Spectra Of Active Galactic Nuclei Research Articles

The X-ray variability of active galactic nuclei: Power spectrum and variance analysis of the Swift/BAT light curves

Aims. We study the X-ray power spectrum of active galactic nuclei (AGN) in order to investigate whether Seyfert I and II power spectra are similar or not and whether AGN variability depends on the mass and accretion rate of black holes as well as to compare the power spectra of AGN with the power spectra of Galactic X-ray black hole binaries. Method. We used 14–195 keV band light curves from the 157-month Swift/BAT hard X-ray survey, and we computed the mean power spectrum and excess variance of AGN in narrow black hole mass and AGN luminosity bins. We fitted a power-law model to the AGN power spectra, and we investigated whether the power spectrum parameters and the excess variance depend on the black hole mass, luminosity, and accretion rate of AGN. Results. We found the Seyfert I and Seyfert II power spectra to be identical, in agreement with AGN unification models. The mean AGN X-ray power spectrum has the same power-law like shape, with a slope of −1 in all AGN irrespective of their luminosity and black hole mass. We did not detect any flattening to a slope of zero at frequencies as low as 10−9 Hz. We detected an anti-correlation between the power spectral density function (PSD) amplitude and the accretion rate, similar to what has been seen in the past in the 2–10 keV band. This implies that the variability amplitude in AGN decreases with an increasing accretion rate. The universal AGN power spectrum is consistent with the mean 2–9 keV band Cyg X-1 power spectrum in its soft state. We detected a small difference in amplitude, but this is probably due to the difference in energy. Conclusions. The mean low-frequency AGN X-ray power spectrum is consistent with the extension of the mean 0.01–25 Hz Cyg X-1 power spectrum in its soft state to lower frequencies. We cannot prove that the mean AGN PSD is analogous to the mean Cyg X-1 PSD in its soft state, as we do not know the location of the high-frequency break in the hard X-ray AGN PSDs. However, if this is the case, then the accretion disc in AGN probably extends to the radius of the innermost circular stable orbit (as is probably the case with the black hole binaries in their soft state). The X-ray corona will then be located on top, illuminating the disc and producing the X-ray reflection and disc reverberation phenomena commonly observed in these objects. Furthermore, the agreement between the PSD amplitude in AGN and the Cyg X-1 (either in the soft or the hard state) over many decades in frequency indicates that the X-ray variability process is probably the same in all accreting objects, irrespective of the mass of the compact object. We plan to investigate this issue further in the near future.

TELAMON: Effelsberg monitoring of AGN jets with very-high-energy astroparticle emission

Aims. We introduce the TELAMON program which is using the Effelsberg 100-m telescope to monitor the radio spectra of active galactic nuclei (AGN) under scrutiny in astroparticle physics, specifically TeV blazars and candidate neutrino-associated AGN. Here, we present and characterize our main sample of TeV-detected blazars. Methods. We analyzed the data sample from the first ∼2.5 yr of observations between August 2020 and February 2023 in the range from 14 GHz to 45 GHz. During this pilot phase, we observed all 59 TeV-detected blazars in the Northern Hemisphere (i.e., Dec > 0°) known at the time of observation. We discuss the basic data reduction and calibration procedures used for all TELAMON data and introduce a sub-band averaging method used to calculate average light curves for the sources in our sample. Results. The TeV-selected sources in our sample exhibit a median flux density of 0.12 Jy at 20 mm, 0.20 Jy at 14 mm, and 0.60 Jy at 7 mm. The spectrum for most of the sources is consistent with a flat radio spectrum and we found a median spectral index (S(ν)∝να) of α = −0.11. Our results on flux density and spectral index are consistent with previous studies of TeV-selected blazars. Compared to the GeV-selected F-GAMMA sample, TELAMON sources are significantly fainter in the radio band. This is consistent with the double-humped spectrum of blazars being shifted towards higher frequencies for TeV-emitters (in particular for high-synchrotron peaked BL Lac type objects), which results in a lower radio flux density. The spectral index distribution of our TeV-selected blazar sample is not significantly different from the GeV-selected F-GAMMA sample. Moreover, we present a strategy to track the light curve evolution of sources in our sample for future variability and correlation analysis.

