accretion disk and the perturbing region. Our results show that two large amplitude outbursts of the H{beta} line observed in 3C 390.3 could be explained by successive occurrences of two bright spots on the approaching side of the disk. These bright spots are either moving, originating in the inner regions of the disk and spiralling outward by crossing small distances during the period of several years, or stationary. In both cases, their widths increase with time, indicating that they most likely decay.

Contribution of a disc component to single-peaked broad lines of active galactic nuclei

We study the disc emission component hidden in the single-peaked broad emission lines (BELs) of active galactic nuclei. We compare the observed broad lines from a sample of 90 Seyfert 1 spectra taken from the Sloan Digital Sky Survey with simulated line profiles. We consider a two-component broad line region model where an accretion disc and a surrounding non-disc region with isotropic cloud velocities generate the simulated BEL profiles. The analysis is mainly based on the measurements of the full widths (at 10 per cent, 20 per cent and 30 per cent of the maximum intensity) and on the asymmetries of the line profiles. Comparing these parameters for the simulated and observed Hα broad lines, we found that the hidden disc emission may be present in BELs even if the characteristic of two-peaked-line profiles is absent. For the available sample of objects (Seyfert 1 galaxies with single-peaked BELs), our study indicates that, in the case of the hidden disc emission in single-peaked broad-line profiles, the disc inclination tends to be small (mostly i < 25°) and that the contribution of the disc emission to the total flux should be smaller than the contribution of the surrounding region.

Spectral optical monitoring of 3C 390.3 in 1995–2007 - I. Light curves and flux variation in the continuum and broad lines

Context. We present the results of the long-term (1995-2007) spectral monitoring of the broad-line radio galaxy 3C 390.3, a well known AGN with double-peaked broad emission lines, usually assumed to be emitted from an accretion disk. Aims. To explore dimensions and structure of the BLR, we analyze the light curves of the broad Hα and Hβ line fluxes and the continuum flux. To detect variations in the BLR, we analyze the Hα and Hβ line profiles, as well as the change in the line profiles during the monitoring period. Methods. We attempt first to find a periodicity in the continuum and Hβ light curves, finding that there is a high probability of measuring quasi-periodical oscillations. Using the line shapes and their characteristics (such as e.g., peak separation and their intensity ratio, or FWHM) of broad Hβ and Hα lines, we discuss the structure of the BLR. We also cross-correlate the continuum flux with Hβ and Hα lines to determine the dimensions of the BLR. Results. During the monitoring period, we found that the broad emission component of the Hα and Hβ lines, and the continuum flux varied by a factor of ≈4―5. We also detected different structure in the line profiles of Hα and Hβ. An additional central component appears to be present and superimposed on the disk emission. In the period of high activity (after 2002), Hβ became broader than Hα and red wing of Hβ was higher than that of Hα. We detected time lags of ∼95 days between the continuum and Hβ flux, and about 120 days between the continuum and Hα flux. Conclusions. Variations in the line profiles, as well as correlation between the line and continuum flux during the monitoring period, are consistent with a disk origin of the broad lines and the possible contribution of some additional region and/or some kind of perturbation in the disk.

Long-term variability of the optical spectra of NGC 4151 - II. Evolution of the broad Hα and Hβ emission-line profiles

Aims. We present the results of a long-term monitoring (11 years, between 1996 and 2006) of Hα and Hβ line variations of the active galactic nucleus of NGC 4151. Methods. High quality spectra (S /N > 50 and R ≈ 8A ) of Hα and Hβ were investigated. During monitoring period, we analyzed line profile variations. Comparing the line profiles of Hα and Hβ, we studied different details (bumps, absorbtion features) in the line profiles. The variations in the different Hα and Hβ line profile segments were investigated. We also analyzed the Balmer decrement for entire lines and for line segments. Results. We found that the line profiles varied strongly during the monitoring period, and exhibited blue and red asymmetries. This is indicative of a complex BLR geometry inside NGC 4151 with, at least, three kinematically distinct regions: one that contributes to the blue line wing, one to the line core and one to the red line wing. The variation may be caused by an accelerating outflow originating very close to the black hole, where the red part may come from a region closer to the black hole than the blue part, which originates in the region with the highest outflow velocities. Conclusions. Taking into account that the BLR of NGC 4151 has a complex geometry (probably affected by an outflow) and that a portion of the broad line emission does not seem to be produced entirely by photoionization, one may ask whether the study of the BLR using reverberation mapping would be worthwhile for this galaxy.

Detailed Analysis of Balmer Lines in a Sloan Digital Sky Survey Sample of 90 Broad-Line Active Galactic Nuclei

In order to contribute to the general effort aiming at the improvement of our knowledge about the physical conditions within the broad-line regions (BLRs) of active galactic nuclei (AGNs), here we present the results achieved by our analysis of the spectral properties of a sample of 90 broad-line-emitting sources, collected from the Sloan Digital Sky Survey database. By focusing our attention mainly on the Balmer series of hydrogen emission lines, which is the dominant feature in the optical wavelength range of many BLR spectra, we extracted several flux and profile measurements, which we related to other source properties, such as optical continuum luminosity, inferred black hole mass, and accretion rate. Using the Boltzmann plot method to investigate the Balmer-line flux ratios as a function of the line profiles, we found that AGNs that emit broader lines typically have larger Hα/Hβ and smaller Hγ/Hβ and Hδ/Hβ line ratios. With the help of some recent investigations, we model the structure of the BLR, and we study the influence of the accretion process on the properties of BLR plasma.

Spectral optical monitoring of 3C 390.3 in 1995–2007 - II. Variability of the spectral line parameters

Context. We present a study of the variability of the broad emission-line parameters of 3C390.3, an active galaxy with the doublepeaked emission-line profiles. We give a detailed analysis of the variation in the broad Hα and Hβ emission-line profiles, the ratios, and the Balmer decrement of different line segments. Aims. We explore the disk structure with an investigation of the variability of the broad-line profiles. This is assumed to emit the broad double-peaked Hβ and Hα emission lines in the spectrum of 3C390.3. Methods. We divided the observed spectra into two periods (before and after the outburst in 2002) and separately analyzed the variation in these two periods. First we analyzed the spectral emission-line profiles of the Hα and Hβ lines and measured the peak positions. Then, we divided the lines into several segments and measured the line-segment fluxes. The Balmer decrement variation for the entire Hα and Hβ fluxes and for the line segments was investigated and discussed. Additionally, we modeled the variations in the line parameters with an accretion disk model and compared our modeled line parameter variations with observed ones. Results. We compared the variability in the observed line parameters with the disk model predictions and found that the variation in line profiles and in the line segments corresponds to the emission of a disk-like broad-line region (BLR). But there is probably another additional emission component that contributes to the Hα and Hβ line center. We found that the variation in the line profiles is caused by the variation in the parameters of the disk-like BLR, first of all in the inner (outer) radius, which can well explain the line parameters variations in Period I. The Balmer decrement across the line profile has a bell-like shape and is affected not only by physical processes in the disk, but also by different emitting disk dimension of the Hα and Hβ line. Conclusions. The geometry of the BLR of 3C390.3 seems to be very complex, and inflows/outflows may be present, but evidently the broad-line region with its disk-like geometry is the dominant emitter.