High Eddington quasars as discovery tools: current state and challenges

Abstract

A landmark of accretion processes in active galactic nuclei (AGN) is the continuum originating from a complex structure, i.e., an accretion disk and a corona around a supermassive black hole. Modelling the broad-band spectral energy distribution (SED) effectively ionizing the gas-rich broad emission line region (BLR) is key to understanding the various radiative processes at play and their importance that eventually leads to the emission from diverse physical conditions. Photoionization codes are a useful tool to investigate two aspects, the importance of the shape of the spectral energy distribution, and the physical conditions in the broad emission line region. In this work, we critically review long-standing issues pertaining to the spectral energy distribution shape and the anisotropic continuum radiation from the central regions around the accreting supermassive black holes (few 10–100 gravitational radii), with a focus on black holes accreting at high rates, possibly much above the Eddington limit. The anisotropic emission is a direct consequence of the development of a geometrically and optically thick structure at regions very close to the black hole due to a marked increase in the accretion rates. The analysis presented in this paper took advantage of the look at the diversity of the type-1 active galactic nuclei provided by the main sequence of quasars. The main sequence permitted us to assess the importance of the Eddington ratio and hence to locate the super Eddington sources in observational parameter space, as well as to constrain the distinctive physical conditions of their line-emitting BLR. This feat is posing the basis for the exploitation of quasars as cosmological distance indicators, hopefully allowing us to use the fascinating super Eddington quasars up to unprecedented distances.

1 Active galactic nuclei as accreting black holes

Active galactic nuclei (AGNs) are among the brightest cosmic objects known to us (Weedman, 1976; Weedman, 1977). They harbour a supermassive black hole (SMBH) at their very centres which due to its immense gravitational potential allows for the infalling of matter. This in-falling matter loses angular momentum while being accreted onto the black hole. This accreted matter manifests in the form of a multi-colour accretion disk which gets heated up and radiates (Shakura and Sunyaev, 1973; Shields, 1978; Czerny and Elvis, 1987; Panda et al., 2018). The photon energy of the dissipated radiation spans a wide range of energies (from sub-eV to hundreds of eVs). The emitted photons then illuminate the material surrounding the accretion disk and lead to the formation and emission of strong, broad emission lines (Schmidt, 1963; Greenstein and Schmidt, 1964; Schmidt and Green, 1983; Osterbrock and Ferland, 2006; Netzer, 2015).

2 Their spectral energy distribution

AGN are observed over the entire range of the electromagnetic spectrum from the radio regime up to MeV-GeV-TeV energy γ-rays (Richards et al., 2006; Harrison, 2014; Yang et al., 2022). The classical view of AGN characterized by an almost flat spectral energy distribution (SED) over many decades in frequency—to juxtapose to the ones of non-active galaxies—has been superseded and extended since long (Malkan and Sargent, 1982; Malkan, 1983), by the recognition that the SED is different for AGN in different accretion states, and is most often characterized by significant features associated with diverse processes and aptly called bump or excess (i.e., IR excess, big blue bump, soft-X excess, etc.)¹.

The “intrinsic” AGN continuum at photon energies high enough to ionize Hydrogen is therefore made of the thermal emission from the accretion disk, the power-law emission from the corona, and soft X-ray excess (Collinson, 2016; Kubota and Done, 2018; Panda et al., 2019c; Ferland et al., 2020). Figure 1 shows templates for quasars believed to radiate at moderate or high Eddington ratio, η ≳ 0.1–0.2 (Population A and extreme Population A, hereafter xA, Marziani and Sulentic 2014a and §3.3.2), along with widely-exploited templates believed to be appropriate for populations of quasars radiating in this range (Mathews and Ferland, 1987; Marziani and Sulentic, 2014a; Ferland et al., 2020). There is a notable similarity between the curves. The SED defined by Marziani and Sulentic (2014a) for sources radiating close to the Eddington limit is in good agreement with the high case of Ferland et al. (2020). There is an increase in big blue bump prominence from the high to the highest case, the latter being associated with extreme values of the Eddington ratios. Note that the soft-X ray excess, located between the optical-UV bump and the peak at the hard X-ray (∼100 keV), is prominent in between keV and 20 keV regions for the SED corresponding to the highest Eddington ratio case (magenta curve in Figure 1) and marginally present in the high case (blue curve in Figure 1). One notices that this feature steepens with Γ > 2 as the Eddington ratio increases (Jin et al., 2012b; Ferland et al., 2020). The feature almost disappears when one transitions to low Eddington ratio sources—see the blue dashed and grey SEDs, where the X-ray bump close to 100 keV is increasingly prominent.