UV FeII emission model of HE 0413−4031 and its relation to broad-line time delays

Context. FeII emission is a well-known contributor to the UV spectra of active galactic nuclei and the modeling of this part may affect the results obtained for the MgII λ2800 emission, which is one of the lines used for black hole mass measurements and cosmological applications. Aims. We tested different FeII emission models when modeling the UV emission of the intermediate-redshift quasar HE 0413−4031 to see how the use of a specific template affects the MgII λ2800 line properties and the measurement of the MgII λ2800 and UV FeII time delays with respect to the continuum. Methods. We used the 11-year monitoring of the selected quasar HE 0413−4031 with the South African Large Telescope (SALT), and we supplemented this monitoring with the near-IR spectrum taken with the SOAR telescope, which gave access to the Hβλ4861 and [OIII] λλ4959, 5007 emission lines at the rest frame and allowed for a precise measurement of the redshift. Results. A new redshift determination (z = 1.39117 ± 0.00017) using [OIII] λλ4959, 5007 gave a very different value than the previous determination based only on the UV FeII pseudocontinuum (z = 1.3764). It favors a different decomposition of the spectrum into MgII and UV FeII emissions. The line characteristics and the time delay of the MgII emission (224−23+21 days) are not significantly affected. However, in comparison with the previous analysis, the rest-frame UV FeII time delay (251−7+9 days) is consistent with the inferred UV FeII line full width at half maximum (FWHM) of 4200 km s−1 that is only slightly smaller than the MgII line FWHM. Hence the FeII-emitting material is more distant than the MgII-emitting gas in HE 0413−4031 by ∼0.023 pc (4700 AU). The inferred velocity shift of both MgII and UV FeII lines with respect to the systemic redshift is now rather low, below 300 km s−1. In addition, we constructed an updated MgII radius-luminosity (R − L) relation from 194 sources, which is more than double the previous sample. The MgII R − L relation is flatter than the UV FeII, optical FeII, and HβR − L relations. While the new decomposition of the spectrum is satisfactory, we see a need to create better FeII templates using the newest version of the code CLOUDY.

Model of a “Warm Corona” as the origin of the soft X-ray excess of active galactic nuclei

Abstract The soft X-ray excess in the spectra of active galactic nuclei is characterized by similar electron temperatures of 0.1–0.3 keV and similar photon indices around 2.2–3, if fitted with inverse Comptonization. It remains a puzzle why both values are not sensitive to the black hole mass nor the accretion rate. Supposing that the scattering-dominated surface layer of an accretion disk can act as a warm corona, we construct a vertical one-zone model to understand what determines its temperature. By solving the equations of (1) the condition for the effective optical depth, (2) the energy balance, and (3) the dominance of the Compton cooling over the bound–free cooling, we could reproduce the basic observational features of the soft excess, provided that anomalous heating (excess heating other than what is expected by local energy dissipation) takes place in the warm corona in agreement with similar studies done so far. The similar temperatures can be understood, since both the anomalous heating and Compton cooling rates are proportional to the dissipation rate of the accretion energy, while similar photon indices are a natural consequence of the fact that observed photons are finally emitted from the layer of Compton y ∼ 1. The soft excess is not observed in black hole binaries, since disk temperatures are too high for the Compton scattering to work as cooling. The derived temperatures are somewhat of an underestimation, however. This may indicate a necessity of multi-zone corona structure. The stability of the warm corona and its consequences are briefly discussed.

