Exploring the AGN Accretion Disks Using Continuum Reverberation Mapping

[Context]: The X-ray spectra of active galactic nuclei (AGN) unveil properties of matter around the super-massive black hole (SMBH). [Aims]: We investigate the X-ray spectra of AGN focusing on Compton reflection and fluorescence. These are two of the most important processes of interaction between primary radiation and circumnuclear material that is located far away from the SMBH, as indicated by the unresolved spectral emission lines (most notably the Fe line) in the X-ray spectra of AGN. Contributions from the inner accretion disk, affected by relativistic effects as expected, have also been detected in several cases. [Methods]: We studied the average X-ray spectrum of a sample of 263 X-ray unabsorbed AGN that yield 419 023 counts in the 2-12 keV rest-frame band distributed among 388 XMM-Newton spectra. [Results]: We fitted the average spectrum using a (basically) unabsorbed power law (representing the primary radiation). From a second model that represents the interaction (through Compton reflection and fluorescence) of this primary radiation with matter located far away from the central engine (e.g. the putative torus), we found that it was very significantly detected. Finally, we added a contribution from interaction with neutral material in the accretion disk close to the central SMBH, which is therefore smeared by relativistic effects, which improved the fit at a 6 sigma. The reflection factors are 0.65 for the accretion disk and 0.25 for the torus. Replacing the neutral disk-reflection with low-ionisation disk reflection, also relativistically smeared, fits the data equally well, suggesting that we do not find evidence for significant ionisation of the accretion disk. [Conclusions]: We detect distant neutral reflection associated with a narrow Fe line in the average spectrum of unabsorbed AGN with z = 0.8. Adding the disk-reflection component associated with a relativistic Fe line improves the data description at a 6 sigma confidence level, suggesting that both reflection components are present. The disk-reflection component accounts for about 70% of the total reflected flux. © ESO, 2014.

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.

The [OII] 3726+3728\AA\ emission line doublet is often used to estimate star formation rates within the host galaxies of active galactic nuclei (AGN), as it is known to be strongly excited by star formation, but is only weakly excited in the broad and narrow line regions of AGN. However, within AGN host galaxies, [OII] can also be excited in low-density gas located at appreciable distances from the nucleus, but still ionized by the AGN. These AGN extended emission line regions (EELRs) can contribute significant flux to integrated spectra, even in the presence of luminous AGN. Here, we identify EELRs by the presence of the [NeV] 3426\AA\ emission line, which, like [OII], is not strongly excited in the inner regions of AGN, but is a prominent emission line in the lower density EELRs. Critically, unlike [OII], [NeV] is not excited by star formation. Therefore, when strong [NeV] is present in an AGN spectrum, the flux from the EELR is not negligible, implying the [OII] flux is contaminated by emission from the EELR, and is not a good measure of star formation. After removing objects with EELRs identified by [NeV], the [OII] flux in the host galaxies of radio-loud AGN is found to be higher than that within radio-quiet AGN, which could either indicate higher star-formation rates, or the presence of moderate-velocity shocks. Being mindful of EELRs for upcoming large-area spectroscopic surveys, particularly those tied to radio continuum surveys, will be important for determining star formation rates in AGN host galaxies.

The Vera C. Rubin Observatory’s Legacy Survey of Space and Time (LSST) will detect an unprecedentedly large sample of actively accreting supermassive black holes with typical accretion disk (AD) sizes of a few light days. This brings us to face challenges in the reverberation mapping (RM) measurement of AD sizes in active galactic nuclei using interband continuum delays. We examine the effect of LSST cadence strategies on AD RM using our metric AGN_TimeLagMetric. It accounts for redshift, cadence, the magnitude limit, and magnitude corrections for dust extinction. Running our metric on different LSST cadence strategies, we produce an atlas of the performance estimations for LSST photometric RM measurements. We provide an upper limit on the estimated number of quasars for which the AD time lag can be computed within 0 < z < 7 using the features of our metric. We forecast that the total counts of such objects will increase as the mean sampling rate of the survey decreases. The AD time lag measurements are expected for >1000 sources in each deep drilling field (DDF; (10 deg2)) in any filter, with the redshift distribution of these sources peaking at z ≈ 1. We find the LSST observation strategies with a good cadence (≲5 days) and a long cumulative season (∼9 yr), as proposed for LSST DDF, are favored for the AD size measurement. We create synthetic LSST light curves for the most suitable DDF cadences and determine RM time lags to demonstrate the impact of the best cadences based on the proposed metric.