FIGURE 1

A weak but statistically significant correlation between hard-X photon index Γ and Eddington ratio has been found (Trakhtenbrot et al., 2017; Panagiotou and Walter, 2020; Liu et al., 2021), and the statistical weaknesses might be explained by the limited range of hard Γ values compared to uncertainties in individual Γ estimates (Wang et al., 2013). In the highest case, the soft and hard X-ray domains are very steep to the point that a turnover at keV as seen in Figure 1 for the Mathews and Ferland (1987) SED may not be anymore required. The difference between the Mathews and Ferland (1987) SED and the extreme case of Ferland et al. (2020) exemplifies this trend. The existence of X-ray weak type-1 AGN and their high prevalence among highly accreting sources (Zappacosta et al., 2020; Laurenti et al., 2022) may also support the absence of a prominent Compton-thin coronal component in super-Eddington sources. The sequence of SEDs in Figure 1 is related to the 4DE1 parameter space (Section 3.3), although its connection to some of the parameters is still incomplete.

A related issue is the location of the high energy downturn around ∼ 100 keV that is required by limits in the measured X-ray background (Mathews and Ferland, 1987). Observations are mostly available up to ∼ 20 keV, and the energy of the downturn is conventionally placed at keV in the SEDs of Figure 1 although it wasn’t actually measured. In recent years measurements by NuSTAR and γ-ray observatories such as SWIFT indicate a dispersion in the actual turnover, from 50 to 200 keV (Fabian et al., 2015; Lubiński et al., 2016). It is currently debated whether the downturn energy may depend on the Eddington ratio, although the trend between Γ and the Eddington ratio suggests that a weak correlation might be possible (Ricci et al., 2018, although see Molina et al., 2019). However, we note that there are studies of multiple sources with cut-off energies measured by NuSTAR where the authors suggest that this cut-off energy is not dependent on the Eddington ratio or the black hole mass (see, e.g., Tortosa et al., 2018; Kamraj et al., 2022).

The focus of Figure 1 is for log  ϵ [Ryd] ≳ − 1. We mention in passing that in the NIR domain, as the low energy tail of the AD emission fades, extrinsic emission is actually reprocessed emission from the dusty torus which surrounds the accretion disk. It becomes the dominant emission at a few μm, along with the polar dust in the direction of SMBH spin axis (Netzer, 2015; Padovani et al., 2017). In the FIR, the SED might be dominated by dust heated by host galaxy star formation, more than by the AGN itself (Kirkpatrick et al., 2015). This is occurring in systems with high accretion rate (e.g., Marziani et al., 2021b). In the case of highly accreting quasars, there is a relatively large prevalence of sources with high radio power, with radio-to-optical ratio ≳ 1 (Ganci et al., 2019; Wang et al., 2022), whose radio emission can be ascribed to star formation processes. Highly-accreting quasars might be predominantly seen as young or rejuvenated active nuclei whose SED is affected by star formation processes (see, e.g., Caccianiga et al., 2015; Ganci et al., 2019).

