Active Galactic Nuclei Parameters Research Articles

A PHYSICALLY BASED MODEL OF THE IONIZING RADIATION FROM ACTIVE GALAXIES FOR PHOTOIONIZATION MODELING

ABSTRACT We present a simplified model of active galactic nucleus (AGN) continuum emission designed for photoionization modeling. The new model oxaf reproduces the diversity of spectral shapes that arise in physically based models. We identify and explain degeneracies in the effects of AGN parameters on model spectral shapes, with a focus on the complete degeneracy between the black hole mass and AGN luminosity. Our reparametrized model oxaf removes these degeneracies and accepts three parameters that directly describe the output spectral shape: the energy of the peak of the accretion disk emission , the photon power-law index of the non-thermal emission Γ, and the proportion of the total flux that is emitted in the non-thermal component . The parameter is presented as a function of the black hole mass, AGN luminosity, and “coronal radius” of the optxagnf model upon which oxaf is based. We show that the soft X-ray excess does not significantly affect photoionization modeling predictions of strong emission lines in Seyfert narrow-line regions. Despite its simplicity, oxaf accounts for opacity effects where the accretion disk is ionized because it inherits the “color correction” of optxagnf. We use a grid of mappings photoionization models with oxaf ionizing spectra to demonstrate how predicted emission-line ratios on standard optical diagnostic diagrams are sensitive to each of the three oxaf parameters. The oxaf code is publicly available in the Astrophysics Source Code Library.

A GROWTH-RATE INDICATOR FOR COMPTON-THICK ACTIVE GALACTIC NUCLEI

ABSTRACT Due to their heavily obscured central engines, the growth rate of Compton-thick (CT) active galactic nuclei (AGNs) is difficult to measure. A statistically significant correlation between the Eddington ratio, λ Edd, and the X-ray power-law index, Γ, observed in unobscured AGNs offers an estimate of their growth rate from X-ray spectroscopy (albeit with large scatter). However, since X-rays undergo reprocessing by Compton scattering and photoelectric absorption when the line of sight to the central engine is heavily obscured, the recovery of the intrinsic Γ is challenging. Here we study a sample of local, predominantly CT megamaser AGNs, where the black hole mass, and thus Eddington luminosity, are well known. We compile results of the X-ray spectral fitting of these sources with sensitive high-energy (E > 10 keV) NuSTAR data, where X-ray torus models, which take into account the reprocessing effects have been used to recover the intrinsic Γ values and X-ray luminosities, L X. With a simple bolometric correction to L X to calculate λ Edd, we find a statistically significant correlation between Γ and λ Edd (p = 0.007). A linear fit to the data yields Γ = (0.41 ± 0.18)log10 λ Edd + (2.38 ± 0.20), which is statistically consistent with results for unobscured AGNs. This result implies that torus modeling successfully recovers the intrinsic AGN parameters. Since the megamasers have low-mass black holes (M BH ≈ 106–107 M ⊙) and are highly inclined, our results extend the Γ–λ Edd relationship to lower masses and argue against strong orientation effects in the corona, in support of AGN unification. Finally this result supports the use of Γ as a growth-rate indicator for accreting black holes, even for CT AGNs.

RADIATION-DRIVEN OUTFLOWS FROM AND RADIATIVE SUPPORT IN DUSTY TORI OF ACTIVE GALACTIC NUCLEI

ABSTRACT Substantial evidence points to dusty, geometrically thick tori obscuring the central engines of active galactic nuclei (AGNs), but so far no mechanism satisfactorily explains why cool dust in the torus remains in a puffy geometry. Near-Eddington infrared (IR) and ultraviolet (UV) luminosities coupled with high dust opacities at these frequencies suggest that radiation pressure on dust can play a significant role in shaping the torus. To explore the possible effects of radiation pressure, we perform three-dimensional radiative hydrodynamics simulations of an initially smooth torus. Our code solves the hydrodynamics equations, the time-dependent multi–angle group IR radiative transfer (RT) equation, and the time-independent UV RT equation. We find a highly dynamic situation. IR radiation is anisotropic, leaving primarily through the central hole. The torus inner surface exhibits a break in axisymmetry under the influence of radiation and differential rotation; clumping follows. In addition, UV radiation pressure on dust launches a strong wind along the inner surface; when scaled to realistic AGN parameters, this outflow travels at and carries M ⊙ yr−1, where M, , and are the mass, UV luminosity, and Eddington luminosity of the central object respectively.