This study analyses JWST MIRI/MRS observations of the infrared (IR) polycyclic aromatic hydrocarbon (PAH) bands in the nuclear (∼0.4″ at 11 μm; ∼75 pc) and circumnuclear regions (inner ∼kpc) of local active galactic nuclei (AGNs) from the Galactic Activity, Torus, and Outflow Survey (GATOS). We examine the PAH properties in the circumnuclear regions of AGNs and the projected direction of AGN-outflows and compare them to those in star-forming regions and the innermost regions of AGNs. This study employs 4.9–28.1 μm sub-arcsecond angular resolution data to investigate the properties of PAHs in three nearby sources (DL ∼ 30 − 40 Mpc). Our findings are aligned with previous JWST studies, demonstrating that the central regions of AGNs display a larger fraction of neutral PAH molecules (i.e. elevated 11.3/6.2 and 11.3/7.7 μm PAH ratios) in comparison to star-forming galaxies. We find that AGNs might affect not only the PAH population in the innermost region, but also in the extended regions up to ∼kpc scales. By comparing our observations to PAH diagnostic diagrams, we find that, in general, regions located in the projected direction of the AGN-outflow occupy similar positions on the PAH diagnostic diagrams as those of the innermost regions of AGNs. Star-forming regions that are not affected by the AGNs in these galaxies share the same part of the diagram as star-forming galaxies. We also examined the potential of the PAH-H2 diagram to disentangle AGN-versus-star-forming activity. Our results suggest that in Seyfert-like AGNs, the illumination and feedback from the AGN might affect the PAH population at nuclear and kpc scales, particularly with respect to the ionisation state of the PAH grains. However, PAH molecular sizes are rather similar. The carriers of the ionised PAH bands (6.2 and 7.7 μm) are less resilient than those of neutral PAH bands (11.3 μm), which might be particularly important for strongly AGN-host coupled systems. Therefore, caution must be applied when using PAH bands as star-formation rate indicators in these systems even at kpc scales, with the effects of the AGN being more important for ionised ones.

Context.Understanding the physics and geometry of accretion and ejection around super massive black holes (SMBHs) is important to understand the evolution of active galactic nuclei (AGN) and therefore of the large scale structures of the Universe.Aims.We aim at providing a simple, coherent, and global view of the sub-parsec accretion and ejection flow in AGN with varying Eddington ratio,ṁ, and black hole mass,MBH.Methods.We made use of theoretical insights, results of numerical simulations, as well as UV and X-ray observations to review the inner regions of AGN by including different accretion and ejection modes, with special emphasis on the role of radiation in driving powerful accretion disk winds from the inner regions around the central SMBH.Results.We propose fiveṁregimes where the physics of the inner accretion and ejection flow around SMBHs is expected to change, and that correspond observationally to quiescent and inactive galaxies; low luminosity AGN (LLAGN); Seyferts and mini-broad absorption line quasars (mini-BAL QSOs); narrow line Seyfert 1 galaxies (NLS1s) and broad absorption line quasars (BAL QSOs); and super-Eddington sources. We include in this scenario radiation-driven disk winds, which are strong in the highṁ, largeMBHregime, and possibly present but likely weak in the moderateṁ, smallMBHregime.Conclusions.A great diversity of the accretion/ejection flows in AGN can be explained to a good degree by varying just two fundamental properties: the Eddington ratioṁand the black hole massMBH, and by the inclusion of accretion disk winds that can naturally be launched by the radiation emitted from luminous accretion disks.