3 Imminent challenges and opportunities

3.1 What an AGN multi-frequency spectrum can reveal to us?

From an observer’s point of view, we can largely quantify the spectrum into two primary components: 1) the emission lines originating from the BLR/NLR clouds; and 2) the AGN continuum, prominent beyond the Lyman limit, that can photoionize the surrounding gas leading to line emission. The ionizing photon flux can be estimated by a careful analysis of the AGN SED, which then gives us a rough idea of the expected line fluxes for the multitude of ionic species (in their various ionization states) that we see in an AGN spectrum. A careful assessment of the density of these ionized clouds and their locations, in addition to the incident photon flux received by them, allows us to predict the strengths of these lines. Important information about density, ionization conditions, and dynamics in the broad line-emitting region of AGN can be inferred from UV spectroscopic observations which are crucial to understanding these line-emitting regions. Past studies of highly-accreting quasars have in turn illustrated the use of certain line diagnostic ratios from observed spectra (e.g., C iv/He ii, Al IIIλ1860/Si III]λ1892, Fe ii/Hβ) in order to estimate these (density, ionization condition, and metallicity) parameters (Negrete et al., 2012; Negrete et al., 2014; Śniegowska et al., 2021; Garnica et al., 2022, and references therein). Curiously, these extreme sources appear to be characterized by values of density, ionization, and metallicity that are extreme but also extremely well-defined: as far as the virialized emitting region is concerned the parameters reach n_H ∼ 10¹³ cm⁻³, log  U ∼ − 2.5, Z ≳ 20Z_⊙².

3.2 Dichotomy in optical and UV emission line profiles

Historically, the BLR clouds were modelled as single clouds where the different lines arise from different parts of the same cloud—a picture that is still widely accepted (Kwan and Krolik, 1981, see the BLR radial structure as shown by Negrete et al., 2012). In the mid-1980s, propositions were made to explain the BLR as two distinct components (Gaskell, 1982; Collin-Souffrin et al., 1988). The broad emission spectrum in AGNs can be divided into two parts: the first set of lines that include Lyα, C iii], C iv, He i, He ii, and N v predominantly emitted by a highly ionized region that presumably has a relatively low density (≲ 10¹⁰ cm⁻³). These are known as High Ionization Lines (HILs). The upper limit to the density of the media emitting these HILs is set by the semi-forbidden CIII] in order not to be collisionally de-excited even if the actual measurement of the blueshifted C iii] is problematic because of the blending with SiIII]λ1892. As a matter of fact, the density of the outflow is poorly constrained, and there is good reason to believe that a “clumpy” scenario (e.g., Takeuchi et al., 2013) might be also appropriate. The second set of lines includes the bulk of the Balmer lines, Mg ii, Fe ii, O i and Ca ii, emitted by a mildly ionized medium having a much higher density (≳ 10¹⁰ cm⁻³). The real scenario is more convoluted and the search for a global unified picture is still ongoing. However, this representation—dichotomy into LILs and HILs originating from the vicinity of the SMBH due to the inherent radiation of the accretion disk—has been instrumental to identify a low-ionization virialized component and the contribution of a high ionization wind (Leighly, 2004; Marziani et al., 2010), that proved to be especially prominent in highly accreting sources, i.e., quasar radiating at maximum radiative output per unit mass (Martínez-Aldama et al., 2019; Panda, 2022). Low-ionization lines retain fairly symmetric profiles that indicate virial motions and therefore that their width is suitable for virial broadening estimation without the need of introducing large corrections (Marziani et al., 2013; 2019; 2022). M_BH estimates remain reliable even if the effect of partially resolved outflows (in radial velocity) has to be taken into account (Negrete et al., 2018; Marziani et al., 2022; Buendia-Rios et al., 2023).

3.3 Quasar main sequence

3.3.1 The Eigenvector 1/main sequence

The study of Boroson and Green (1992) brought together the spectral diversity of Type-1 AGNs under a single framework. Their paper is fundamental for two reasons: 1) it provides one of the first templates for fitting the Fe ii pseudo-continuum. The Fe ii emission manifests itself as a pseudo-continuum owing to the many, blended multiplets over a wide wavelength range, extracted from the spectrum of a prototypical Narrow Line Seyfert Type-1 (NLS1) source, I Zw 1; and more importantly, 2) it introduced the Eigenvector 1 (E1) sequence to unify the diverse group of AGNs. They used principal component analysis—a conventional dimensionality reduction technique—on observed properties of a sample of optically bright quasars to obtain a sequence. Of special importance is the optical plane that shows the connection between the FWHM of the broad Hβ and the strength of the Fe ii blend between 4,434 and 4,684 Å to the Hβ (or R_FeII). This optical plane of the Eigenvector 1 (or of the “main sequence” of quasars) was eventually included in a 4D parameter space (4DE1) that encompasses high-ionization broad line blueshifts (Sulentic et al., 2000b; 2007), and soft-X photon index (Sulentic et al., 2000b; Bensch et al., 2015). The 4DE1 additional parameters are related to wind prominence and accretion status. It is therefore not unexpected that the main sequence might be primarily driven by the Eddington ratio, i.e., the ratio between radiation and gravitational forces (e.g., Sulentic et al., 2000a; Marziani et al., 2001; Boroson, 2002; Shen and Ho, 2014; Marziani et al., 2018b) affecting several BLR physical properties (Panda et al., 2018; Panda et al., 2019a; Panda et al., 2019c; Panda, 2021a).