The coronal parameters of local Seyfert galaxies

AbstractOne of the open problems for AGN is the nature of the primary X‐ray emission: It is likely due to Comptonization of soft UV photons, but the optical depth and temperature of the emitting corona were largely unknown before the launch of the Nuclear Spectroscopic Telescope Array (NuSTAR). It is the first focusing hard X‐ray telescope on orbit, ∼ 100 times more sensitive in the 10–79 keV band compared to previous observatories, enabling the study of AGN at high energies with high precision. We present and discuss the results on the hot corona parameters of active galactic nuclei that have been recently measured with NuSTAR (often in coordination with XMM‐Newton, Suzaku, or Swift) with unprecedented accuracy, in a number of local Seyfert galaxies. (© 2016 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

A degeneracy in DRW modelling of AGN light curves

Individual light curves of active galactic nuclei (AGNs) are nowadays successfully modelled with the damped random walk (DRW) stochastic process, characterized by the power exponential covariance matrix of the signal, with the power $\beta=1$. By Monte Carlo simulation means, we generate mock AGN light curves described by non-DRW stochastic processes ($0.5\leq\beta\leq 1.5$ and $\beta\neq1$) and show they can be successfully and well-modelled as a single DRW process, obtaining comparable goodness of fits. A good DRW fit, in fact, may not mean that DRW is the true underlying process leading to variability and it cannot be used as a proof for it. When comparing the input (non-DRW) and measured (DRW) process parameters, the recovered time scale (amplitude) increases (decreases) with the increasing input $\beta$. In practice, this means that the recovered DRW parameters may lead to biased (or even non-existing) correlations of the variability and physical parameters of AGNs if the true AGN variability is caused by non-DRW stochastic processes. The proper way of identifying the processes leading to variability are model-independent structure functions and/or power spectral densities and then using such information on the covariance matrix of the signal in light curve modelling.

The broad wing of the [O III] λ5007 emission line in active galactic nuclei

We use a sample of type 2 active galactic nuclei (AGNs) from SDSS DR7 in which the [O III] λ5007 emission line can be modeled by two Gaussian components, a broad wing plus a narrow core, to investigate the origin of the broad wing and the connection between the velocity shift of the broad wing and the physical parameters of AGNs, as well as their host galaxies. We find that the flux of the wing components is statistically roughly equal to that of the core components. However, the velocity shift of the wing component has only weak, if any, correlations with the physical properties of AGNs and the host galaxies, such as bolometric luminosity, the Eddington ratio, the mass of supermassive black holes, D4000, HδA or stellar mass. Comparing the velocity shift from our type 2 AGN sample to that from the type 1 sample in Zhang et al., we suggest that the [O III] broad wing originates from outflow.

Efficiency tests for estimating the gas and stellar population parameters in Type 2 objects

We investigated the efficiency of estimating characteristics of stellar populations (SP) and Active Galactic Nuclei (AGN) emission using ULySS code. To analyze simultaneously AGN and SP components in the integrated spectrum of Type 2 active galaxies, we modeled the featureless continuum (FC) and emission lines, and we used PEGASE.HR stellar population models provided by ULySS. In order to validate the method, we simulated over 7000 integrated spectra of Seyfert 2 galaxies. Spectra were generated using different characteristics of the featureless AGN continuum, signal-to-noise ratio (SNR), spectral ranges, properties of emission lines and single stellar population (SSP) model whose initial mass function (IMF) and abundance pattern is similar to the solar neighborhood. Simulated spectra were fitted with ULySS to evaluate the ability of the method to extract SP and AGN properties. We found that the analysis with ULySS can efficiently restore the characteristics of SP in spectra of Seyfert 2 AGNs, where signal-to-noise ratio is higher than 20, and where SP contributes with more than 10% to the total flux. Degeneracies between AGN and SP parameters increase with increasing the AGN continuum fraction, which points out the importance of simultaneous fitting of the FC and SP contributions.

Spins of Supermassive Black Holes and the Magnetic Fields of Accretion Disks in Active Galactic Nuclei with Maser Emission

The major parameters of active galactic nuclei (AGN) with maser emission— the magnetic field in the maser emission region and the spin of the central supermassive black hole (SMBH)— are examined. The spin is calculated using two physical models for the generation of relativistic jets: the Blandford-Znajek (BZ) and hybrid Meier models. These calculations indicate that some AGN have retrograde rotation of the black hole. The magnetic field in the maser emission region is calculated using generally accepted models for the accretion disk. Our estimates show that the magnetic field in the megamaser emission region is bounded above by 1 G, in agreement with observational data.