Euclid preparation XXXVIII. Spectroscopy of active galactic nuclei with NISP

The statistical distribution and evolution of key properties of active galactic nuclei (AGN), such as their accretion rate, mass, and spin, remains a subject of open debate in astrophysics. The ESA space mission, launched on July $ 2023, promises a breakthrough in this field. We create detailed mock catalogues of AGN spectra from the rest-frame near-infrared down to the ultraviolet ---including emission lines--- to simulate what will observe for both obscured (type 2) and unobscured (type 1) AGN. We concentrate on the red grisms of the NISP instrument, which will be used for the wide-field survey, opening a new window for spectroscopic AGN studies in the near-infrared. We quantify the efficiency in the redshift determination as well as in retrieving the emission line flux of the H alpha N ii complex, as is mainly focused on this emission line, given that it is expected to be the brightest one in the probed redshift range. Spectroscopic redshifts are measured for 83<!PCT!> of the simulated AGN in the interval where the H alpha is visible (i.e. $0.89<z<1.83$ at a line flux of $>2 $\,erg $ cm$^ $, encompassing the peak of AGN activity at $z within the spectral coverage of the red grism. Outside this redshift range, the measurement efficiency decreases significantly. Overall, a spectroscopic redshift is correctly determined for about 90<!PCT!> of type 2 AGN down to an emission line flux of roughly $3 $\,erg $ cm$^ $, and for type 1 AGN down to $8.5 $\,erg $ cm$^ $. Recovered values for black hole mass show a small offset with respect to the input values by about 10<!PCT!>, but the agreement is good overall. With such a high spectroscopic coverage at $z<2$, we will be able to measure AGN demography, scaling relations, and clustering from the epoch of the peak of AGN activity down to the present-day Universe for hundreds of thousands of AGN with homogeneous spectroscopic information.

Time Evolving Photo Ionisation Device (TEPID): A novel code for out-of-equilibrium gas ionisation

Context. Photoionisation is one of the main mechanisms at work in the gaseous environment of bright astrophysical sources. A great deal of information on the gas physics, chemistry and kinematics, and on the ionising source itself, can be gathered through optical to X-ray spectroscopy. While several public time equilibrium photoionisation codes are readily available and can be used to infer average gas properties at equilibrium, time-evolving photoionisation models have only very recently started to become available. They are needed when the ionising source varies faster than the typical gas equilibration timescale. Using equilibrium models to analyse spectra of non-equilibrium photoionised gas may lead to inaccurate results, and prevents a solid assessment of gas density, physics, and geometry. Aims. Our main objective is to present and make available the Time-Evolving PhotoIonisation Device (TEPID), a new code that self-consistently solves time evolving photoionisation equations (both thermal and ionisation balance) and accurately follows the response of the gas to changes in the ionising source. Methods. TEPID self-consistently follows the gas temperature and ionisation in time by including all the main ionisation/recombination and heating/cooling mechanisms. The code takes in input the ionising light curve and spectral energy distribution and solves the time-evolving equations as a function of gas electron density and of time. The running time is intelligently optimised by an internal algorithm that initially scans the input light curve to set a time-dependent integration frequency. The code is built in a modular way, can be applied to a variety of astrophysical scenarios and produces time-resolved gas absorption spectra to fit the data. Results. To describe the structure and main features of the code, we present two applications of TEPID to two dramatically different astrophysical scenarios: the typical ionised absorbers observed in the X-ray spectra of active galactic nuclei (e.g. warm absorbers and ultra-fast outflows), and the circumburst environment of a gamma-ray burst. For both cases we show how the gas energy and ionisation balances vary as a function of time, gas density and distance from the ionising source. We show that time-evolving photoionisation leads to unique ionisation patterns that cannot be reproduced by stationary photoionisation codes when the gas is out of equilibrium. This demonstrates the need for codes such as TEPID in view of the unprecedented capabilities that will be offered by the upcoming high-resolution X-ray spectrometers on board missions like XRISM or Athena.