3.3.2 Population A and population B

A classification based on the width of the Hβ emission line profile in an AGN spectrum was introduced by Sulentic et al. (2000a). Population A includes local NLS1s as well as more massive high accretors which are mostly classified as radio-quiet (e.g., Marziani and Sulentic, 2014a) and that have FWHM(Hβ) ≤4,000 km s⁻¹. On the contrary, Population B sources are those with broader Hβ (≥4,000 km s⁻¹), and are, at a large prevalence, “jetted” sources (e.g., Padovani et al., 2017). The Eigenvector 1 sequence of Boroson and Green (1992) was extended to cover the soft-X ray domain (e.g., Sulentic et al., 2000b), assessing a relation between the soft-X photon index Γ_soft, the Hβ line width, and the Fe ii prominence (see also later developments on the relation between X-ray and optical spectra by Grupe, 2004; Grupe et al., 2010; Ai et al., 2011; Jin et al., 2012b; Bensch et al., 2015; Ojha et al., 2020).

Sources with higher values of soft X-ray excess (corresponding to a value of the soft-X photon index³ (Γ_soft ≈ 3–4) concentrate among the highly accreting Pop. A quasars (Grupe, 2004; Sulentic et al., 2008), while Pop. B quasars typically have Γ_soft ≈ 2. The cut-off in the FWHM of Hβ at 4,000 km s⁻¹ was suggested by Sulentic et al. (2000a) who found that low z AGN (z ≲ 1) properties appear to change more significantly at this broader line-width cutoff (see also Collin et al., 2006; Ferland et al., 2020).

The usefulness of a fixed FWHM limit—let it be 2000 km s⁻¹ or 4,000 km s⁻¹ is questionable, as the FWHM is dependent on M_BH (or luminosity), viewing angle, and Eddington ratio (Marziani et al., 2018b). It makes sense if the limit is applied to samples in a narrow range of luminosity or M_BH. However, we re-iterate the question posed a few years ago (Sulentic and Marziani, 2015):

Are populations A and B simply two extreme ends of the main sequence or do they represent two distinct quasar populations? Or are they tied via a smooth transition in the accretion mode?

The issue is very relevant to our quest to use quasars as standard, or standardizable candles, since the shape of the emission line profiles and continuum strength is directly connected to the central engine, especially to the black hole mass, and, the accretion rate, in addition to the black hole spin and, the angle at which the central engine is viewed by a distant observer (Wang et al., 2014b; Czerny et al., 2017; Panda et al., 2017; Marziani et al., 2018b; Panda et al., 2019c; Panda, 2021b). Looking at the black hole mass vs. luminosity diagram, type-1 AGN in optically selected surveys are distributed along a relatively narrow strip with 0.01 ≲ L/L_Edd ≲ 1, over a range of black hole masses M_BH exceeding 4 dexes (see, e.g., the Figure 15 of D’Onofrio et al., 2021). For L/L_Edd ≲0.01, accretion is expected to enter into a radiatively inefficient domain, in addition to selection effects that disfavour the lowest accretors at a given M_BH. At the other end of the L/L_Edd range, sources that radiate at L/L_Edd ≫ 1 may simply not exist, as radiative efficiency is expected to decrease at a very high accretion rate, yielding to an asymptotic behaviour for the Eddington ratio toward a limiting value of order unity (; Mineshige et al., 2000; Watarai et al., 2000; Sadowski, 2011)⁴. Perhaps the following scheme is already something more than a working hypothesis: Population B is associated with modest accretion rates, and the continuum may be fit by refined α-disk models (Shakura and Sunyaev, 1973; Laor and Netzer, 1989). At some threshold of the radiative efficiency, η ≳ 0.1 the inner, advection-dominated region of the disk starts to have a significant role in the geometry of the BLR. At a very high accretion rate, this effect may be extreme, with collimation of ionized outflows and shielding of the low-ionization emitting region from the luminous continuum that is instead seen by an observer oriented at a small angle with respect to the disk axis (Wang et al., 2014c; Giustini and Proga, 2019, Panda and Marziani in preparation). It is also debatable to which extent accretion might be super-Eddington, as a large fraction of the infalling mass that would be accretion matter for the black hole might be actually expelled from the black hole gravitational sphere of influence (e.g., Carniani et al., 2015; Marziani et al., 2016; Vietri et al., 2018).