CONNECTION BETWEEN MID-INFRARED EMISSION PROPERTIES AND NARROW-LINE REGION OUTFLOWS IN TYPE 1 ACTIVE GALACTIC NUCLEI

The location of warm dust producing the Mid-infrared (MIR) emission in Type 1 Active Galactic Nuclei (AGNs) is complex and not yet fully known. We explore this problem by studying how the MIR covering factor (CF_{MIR} =L_{MIR}/L_{bol}) correlates with the fundamental parameters of AGN accretion process (such as L_{bol}, black hole mass MBH, and Eddington ratio L/LEdd) and the properties of narrow emission lines (as represented by [O III] 5007), using large data sets derived from the Sloan Digital Sky Spectroscopic Survey (SDSS) and the Wide Infrared Sky Survey (WISE). Firstly we find that the luminosity of the [O III] wing component (Lwing) correlates more tightly with the continuum luminosity (L5100) than the luminosity of the line core component (Lcore) does, which is in line with our previous conclusion that the wing component, generally blueshifted, originates from the polar outflows in the inner narrow-line region (NLR). We then find that the MIR CF shows the strongest correlation with Lwing/L_{bol} rather than with Lcore/L_{bol} or the above fundamental AGN parameters, and the correlation becomes stronger as the infrared wavelength increases. We also confirm the anti-correlations of CF_{MIR} with L_{bol} and MBH, and the lack of dependence of CF_{MIR} on the Eddington ratio. These results suggest that a large fraction of the warm dust producing MIR emission in AGNs is likely embedded in polar outflows in the NLR instead of in the torus.

Stellar Population in the Sample of Type 2 Active Galactic Nuclei

We analyse simultaneously stellar populations (SPs) in active galactic nuclei (AGNs), AGN featureless continuum and emission lines, using NBurst code, adopted to fit the total spectrum of an active galaxy. In this paper we present the investigations of the accuracy, limitations and applications of the code in a research of stellar populations in narrow emission line galaxies (Type 2). To validate the method, we simulated several thousands of line-of-sight integrated spectra of Type 2 galaxies. We fitted simulated spectra with NBurst and found that code can extract SP and AGN parameters with a high precision. Additionally, we used the method to investigate SP and AGN spectral parameters of a sample of Type 2 AGNs. Especially we investigate the properties of stellar population in the first kiloparsec.

Numerical Investigations of a New N-body Simulation Method

Numerical investigation of a new similarity method (the Aldar-Kose method) for N-body simulations is described. Using this method we have carried out numerical simulations for two tasks: 1) calculation of the temporal behavior of different physical parameters of active galactic nuclei (AGN) containing a super massive black hole (SMBH), an accretion disk, and a compact stellar cluster; 2) calculation of the stellar capture rate to the central SMBH without accretion disk. The calculations show good perspectives for applications of the similarity method to optimize the evolution model calculations of large stellar systems and of AGN.

THE BLUESHIFTING AND BALDWIN EFFECTS FOR THE [O III] λ5007 EMISSION LINE IN TYPE 1 ACTIVE GALACTIC NUCLEI

We use homogeneous samples of radio-quiet Seyfert 1 galaxies and QSOs selected from the Sloan Digital Sky Survey to investigate the connection between the velocity shift and the equivalent width (EW) of the [O III] λ5007 emission line, and their correlations with physical parameters of active galactic nuclei (AGNs). We find a significant and negative correlation between the EW of the core component, EW(core), and the blueshift of either the core (the peak), the wing, or the total profile of [O III] emission; it is fairly strong for the blueshift of the total profile in particular. However, both quantities (EW and velocity shift) generally have only weak, if any, correlations with fundamental AGN parameters such as the nuclear continuum luminosity at 5100 A (L 5100), black hole mass (M BH), and the Eddington ratio (L/L Edd); these correlations include the classical Baldwin effect of EW(core), an inverse Baldwin effect of EW(wing), and the relationship between velocity shifts and L/L Edd. Our findings suggest that both the large object-to-object variation in the strength of [O III] emission and the blueshift-EW(core) connection are not governed primarily by fundamental AGN parameters such as L 5100, M BH, and L/L Edd. We propose that the interstellar medium conditions of the host galaxies play a major role instead in the diversity of the [O III] properties in active galaxies. This suggests that the use of [O III] λ5007 luminosity as a proxy of AGN luminosity does not depend strongly on the above-mentioned fundamental AGN parameters.

WHAT CONTROLS THE Fe II STRENGTH IN ACTIVE GALACTIC NUCLEI?