Influence of the optical Fe II quasi-continuum on measuring the spectral parameters of active galactic nuclei

Aims. We explore the influence of optical Fe II quasi-continuum on the measured spectral parameters in the λλ4150−5500 Å range for the spectra of Type 1 active galactic nuclei (AGNs). Methods. We assume that the broad line region is composed of two sub-regions: the very broad line region (VBLR) and the intermediate line region (ILR). We constructed a large set of synthetic AGN spectra by taking different portions of the VBLR and ILR contributions, where initially the VBLR and ILR model spectra were constructed on the basis of prototypes of two observed spectra with dominant VBLR (i.e. ILR) emission. To investigate the influence of the optical Fe II quasi-continuum on the AGN measured spectral parameters, we fit the power-law continuum and emission lines in a set of model spectra, as commonly done for observed AGN spectra. We then compared the spectral parameters obtained after the fitting procedure with those of the model. Results. We find that the optical Fe II quasi-continuum can be very strong in the case of spectra with strong and very broad Fe II lines and it is difficult to fully separate it from the power-law continuum. This gives the effect of a slightly underestimated Hβ width and underestimated fluxes of the Hβ and Fe II lines, while the continuum flux is then slightly overestimated. The most affected spectral parameters are the line equivalent widths (EWs), especially EW Fe II, which may be strongly underestimated. We discuss the possible underlying physics in the quasar main sequence, as implied by the results of our spectral modelling. We find that the set of AGN model spectra assuming different ILR and VBLR contributions can aptly reproduce the quasar main sequence, that is, the full width at half maximum Hβ versus Fe II/Hβ anti-correlation, where both parameters in this anti-correlation are strongly dependent on the ILR and VBLR contribution rate.

Testing Physical Scenarios for the Reflection Features of Type-1 AGNs using XMM-Newton and NuSTAR Simultaneous Observations

Above ∼3 keV, the X-ray spectrum of active galactic nuclei (AGNs) is characterized by the intrinsic continuum and Compton reflection features. For type-1 AGNs, several regions may contribute to the reflection. To investigate the nature of the reflecting medium, we perform a systematic analysis of the reflector using XMM-Newton and Nuclear Spectroscopic Telescope Array observations of a sample of 22 type-1 AGNs. We create a baseline model that includes Galactic absorption and an intrinsically absorbed power law, plus a reflection model. We test a set of nine reflection models in a subsample of five objects. Based on these results, we select three models to be tested on the entire sample, accounting for distinct physical scenarios: neutral/distant reflection, ionized/relativistic reflection, and neutral/distant+ionized/relativistic reflection, namely, a hybrid model. We find that 18 sources require the reflection component to fit their spectra. Among them, 67% prefer the hybrid model. Neutral and ionized models are equally preferred by three sources. We conclude that both the neutral/distant reflector most probably associated with the inner edges of the torus and the ionized/relativistic reflector associated with the accretion disk are required to describe the reflection in type-1 AGNs.

Star Formation in Self-gravitating Disks in Active Galactic Nuclei. III. Efficient Production of Iron and Infrared Spectral Energy Distributions

Strong iron lines are a common feature of the optical spectra of active galactic nuclei (AGNs) and quasars from z ∼ 6−7 to the local universe, and [Fe/Mg] ratios do not show cosmic evolution. During active episodes, accretion disks surrounding supermassive black holes (SMBHs) inevitably form stars in the self-gravitating part, and these stars accrete with high accretion rates. In this paper, we investigate the population evolution of accretion-modified stars (AMSs) to produce iron and magnesium in AGNs. The AMSs, as a new type of star, are allowed to have any metallicity but without significant loss from stellar winds, since the winds are choked by the dense medium of the disks and return to the core stars. Mass functions of the AMS population show a pile-up or cutoff pile-up shape in top-heavy or top-dominant forms if the stellar winds are strong, consistent with the narrow range of supernovae (SNe) explosions driven by the known pair-instability. This provides an efficient way to produce metals. Meanwhile, SN explosions support an inflated disk as a dusty torus. Furthermore, the evolving top-heavy initial mass functions lead to bright luminosity in infrared bands in dusty regions. This contributes a new component in infrared bands, which is independent of the emissions from the central part of accretion disks, appearing as a long-term trending of the NIR continuum compared to optical variations. Moreover, the model can be further tested through reverberation mapping of emission lines, including LIGO/LISA detections of gravitational waves and signatures from spatially resolved observations of GRAVITY+/VLTI.