3.3.3 Narrow-line Seyfert 1s—A special class of AGNs?

Narrow Line Seyfert Type-1 galaxies (or NLS1s)⁵ are a class of Type-1 AGNs that are characterized with “narrower” broad emission lines: FWHM(Hβ_broad) ≤ 2,000 km s⁻¹, along with the ratio of [Oiii]λ5007 to the Hβ less than 3 (Osterbrock and Pogge, 1985; Goodrich, 1989). In addition to these properties, the NLS1s often exhibit strong Fe ii emission and the relative strength of the optical Fe ii (within 4,434–4,684 Å) to the Hβ, or R_FeII ≳ 1 (Sulentic et al., 2000a; Marziani et al., 2018b; Panda et al., 2019c; Rakshit et al., 2020). NLS1s have been used to analyze the Fe ii emission since the late 1970s (Phillips, 1978) and have been regarded among the most noticeable cooling agents of the BLR, emitting about ∼25% of the total energy in the BLR (Wills et al., 1985; Marinello et al., 2016). The Fe ii is a strong contaminant owing to a large number of emission lines and without proper modelling and subtraction, it may lead to a wrong description of the physical conditions in the BLR (Verner et al., 1999; Sigut and Pradhan, 2003; Baldwin et al., 2004; Sigut et al., 2004; Panda, 2021b). NLSy1s tend to be more variable than their “broader” counterparts in the X-ray regime (Leighly, 1999; Grupe, 2004; McHardy et al., 2006), although the scales of their variability are not as pronounced in the optical and infrared regime (Giannuzzo et al., 1998; Ai et al., 2013).

This is a summary of the conventional view of NLSy1s. A more exhaustive view is reached by the contextualization offered by the Eigenvector 1 Main Sequence. More prominently, the parameter R_FeII is central to the E1 schema as it is the dominant variable in the principal component analysis presented by Boroson and Green (1992). The PCA analysis led for the first time to the appreciation of the Fe ii relevance in large samples of quasar spectra. NLSy1s showing significant Fe ii emission (R_FeII ≳ 0.5) were described by Sulentic et al. (2000a) “as drivers of all Eigenvector 1 correlations.” Indeed, NLS1s with high accretion rates are typically shown to have a soft-X-ray excess (Arnaud et al., 1985) in their broadband SED (Jin et al., 2012a; b; Kubota and Done, 2018; Ferland et al., 2020). NLS1s also show stronger blueshifts (blueward asymmetries) especially in the HILs (e.g., Sulentic et al., 2000a; Sulentic et al., 2007; Leighly and Moore, 2004), as well as higher R_FeII implying high L/L_Edd (e.g., Du et al., 2016b). The E1 is now well understood to be associated with important parameters of the accretion process in the AGNs (Sulentic et al., 2000a; Shen and Ho, 2014; Marziani et al., 2018b; Panda et al., 2019c; Du and Wang, 2019; Martínez-Aldama et al., 2021), although the nature of the connection between R_FeII and L/L_Edd remains unclear to date. NLS1s typically host black holes with lower masses (≲ 10⁷ M_⊙) and tend to be less luminous and have low radio jet power—which has led many authors (Sulentic et al., 2000a; Mathur, 2000; Berton et al., 2017; Fraix-Burnet et al., 2017) to link them to an evolutionary scheme of BHs. These authors have suggested that the NLS1s are the younger versions of more evolved, more massive SMBHs that constitute the bulk of the population of AGNs.