We used a large, homogeneous sample of 4178 z <= 0.8 Seyfert 1 galaxies and QSOs selected from the Sloan Digital Sky Survey to investigate the strength of Fe II emission and its correlation with other emission lines and physical parameters of active galactic nuclei. We find that the strongest correlations of almost all the emission-line intensity ratios and equivalent widths (EWs) are with the Eddington ratio (L/L_{Edd}), rather than with the continuum luminosity at 5100\AA\ (L_{5100}) or black hole mass (M_{BH}); the only exception is the EW of ultraviolet Fe II emission, which does not correlate at all with broad-line width, L_{5100}, M_{BH}, or L/L_{Edd}. By contrast, the intensity ratios of both the ultraviolet and optical Fe II emission to Mg II \lambda 2800 correlate quite strongly with L/L_{Edd}. Interestingly, among all the emission lines in the near-UV and optical, the EW of narrow optical Fe II emission has the strongest correlation with L/L_{Edd}. We suggest that the variation of the emission-line strength in active galaxies is regulated by L/L_{Edd} because it governs the global distribution of the column density of the clouds gravitationally bound in the line-emitting region, as well as its overall gas supply. The systematic dependence on L/L_{Edd} must be corrected when using the FeII/MgII intensity ratio as a measure of the Fe/Mg abundance ratio to study the history of chemical evolution in QSO environments.

INTERPLAY BETWEEN STELLAR SPIRALS AND THE INTERSTELLAR MEDIUM IN GALACTIC DISKS

We use homogeneous samples of radio-quiet Seyfert 1 galaxies and QSOs selected from the Sloan Digital Sky Survey to investigate the connection between the velocity shift and the equivalent width (EW) of the [OIII] 5007 emission line, and their correlations with physical parameters of active galactic nuclei (AGNs). We find a significant and negative correlation between the EW of the core component, EW(core), and the blueshift of either the core (the peak), the wing, or the total profile of [OIII] emission; it is fairly strong for the blueshift of the total profile particularly. However, both quantities (EW and velocity shift) generally have only weak, if any, correlations with fundamental AGN parameters such as the nuclear continuum luminosity at 5100 L_{5100}, black hole mass (M_{BH}), and the Eddington ratio (L/L_{Edd}); these correlations include the classical Baldwin effect of EW(core), an inverse Baldwin effect of EW(wing), and the relationship between velocity shifts and \lratio. Our findings suggest that both the large object-to-object variation in the strength of [OIII] emission and the blueshift--EW(core) connection are not governed primarily by fundamental AGN parameters such as L_{5100}, M_{BH} and L/L_{Edd}. We propose that the ISM conditions of the host galaxies play a major role instead in the diversity of the [OIII] properties in active galaxies. This suggests that the use of[OIII] 5007 luminosity as proxy of AGN luminosity does not depend strongly on the above-mentioned fundamental AGN parameters.

The evolution of black hole mass and spin in active galactic nuclei

Observations show that the central black hole in galaxies has a mass M of only ∼10−3 of the stellar bulge mass. Thus, whatever process grows the black hole also promotes star formation with far higher efficiency. We interpret this in terms of the generic tendency of active galactic nucleus (AGN) accretion discs to become self-gravitating outside some small radius Rsg∼ 0.01–0.1 pc from the black hole. We argue that mergers consist of sequences of such episodes, each limited by self-gravity to a mass ΔMepisode∼ 10−3M, with angular momentum characteristic of the small part of the accretion flow which formed it. In this picture, a major merger with ΔMmerger∼M gives rise to a long series of low-mass accretion disc episodes, all chaotically oriented with respect to one another. Thus, the angular momentum vector oscillates randomly during the accretion process, on mass-scales ∼ 103 times smaller than the total mass accreted in a major merger event. We show that for essentially all AGN parameters, the disc produced by any accretion episode of this type has lower angular momentum than the hole, allowing stable co- and counter-alignment of the discs through the Lense–Thirring effect. A sequence of randomly oriented accretion episodes as envisaged above then produces accretion discs stably co- or counter-aligned with the black hole spin with almost equal frequency. Accretion from these discs very rapidly adjusts the hole's spin parameter to average values (the precise range depending slightly on the disc vertical viscosity coefficient α2) from any initial conditions, but with significant fluctuations (Δa∼±0.2) about these. We conclude that (i) supermassive black holes (SMBH) should on average spin moderately, with the mean value decreasing slowly as the mass increases; (ii) SMBH coalescences leave little long-term effect on ⁠; (iii) SMBH coalescence products in general have modest recoil velocities, so that there is little likelihood of their being ejected from the host galaxy; (iv) black holes can grow even from stellar masses to ∼5 × 109M⊙ at high redshift z∼ 6; and (v) jets produced in successive accretion episodes can have similar directions, but after several episodes the jet direction deviates significantly. Rare examples of massive holes with larger spin parameters could result from prograde coalescences with SMBHs of similar mass, and are most likely to be found in giant ellipticals. We compare these results with observation.