Fantastic Fits with fantasy of Active Galactic Nuclei Spectra: Exploring the Fe ii Emission near the Hα Line

In this study, a refined approach for multicomponent fitting of active galactic nuclei (AGNs) spectra is presented utilizing the newly developed Python code Fully Automated pythoN Tool for AGN Spectra analYsis (fantasy). AGN spectra are modeled by simultaneously considering the underlying broken power-law continuum, predefined emission line lists, and an Fe ii model, which is here extended to cover the wavelength range 3700–11000 Å. The Fe ii model, founded solely on atomic data, effectively describes the extensive emission of the complex iron ion in the vicinity of the Hγ and Hβ lines, as well as near the Hα line, which was previously rarely studied. The proposed spectral fitting approach is tested on a sample of high-quality AGN spectra from the Sloan Digital Sky Survey Data Release 17. The results indicate that when Fe ii emission is present near Hβ, it is also detected redward from Hα, potentially contaminating the broad Hα line wings and thus affecting the measurements of its flux and width. The production of Fe ii emission is found to be strongly correlated with Eddington luminosity and appears to be controlled by a similar mechanism as the hydrogen Balmer lines. The study highlights the benefits of fitting type 1 AGN spectra with the fantasy code, pointing that it may be used as a robust tool for analyzing a large number of AGN spectra in the coming spectral surveys.

The universal shape of the X-ray variability power spectrum of AGN up to z ∼ 3

Aims. We study the ensemble X-ray variability properties of active galactic nuclei (AGN) over large ranges of timescale (20 ks ≤ T ≤ 14 yr), redshift (0 ≤ z ≲ 3), luminosity (1040 erg s−1 ≤ LX ≤ 1046 erg s−1), and black hole (BH) mass (106 ≤ M⊙ ≤ 109). Methods. We propose the use of the variance-frequency diagram as a viable alternative to the study of the power spectral density (PSD), which is not yet accessible for distant, faint, and/or sparsely sampled AGN. Results. We show that the data collected from archival observations and previous literature studies are fully consistent with a universal PSD form, which does not show any evidence for systematic evolution of shape or amplitude with redshift or luminosity, even if there may be differences between individual AGN at a given redshift or luminosity. We find new evidence that the PSD bend frequency depends on BH mass and possibly on accretion rate. We finally discuss the implications for current and future AGN population and cosmological studies.

Ray-tracing Simulations and Spectral Models of X-Ray Radiation in Dusty Media

Dust can play an important role in shaping the X-ray spectra and images of astrophysical sources. In this work we report on the implementation of dust in the ray-tracing platform RefleX. We illustrate the different effects associated with the interaction between X-ray photons and dust grains, such as dust scattering, near-edge X-ray absorption fine structures, and shielding. We show how the cross sections of the photon–gas interaction change depending on the fraction of metals in dust grains (i.e., the dust depletion factor). We compare RefleX simulations to the most widely used absorption model that includes dust and show how X-ray spectra are affected by the presence of dust in the absorbing/reprocessing medium for different geometries. We also show how RefleX can be used to reproduce the dust scattering halos observed in Galactic sources, and we release the first torus X-ray spectral model that considers dust absorption and scattering (RXTorusD), to reproduce the spectra of active galactic nuclei (AGNs). RXTorusD also considers other physical processes that are not included in the most widely used AGN torus models, such as Rayleigh scattering and scattering on molecular gas, which can lead to remarkable differences in the predicted X-ray spectra for the same set of geometrical and physical parameters.