Continuum reverberation mapping is widely used in studying the accretion disks of active galactic nuclei (AGNs). Some indirect evidence and simulations have indicated that the diffuse continuum, especially the strong Balmer continuum from the broad-line region, may contribute to the continuum in the u/U band. Here, we present direct evidence for this contribution. In this work, we apply the ICCF-Cut method to continuum reverberation mapping to extract the possible diffuse continuum light curves of six AGNs, using high-cadence, high-quality, and multiband observations. We find the existence of an outer component out of the accretion disk for each of the six AGNs in the Swift U band. Meanwhile, similar results can be derived with the JAVELIN Photometric Reverberation Mapping Model for four of them. The lags of the outer components are consistent with the predicted Balmer continuum lags, which are about half of the Hβ lag values. Our result directly reinforces the understanding that an outer component, especially the Balmer continuum in the rest-frame u/U band, can contribute significantly to the continuum reverberation lags of AGNs.

Despite the long history of studies of active galactic nuclei (AGN), details of the structure of the accretion flow onto supermassive black holes are far from clear. Work presented in this thesis is directed at unveiling properties of AGN structure through broadband X-ray spectroscopy, with particular emphasis on the hard X-ray band (photon energies above 10 keV). With its unprecedented sensitivity in this energy band, the NuSTAR telescope provides the key observational diagnostics of the properties of the AGN X-ray source, the corona, and the surrounding gas in the accretion disk, the broad-line region, and the torus. The first study presented in this thesis focuses on measurements of the optical depth and the temperature of the plasma in the corona of an obscured AGN. Fitting theoretical spectral models for coronal emission to the NuSTAR data constrained these two basic physical parameters under the assumption of either spherical or disk-like geometry for the corona. The remainder of the thesis is dedicated to studies of the anisotropic obscuring structure broadly referred to as the torus. One of them is a case study of three heavily obscured AGN with spectra dominated by the X-ray light scattered and reprocessed in the torus, where it is possible to constrain one of the basic torus properties -- its globally averaged column density. The following study presents the calculation of a new spectral model for reprocessing of the intrinsic X-ray continuum within the torus. Its added flexibility compared to previously available models allows for both the average column density of the torus and its covering factor to be constrained from broadband X-ray spectra of a wide variety of AGN. The final part of the thesis in based on a large survey of the local obscured AGN population performed with NuSTAR. Spectral modeling of more than a hundred individual AGN, including both old and new spectral models, is presented. From analyses of the X-ray data for a large and representative AGN sample, for the first time, it is found that their tori preferentially have high covering factors and average column densities close to unity optical depth for Compton scattering.

The line profiles like that of the fluorescent Fe K or Co K lines in the X-ray spectra of the active galactic nuclei (AGN) reflect characteristics of the central regions of these objects. These lines can be formed in the accretion disks around central supermassive black holes and their shapes are connected with the central black hole spin and the accretion disk inclination angle to the line-of-the-sight. If an AGN is a source of a gravitational lens system with microlensing events, one can get an additional important information about both the accretion disk parameters and gravitational lens parameters as well. Microlensing processes were observed in such gravitational lens systems, as PKS 1830-211, B0218+357, RX J1131-1231 i HE1104-1805, Q2237+0305 and we can suspect to observe there also the spectral appearances of microlensing. Here we performed the numerical simulations of the microlensed relativistic spectral line profiles formed in the AGN accretion disks. Using the inear caustic model we show that the time dependence of the profile is determined essentially by the angle between to the disk axis and the caustic. This gives us an opportunity to assess this orientation. Microlens caustics magnify some parts of the accretion disk more prominently than others. Due to the Doppler effects and differences in the rotation direction this leads to the frequency-dependent magnification which distorts the profile of a relativistic spectral line. Such deformations are variable with time due to relative motions of the source and the microlens, and they can give us possibility to obtain some additional information about the disk brightness profile and caustic orientation relatively to the disk. Here we consider the thin disk model, Schwarzschild black hole, and the linear caustic approximation as well. The numerical simulations of the relativistic emission line profiles distorted by strong gravitational microlensing effect were performed for several different orientations of the linear caustic relatively to the disk, as well as several inclinations of the disk to the line-of-the-sight. Basic presumptions for the numerical modeling were the following: (a) AGN is a source in the gravitational lens system and it its inner parts the luminescent emission lines with relativistic profiles are being emitted; (b) this line is formed in the thin accretion disk quite far away from the central black hole and can be calculated with no taking into account the relativistic effects; (c) the caustic can be considered as a linear one. We show that the relative orientation of the caustic and the disk can be determined from emission lines profiles. Our numerical simulations demonstrate that the difference between profiles corresponding to different caustic orientations appears to be more prominent during the first half of the strong microlensing event, namely, before the crossing the disk center, and this dependence is irrespective to the accretion disk brightness profile. We show that for the spectral accuracy level high enough we have a perspective to determine the caustic orientation from the observational data.