Study of the physical conditions in active galactic nuclei. A method for estimating the physical parameters of radio sources

A new method for estimating the physical parameters of active galactic nuclei involving the analysis of observations of the compact radio sources in them is proposed. The method is based on an inhomogeneous model for a synchrotron radio source. Theoretical spectra of the radio sources are obtained via numerical solution of the transfer equation. Due to the paucity of observational data, only interval estimates of the magnetic field strength and the energy densities of the magnetic field and relativistic particles can be obtained. A mechanism for the formation of flat radio spectra is proposed.

Estimates of AGN Black Hole Mass and Minimum Variability Timescale

Black hole mass is one of the fundamental physical parameters of active galactic nuclei (AGNs), for which many methods of estimation have been proposed. One set of methods assumes that the broad-line region (BLR) is gravitationally bound by the central black hole potential, so the black hole mass can be estimated from the orbital radius and the Doppler velocity. Another set of methods assumes the observed variability timescale is determined by the orbital timescale near the innermost stable orbit around the Schwarzschild black hole or the Kerr black hole, or by the characteristic timescale of the accretion disk. We collect a sample of 21 AGNs, for which the minimum variability timescales have been obtained and their black hole masses (M,) have been well estimated from the stellar velocity dispersion or the BLR size-luminosity relation. Using the minimum variability timescales we estimated the black hole masses for 21 objects by the three different methods, the results are denoted by M-s, M-k and M-d, respectively. We compared each of them with M-sigma individually and found that: (1) using the minimum variability timescale with the Kerr black hole theory leads to small differences between M-sigma and M-k, none exceeding one order of magnitude, and the mean difference between them is about 0.53 dex; (2) using the minimum variability timescale with the Schwarzschild black hole theory leads to somewhat larger difference between M-sigma and M-s, larger than one order of magnitude for 6 of the 21 sources, and the mean difference is 0.74 dex; (3) using the minimum variability timescale with the accretion disk theory leads to much larger differences between M-sigma and M-d, for 13 of the 21 sources the differences are larger than two orders of magnitude; and the mean difference is as high as about 2.01 dex.

The scaling of the X-ray variability with black hole mass in active galactic nuclei

The relation between the 2–10 keV, long-term, excess variance and active galactic nuclei (AGN) black hole mass is considered in this work. A significant anticorrelation is found between these two quantities in the sense that the excess variance decreases with increasing black hole mass. This anticorrelation is consistent with the hypothesis that the 2–10 keV power spectrum in AGN follows a power law of slope −2 at high frequencies. It then flattens to a slope of −1 below a break frequency, νhfb, until a second break frequency, νlfb, below which it flattens to a slope of 0. The ratio νhfb/νlfb is equal to 10–30, similar to the ratio of the respective frequencies in Cyg X-1. The power spectrum amplitude in the (frequency × power) space does not depend on black hole mass. Instead, it is roughly equal to 0.02 in all objects. The high-frequency break decreases with increasing black hole mass according to the relation νhfb= 1.5 × 10−6/(M/107 M⊙) Hz, in the case of ‘classical’ Seyfert 1 galaxies. The excess variance of NGC 4051, a narrow-line Seyfert 1 object, is larger than is expected for its black hole mass and X-ray luminosity. This can be explained if its νhfb is 20 times larger than the value expected in the case of a ‘classical’ Seyfert 1 with the same black hole mass. Finally, the excess variance versus X-ray luminosity correlation is a by-product of the excess variance versus black hole mass correlation, with AGN accreting at ∼ 0.1–0.15 times the Eddington limit. These results are consistent with recent results from the power spectral analysis of AGN. However, as they are based on data from a few objects only, further investigation is necessary to confirm that there is indeed a ‘universal’ power spectrum shape in AGN (in the sense that the value of the power spectrum parameters of most AGN will be distributed around the ‘canonical’ slope, and amplitude values listed above). One way to achieve this is to determine the excess variance versus black hole relation more accurately, using data from many more objects. This will be possible in the near future, because it is easier to measure the excess variance of archival light curves than to estimate their power spectrum. The excess variance versus black hole relation can therefore play an important role in the study of the X-ray variability scaling with black hole mass in AGN.