An empirical relation to estimate host galaxy stellar light from AGN spectra

ABSTRACT Measurement of black hole mass for low-z (z≤ 0.8) Active Galactic Nuclei (AGNs) is difficult due to the strong contribution from host galaxy stellar light necessitating detailed spectral decomposition to estimate the AGN luminosity. Here, we present an empirical relation to estimate host galaxy stellar luminosity from the optical spectra of AGNs at z ≤ 0.8. The spectral data were selected from the fourteenth data release of the Sloan Digital Sky Survey (SDSS-DR14) quasar catalogue having a signal-to-noise ratio at 5100 Å (SNR5100) >10 containing 11 415 quasars. The median total luminosity (log (Ltotal/[erg s−1])), stellar luminosity (log (Lstar/[erg s−1])), and AGN continuum luminosity ((log Lcont/[erg s−1])) in our sample are 44.52, 44.06, and 44.30, respectively. We fit the AGN power-law continuum, host galaxy, and iron blend contribution, simultaneously over the entire available spectrum. We found the host galaxy fraction to anticorrelate with continuum luminosity and can be well-represented by a polynomial function, which can be used to correct the stellar light contribution from AGN spectra. We also found anticorrelation between host galaxy fraction and iron strength, Eddington ratio, and redshift. The empirical relation gives comparable results of host-fraction with the image decomposition method.

The entropy of galaxy spectra: how much information is encoded?

Abstract The inverse problem of extracting the stellar population content of galaxy spectra is analysed here from a basic standpoint based on information theory. By interpreting spectra as probability distribution functions, we find that galaxy spectra have high entropy, thus leading to a rather low effective information content. The highest variation in entropy is unsurprisingly found in regions that have been well studied for decades with the conventional approach. We target a set of six spectral regions that show the highest variation in entropy – the 4000 Å break being the most informative one. As a test case with real data, we measure the entropy of a set of high-quality spectra from the Sloan Digital Sky Survey, and contrast entropy-based results with the traditional method based on line strengths. The data are classified into star-forming (SF), quiescent (Q), and active galactic nucleus (AGN) galaxies, and show – independently of any physical model – that AGN spectra can be interpreted as a transition between SF and Q galaxies, with SF galaxies featuring a more diverse variation in entropy. The high level of entanglement complicates the determination of population parameters in a robust, unbiased way, and affects traditional methods that compare models with observations, as well as machine learning (especially deep learning) algorithms that rely on the statistical properties of the data to assess the variations among spectra. Entropy provides a new avenue to improve population synthesis models so that they give a more faithful representation of real galaxy spectra.

Forecasts for cosmological measurements based on the angular power spectra of AGN and clusters of galaxies in the SRG/eROSITA all-sky survey

Context. The eROSITA X-ray telescope aboard the Spectrum-Roentgen-Gamma (SRG) orbital observatory, in the course of its all-sky survey, is expected to detect about three million active galactic nuclei (AGNs) and approximately one hundred thousand clusters and groups of galaxies. Such a sample, clean and uniform, complemented with redshift information, will open a new window into the studies of the large-scale structure (LSS) of the Universe and the determination of its cosmological parameters. Aims. The purpose of this work is to assess the prospects of cosmological measurements with the eROSITA sample of AGNs and clusters of galaxies. We assumed the availability of photometric redshift measurements for eROSITA sources and explored the impact of their quality on our forecasts. Methods. As the LSS probe, we use the redshift-resolved angular power spectrum of the density fluctuations of objects. We employed a Fisher-matrix formalism and assumed flat ΛCDM cosmology to forecast the constraining power of eROSITA samples of AGNs and clusters of galaxies. We computed the LSS-relevant characteristics of AGNs and clusters in the framework of the halo model and their X-ray luminosity functions. As the baseline scenario, we considered the full four-year all-sky survey and investigated the impact of reducing the survey length to two years. Results. We find that the accuracy of photometric redshift estimates has a more profound effect on cosmological measurements than the fraction of catastrophic errors. Under realistic assumptions about the photometric redshift quality, the marginalised errors on the cosmological parameters achieve 1 − 10% accuracy depending on the cosmological priors used from other experiments. The statistical significance of Baryon acoustic oscillation detection in angular power spectra of AGNs and clusters of galaxies considered individually achieves 5 − 6σ. Our results demonstrate that the eROSITA sample of AGNs and clusters of galaxies used in combination with currently available photometric redshift estimates will provide cosmological constraints on a par with dedicated optical LSS surveys.