As active galactic nuclei (AGN) ‘turn on’, some stars end up embedded in accretion discs around supermassive black holes (SMBHs) on retrograde orbits. Such stars experience strong headwinds, aerodynamic drag, ablation, and orbital evolution on short time-scales. The loss of orbital angular momentum in the first ∼0.1 Myr of an AGN leads to a heavy rain of stars (‘starfall’) into the inner disc and on to the SMBH. A large AGN loss cone (θAGN, lc) can result from binary scatterings in the inner disc and yield tidal disruption events (TDEs). Signatures of starfall include optical/UV flares that rise in luminosity over time, particularly in the inner disc. If the SMBH mass is $M_{\rm SMBH} \gtrsim 10^{8}\, \mathrm{M}_{\odot }$, flares truncate abruptly and the star is swallowed. If $M_{\rm SMBH}\lt 10^{8}\, \mathrm{M}_{\odot }$, and if the infalling orbit lies within θAGN, lc, the flare is followed by a TDE that can be prograde or retrograde relative to the AGN inner disc. Retrograde AGN TDEs are overluminous and short-lived as in-plane ejecta collide with the inner disc and a lower AGN state follows. Prograde AGN TDEs add angular momentum to inner disc gas and so start off looking like regular TDEs but are followed by an AGN high state. Searches for such flare signatures test models of AGN ‘turn on’, SMBH mass, as well as disc properties and the embedded population.

It has been recently proposed that the broad line region in active galactic\nnuclei originates from dusty clouds driven from the accretion disk by radiation\npressure, at a distance from the black hole where the disk is cooler than the\ndust sublimation temperature. We test this scenario by checking the consistency\nof independent broad line region and accretion disk reverberation measurements,\nfor a sample of 11 well studied active galactic nuclei. We show that\nindependent disk and broad line region reverberation mapping measurements are\ncompatible with a universal disk temperature at the H{\\beta} radius of\nT[R(H{\\beta})]=1670(231) K which is close to typical dust sublimation\ntemperatures.\n

We report the discovery of the first transient relativistic iron K α line in an active galactic nucleus (AGN), J1047+5907. The line was detected 21.5 days (rest frame) after an X-ray coronal flare was observed in 2008, and it exhibits significant broadening consistent with relativistic reflection from the accretion disk in the vicinity of the central supermassive black hole (SMBH). The line has a width of ∼300 eV, corresponding to a Keplerian velocity of 14,000 km s −1 , at a distance of 5–41 light-days from the SMBH, strongly implying that the observed coronal flare triggered the emergence of the line. This event provides rare direct evidence of the response of the accretion disk to impulsive coronal illumination and offers a new method to probe the SMBH and disk physics. The relativistic modeling favors a broadened line produced by distant reflection from an accretion disk around a rapidly spinning black hole viewed at an intermediate inclination, consistent with other observations. Systematic monitoring of type 1 AGN following strong X-ray flares may open a new observational window into the innermost regions of AGN, enabling constraints on the physics of SMBH and its accretion disk at different radii that are otherwise challenging to access.