3.4 Accretion parameters

The estimation of black hole masses is perhaps the most sought-after analysis when it comes to AGN studies (Vestergaard and Peterson, 2006; Shen et al., 2011). AGNs show variations in their continuum and emission line intensities that can range in the order of minutes/days for the continuum to days/weeks/months timescales for the BLR region (Ulrich et al., 1997). This crucial feature led to the estimation of black hole masses for a few hundred nearby AGNs and relatively distant quasars⁶ using the technique of reverberation mapping (Blandford and McKee, 1982; Peterson, 1988; Peterson, 1993; Peterson et al., 2004) with the knowledge of the location of the line emitting region from the central SMBH⁷. Coupling the information of the velocity broadening from single/multi-epoch spectroscopy (Kaspi et al., 2000; Bentz et al., 2013; Du et al., 2014) with a basic knowledge of the geometry of the emitting region (Pancoast et al., 2014; Li et al., 2016; Li et al., 2018), we are well poised to derive the black hole masses using the virial relation (Peterson et al., 2004): M_BH ∼ f_Sr_BLRδv²/G. There remains still a considerable level of uncertainty in the virial factor f_S (Marziani and Sulentic, 2012; Shen, 2013): the geometry and dynamics of the emitting regions remain poorly constrained. In dealing with M_BH estimate we actually encounter several difficulties associated with the lack of spherical symmetry of the BLR velocity field (McLure and Jarvis, 2002; Decarli et al., 2011), and with the possibility of anisotropic continuum emission. The uncertainties in the M_BH estimation affect the L/L_Edd estimates as well, with the complication that L/L_Edd estimates from optical/UV data depend on a bolometric correction that in turn depends not only on accretion state as evinced from Figure 1 but also on luminosity (e.g., Netzer, 2019, and references therein) as well as on viewing angle (e.g., Runnoe et al., 2013). Curiously, if the assumption of a highly flattened Hβ emitting region is correct, there could be a way out right for super-Eddington sources exploiting the computation of the “virial luminosity” from line widths (Section 4; Eq. 1).

The ratios between the virial and the redshift-based luminosity can be explained entirely by orientation effects (Negrete et al., 2018), thereby making it possible to derive an estimate of the viewing angle for each individual source. This method has been barely explored, but it has in principle the ability to constrain the disk/wind scenario in extreme Population A. We will discuss in §4.3 the anisotropy in continuum emission for xA sources—these refer to the sources that exhibit strong Fe ii emission (R_FeII ≳1) and are associated with accretion rates close to the Eddington limit, in the optical plane of the Eigenvector 1 main sequence diagram (Marziani et al., 2018a; Panda et al., 2019c).

4 An avenue for cosmological studies?

A better understanding of the AGN’s inner workings can pave the road to far-reaching applications. One of them is the standardization of quasars (or QSOs) for measuring cosmological parameters. Two methods that involve quasar intrinsic properties resort to a law analogous to that of Faber-Jackson, connecting velocity dispersion and luminosity (Section 4.1), and the radius-luminosity scaling laws (Section 4.2). Both methods face challenges.

4.1 Eddington standard candles?

The application of quasars radiating at or above the Eddington limit has been proposed for several years although the method has not yet been exploited to its full potential (Marziani et al., 2021b; Dultzin et al., 2020, and references therein). The method is conceptually simple: the accretion luminosity of a quasar is proportional to the a power of the line width,⁸ i.e., L ∝FWHMⁿ. The value of the exponent n = 4 comes from the virial relation for the black hole mass, the assumptions of constant Eddington ratio (L/L_Edd ∝ L/M_BH), and of BLR radius rigorously scaling with luminosity as r ∝ L^0.5. The last assumption is likely to be verified for sources radiating close to the Eddington limit: they are identified by spectral similarity (R_FeII), and so SED and the physical properties of the emitting regions need to be similar.