Tell-tale Spectral Signatures of MHD-driven Ultrafast Outflows in AGNs

We aim to explore spectral signatures of the predicted multi-ion ultrafast outflows (UFOs) in the broadband X-ray spectra of active galactic nuclei by exploiting an accretion disk-wind model in the context of a simple magnetohydrodynamic (MHD) framework. We are focused primarily on examining the spectral dependences on a number of key properties: (1) ionizing luminosity ratio λ ion, (2) line-of-sight wind density slope p, (3) optical/UV-to-X-ray strength α OX, (4) inclination θ, (5) X-ray photon index Γ, and (6) wind density factor f D . With an emphasis on radio-quiet Seyferts in the sub-Eddington regime, multi-ion UFO spectra are systematically calculated as a function of these parameters to show that MHD-driven UFOs imprint a unique asymmetric absorption line profile with a pronounced blue tail structure on average. Such a characteristic line signature is generic to the simplified MHD disk-wind models presented in this work due to their specific kinematics and density structure. The properties of these absorption line profiles could be utilized as a diagnostics to distinguish between different wind-driving mechanisms or even the specific values of given MHD wind parameters. We also present high-fidelity microcalorimeter simulations in anticipation of the upcoming XRISM/Resolve and Athena/X-IFU instruments to demonstrate that such a “tell-tale” sign may be immune to a spectral contamination by the presence of additional warm absorbers and partially covering gas.

Reconstructing AGN X-ray spectral parameter distributions with Bayesian methods

X-ray spectra of active galactic nuclei (AGN) consist of several different emission and absorption components. To determine the spectral parameters, these components are often fitted manually with models chosen on a case-by-case basis. However, this approach has two problems. First, it becomes very hard for a survey with a large number of sources. Second, when the signal-to-noise ratio (S/N) is low, there is a tendency to adopt an overly simplistic model, biasing the parameters and making their uncertainties unrealistic. We developed a Bayesian method for automatically fitting AGN X-ray spectra obtained by XMM-Newton with a consistent and physically motivated model. Our model includes all spectral components, even when the data quality is low. We used a physical model for the X-ray background and an empirical model for the non-X-ray background. Noninformative priors were applied on the parameters of interest, the photon index (Γ) and the hydrogen column density (NH), while informative priors obtained from deep surveys were used to marginalize over the parameter space of the nuisance parameters. To improve speed, we developed a specific spectral extraction and fitting procedure. We tested this method using a realistic sample of 5000 spectra, which was simulated based on our source model, reproducing typical population properties. Spectral parameters were randomly drawn from the priors, taking the luminosity function into account. Well-constrained or meaningful posterior probability density distributions (PDFs) were obtained for the most relevant spectral parameters, for instance, NH, Γ, and LX, even at low S/N, but in this case, we were unable to constrain the parameters of secondary components such as the reflection and soft excess. As a comparison, a maximum-likelihood approach with model selection among six models of different complexities was also applied to this sample. We find clear failures in the measurement of Γ in most cases, and of NH when the source is unabsorbed (NH < 1022 cm−2). The results can hardly be used to reconstruct the parent distributions of the spectral parameters, while our Bayesian method provides meaningful multidimensional posteriors that will be used in a subsequent paper to infer the population.

Spiral arms in broad-line regions of active galactic nuclei

As a major feature of the spectra of active galactic nuclei, broad emission lines deliver information on the kinematics and spatial distributions of ionized gas located in the so-called broad-line regions (BLRs) surrounding the central supermassive black holes (SMBHs). There is growing evidence for the appearance of spiral arms in BLRs. It has been shown through reverberation mapping (RM) campaigns that the characterized radius of BLRs overlaps with that of self-gravitating regions of accretion disks. In the framework of the WKB approximation, we show robust properties of observational features of the spiral arms. The resulting spiral arms lead to the characterization of various profiles of the broad emission line. We calculated the RM and differential interferometric features of BLRs withm = 1 mode spiral arms. These features can be detected with high-quality RM and differential interferometric observations via such instruments as GRAVITY on board the Very Large Telescope Interferometer (VLTI). The WKB approximation will be relaxed and universalized in the future to explore more general cases of density wave signals in RM campaigns and differential spectroastrometric observations.