Active galactic nuclei (AGNs) are objects hosting in their center a super-massive black hole (SMBH) with an accretion disk surrounded by gas and dust. The mass of an SMBH can be derived from the dynamics of the gas gravitationally bounded to the SMBH. This is the case for the broad line region (BLR), i.e. a photoionized gas in the vicinity of an SMBH that emits broad emission lines (BELs), which properties can be used to estimate the mass of the SMBH. In spite of a number of papers devoted to the BLR research, its true nature is not well known. Therefore, it is still important to investigate the BLR structure (size, geometry, physics, etc.), where one of the aims is to better constrain the mass of the SMBH in the center of AGNs. The BELs are the only signatures of the BLR physics and geometry. They can be clearly identified in AGN spectra and they often show complex profiles. Their fluxes, profiles and ratios can provide much information about the BLR geometry and physics. Moreover, the BELs and continuum flux are very often varying in AGNs. Therefore, an investigation of the BEL flux and profile variability during a long period is another useful tool for mapping the geometrical and dynamical structure of the BLR. In this review we present and discuss some tools and techniques for studying the structure of the BLR using broad emission line properties.

Supermassive black holes in the centers of active galactic nuclei (AGN) are surrounded by broad-line regions (BLRs). The broad emission lines seen in the AGN spectra are emitted in this spatially unresolved region. We intend to obtain information on the structure and geometry of this BLR based on observed line profiles. We modeled the rotational and turbulent velocities in the line-emitting region on the basis of the line-width FWHM and line dispersion sigma_line of the variable broad emission lines in NGC5548. Based on these velocities we estimated the height of the line-emitting regions above the midplane in the context of their distances from the center. The broad emission lines originate at distances of 2 to 27 light days from the center. Higher ionized lines originate in the inner region (lesser equal 13 light days) in specific filamentary structures 1 to 14 light days above the midplane. In contrast, the Hbeta line is emitted in an outer (6 - 26 light days), more flattened configuration at heights of 0.7 to 4 light days only above the midplane. The derived geometry of the line-emitting region in NGC5548 is consistent with an outflowing wind launched from an accretion disk.

Accretion disks around supermassive black holes (SMBH) in the centers of galaxies are the central engines of active galactic nuclei (AGN), which are observable over the entire electromagnetic spectrum and out to the beginning of galaxy formation. The gradual assembly of galaxies is believed to have resulted in SMBH today. The growth of SMBH is largely hidden by dust, and possibly by large time delays between galaxy mergers and the feeding of the central monster, so the connection between galaxy assembly and SMBH growth is currently at best circumstantial. Facilities like HST WFC3 and the James Webb Space Telescope (JWST) are needed to trace this process from the epoch of reionization to the present.Using panchromatic deep HST WFC3+ACS imaging data, grism spectra, and ground‐based spectroscopy in GOODS and the Hubble Ultra Deep Field (HUDF), we address this issue through the epoch‐dependent rate of major mergers in massive galaxies in the HUDF, and through SED‐fitting of objects with and without (known) AGN in GOODS. On average, the field galaxy population at z = 1–6 has an underlying star‐forming SED with typical ages of 0.1–0.2 Gyr. However, most AGN‐dominated objects at z = 0.5–1.5 have an underlying stellar SED age of ∼1 Gyr on average. This suggests that AGN growth and SMBH feeding may become visible 0.5–1 Gyr after the dynamical event which triggers the dominant starburst at these redshifts. This may also be reflected in the peak in the massive galaxy major merger rate compared to the peak in the redshift distribution of weak AGN. Finally, we discuss how JWST will expand on this topic in the next decade from the epoch of first light to the present.