More in detail, the equation connecting luminosity and line width can be written as:

Where we have evidenced the main physical factors entering the estimate of the virial luminosity from measurements of radial velocity dispersion δv_r (FWHM, σ). The effect of orientation can be quantified by assuming that the line broadening is due to an isotropic component δv_iso + a flattened component whose virial velocity field projection along the line of sight isthat implies:

The factor is the ratio between the SED fraction of the ionizing continuum and the average energy of the ionizing photons. The factor is the product density times ionization parameter (n_HU ∼ 10^9.6cm⁻³, Padovani and Rafanelli, 1988; Matsuoka et al., 2008; Negrete et al., 2012). The two factors come from the definition of the photoionization radius of the BLR (Wandel et al., 1999). Deviations between the virial estimates and luminosity estimated from redshift and assumed concordance cosmology can be fully explained by the effect of orientation (Negrete et al., 2018). The distributions of the viewing angles from the Negrete et al. (2018) sample based on Hβ at low z peaks at about 17°, with only a very small fraction of quasars, is observed at θ ≳ 30°. This means that the effect of kinematic anisotropy on the computation of the virial luminosity should introduce a significant dispersion, as it is ∝ 1/sin²θ.

4.2 Scatter in the R-L relation, standardizing QSOs for cosmological studies

An important result of the reverberation mapping studies is from the empirical power-law relation between the BLR radius (or light travel time delay) and the luminosity, R_BLR L₅₁₀₀^α where τ is the time delay in response to continuum variation of a suitable line, mostly Hβ.⁹Bentz et al. (2013) found a best-fit for a sample of 41 AGNs covering four orders of magnitude in luminosity with a power-law slope value, , very close to the theoretical value, α = 0.5 needed to preserve spectral similarity (Davidson, 1977; Wills et al., 1985; Osterbrock and Ferland, 2006). This function is shown using a dashed line in the left panel of Figure 2. One can then combine the R_BLR-L₅₁₀₀ relation with the line widths for the broad emission lines estimated from single/multi-epoch spectroscopy to estimate the black hole masses which makes it especially useful for large statistical surveys of sources throughout cosmic history (Vestergaard and Peterson, 2006; Shen et al., 2011).

FIGURE 2

Recent observations have led to populate the R_BLR-L₅₁₀₀ observational space and take the total count over 100, especially the sources monitored under the SEAMBH project (Super-Eddington Accreting Massive Black Holes, Du et al., 2014; Wang et al., 2014a; Hu et al., 2015; Du et al., 2015; Du et al., 2016a; Du et al., 2018), and from the SDSS-RM campaigns (Grier et al., 2017; Shen et al., 2019). But this has introduced us to a new challenge - the inherent dispersion in the R_BLR-L₅₁₀₀ relation after the introduction of these new sources. Figure 2A is an abridged version from Martínez-Aldama et al. (2019); Panda (2021a) where the R_BLR-L₅₁₀₀ observational space for 117 reverberations mapped AGNs is shown. The sources are coloured with respect to their Eddington ratios (L_bol/L_Edd). The best-fit relation for this sample is, log R_Hβ = 0.387×(log  L₅₁₀₀) - 15.702, with a Spearman’s correlation coefficient (ρ) = 0.733 and p-value = 2.733 × 10⁻²¹, thus making the overall slope of the relation much shallower than obtained from the previous studies by Bentz et al. (2013) and bringing the validity of the empirical R_BLR-L₅₁₀₀ relation into question. But interestingly, the sources that eventually led to the increase in the scatter in the relation show a trend with the Eddington ratio - the larger the dispersion of a source from the empirical R_BLR-L₅₁₀₀ relation, the higher its Eddington ratio! In Martínez-Aldama et al. (2019), we found that this dispersion can be accounted for in the standard R_BLR-L₅₁₀₀ relation with an added dependence on the Eddington ratio (L_bol/L_Edd). This is highlighted in the Figure 2B, where the difference between the observed time delays and that estimated from the empirical R_BLR-L₅₁₀₀ relation (i.e., ΔR_BLR; Bentz et al., 2013), for the sources shown in the left panel, are plotted against their respective L₅₁₀₀. One can appreciate the drop in the ΔR_BLR value, especially for sources with high Eddington ratios indicating that sources with shorter “observed” time lag especially at the high luminosity end of the scaling relation exhibit high accretion rates. In other words, these sources are expected to host SMBHs with low BH masses (Du et al., 2014; Du et al., 2016a). Although we note that there are some sources which, albeit being of high Eddington nature, are observed to have R_BLR sizes comparable to the predicted value from Bentz et al. (2013). Du and Wang (2019) exploited this further in their work and realized that with an additional correction term, the relation can be reverted back to the original relation with a slope ∼0.5. This additional correction term is an observational parameter, the relative strength between the optical Fe ii emission and the corresponding Hβ emission (R_FeII) that has been shown in earlier studies to be a reliable observational proxy for the Eddington ratio (Sulentic et al., 2000a; Marziani et al., 2001; Shen and Ho, 2014; Marziani et al., 2018b; Panda et al., 2019c; Du and Wang, 2019; Martínez-Aldama et al., 2021) which we touched upon in earlier sections. The relation takes the form (Du and Wang, 2019):