X-ray illuminated accretion discs and quasar microlensing disc sizes

Aims. We study the half-light radius versus black hole mass as well as the luminosity versus black hole mass relations in active galactic nuclei (AGN) when the disc is illuminated by the X-ray corona. Methods. We used KYNSED, a recently developed spectral model for studying broadband spectral energy distribution in AGN. We considered non-illuminated Novikov-Thorne discs and X-ray illuminated discs based on a Novikov-Thorne temperature radial profile. We also considered the case where the temperature profile is modified by a colour-correction factor. In the case of X-ray illumination, we assumed that the X-ray luminosity is equal to the accretion power that is dissipated to the disc below a transition radius and we computed the half-light radius and the disc luminosity for many black hole masses, as well as a wide range of accretion rates, black hole spins, X-ray luminosities and heights of the corona. Results. The half-light radius of X-ray illuminated radii can be up to ∼3.5 times greater than the radius of a standard disc, even for a non-spinning black hole, based on a wide range of model parameters – as long as the transition radius is larger than three times the radius of the innermost stable circular orbit and the coronal height is greater than ∼40 Rg. This result is due to the fact that the absorbed X-rays act as a secondary source of energy, increasing the disc temperature, and mainly at large radii. Non-illuminated discs are consistent with observations, but only at the 2.5σ level. On the other hand, X-ray illuminated discs can explain both the half-light radius-black hole mass as well as the luminosity-black hole mass relation in AGN, for a wide range of physical parameters. The range of the parameter space is broader in the case where we consider the colour-correction factor. X-ray illuminated discs can explain the data when we observe gravitationally lensed quasars mainly face-on, but also if the mean inclination angle is 60°. In addition, we show that the observed X-ray luminosity of the gravitationally lensed quasars is fully consistent with the X-ray luminosity that is necessary for heating the disc. Conclusions. X-ray disc illumination was proposed many years ago to explain various features that are commonly observed in the X-ray spectra of AGN. Recently, we showed that X-ray illumination of the accretion disc can also explain the observed UV/optical time-lags in AGN, while in this work, we show that the same model can also account for the quasar micro-lensing disc size problem. These results support the hypothesis of the disc X-ray illumination in AGN.

H i 21-cm absorption in radio-loud AGN with double-peaked [O iii] emission

ABSTRACT Different physical processes in galaxy evolution, such as galaxy mergers that lead to coalescence of dual Active Galactic Nuclei (AGN) and outflows emanating from the narrow line region, can leave their imprint on the optical spectra of AGN in the form of double-peaked narrow emission lines. To investigate the neutral gas in the centres of such AGN, we have conducted a pilot survey of H i 21-cm absorption, using the upgraded Giant Metrewave Radio Telescope (uGMRT), in radio-loud AGN whose optical spectra show double-peaked [O iii] emission lines at z ≤ 0.4 (median z ≈ 0.14). Among the eight sources for which we could obtain clean spectra, we detect H i 21-cm absorption in three sources (detection rate of $38^{+36}_{-20}$ per cent) and find tentative indication of absorption in two other sources. The detection rate of H i 21-cm absorption is tentatively higher for the systems that show signatures of interaction or tidal disturbance (≳ 50 per cent) in the ground-based optical images than that for the systems that appear single and undisturbed (≈25 per cent). This is consistent with the high incidence of H i 21-cm absorption observed in z ≤ 0.2 galaxy mergers. Higher spatial resolution spectroscopy is required to confirm the origin of the H i absorbing gas, i.e. either gas infalling on to the radio-loud AGN, outflowing gas ejected by the AGN, or gas in rotation on the galactic-scale or circumnuclear discs.