Where the flux (F) can be independently estimated from the observed AGN spectrum for a given source. With this correction in terms of an observable quantity, i.e., R_FeII, we avoid the circularity problem which was present when the correction was explicitly made in terms of the Eddington ratio (Martínez-Aldama et al., 2019), and can have a robust estimate of the luminosity distance (D_L). This then allows us to construct a Hubble diagram using quasars where we know their luminosity distance and their redshifts, and test the validity of the standard cosmological model and their alternatives (see, e.g., Haas et al., 2011; Watson et al., 2011; Czerny et al., 2013; Czerny et al., 2019; Czerny et al., 2021; Khadka et al., 2021; Khadka et al., 2022; Zajaček et al., 2021). Super-Eddington accreting sources are expected to be preferentially selected with increasing redshift in flux-limited sample (Sulentic et al., 2014). A combination of this selection bias and intrinsic Eddington ratio evolution (e.g., Cavaliere and Vittorini, 2000; Hopkins et al., 2006) makes for high redshift quasar spectra often resembling lower-z extreme Population A spectra (an effect predicted by Sulentic et al. 2000a). Their inclusion is thus vital in extending the R_BLR-L₅₁₀₀ relation to higher luminosity regime as shown in Figure 3 where, κ = 1.65 ± 0.06, α = 0.45 ± 0.03, and γ = −0.35 ± 0.08. Clearly, the introduction of the R_FeII term and for sources with strong Fe ii emission, is able to account for their shorter time-lags and hence, smaller R_BLR sizes. In addition, we are able to recover the slope (α) closer to the theoretical predictions, i.e., 0.5 (Davidson, 1977; Davidson and Netzer, 1979) and hence, we can safely use the modified R_BLR-L₅₁₀₀ relation (including the correction term wrt R_FeII) to independently estimate the monochromatic luminosity for a given source, and couple the information from the flux obtained from direct observations, to eventually estimate the luminosity distance to the said source:

FIGURE 3

Where the flux (F) can be independently estimated from the observed AGN spectrum for a given source. In this way, we can avoid circularity and can have a robust estimate of the luminosity distance.

4.3 Anisotropic radiation from the accretion disk

An equally important aspect in this regard is the ionizing continuum produced by the central engine. The characterization of the ionizing SED that comes from regions closer than the BLR is important for our study of the emission lines. From the photo-ionization point of view, this fraction of the broad-band SED is closely related to the number of ionizing photons that eventually leads to the line production that has permitted the estimation of a photoionization radius of the BLR (Wandel et al., 1999; Negrete et al., 2014; Martínez-Aldama et al., 2015; Panda, 2021b; Panda, 2022).

We tested the variation in the low-ionization part of the BLR by accounting for the changes in the shape of the ionizing continuum (the SED) and the location of the Hβ-emitting BLR from the central ionizing source (or R_BLR) from the reverberation mapping, in the context of Main Sequence of Quasars (Panda, 2022). In this and previous work (Panda, 2021b), we have found that in order to estimate the correct physical conditions for these low-ionization lines emitting regions in the BLR, it is not sufficient to only retrieve the flux ratios (e.g., R_FeII) but to also have an agreement with the corresponding modelled and observed line strengths (or line equivalent widths, EWs). Compared to the results that are directly obtained from the photoionization theory, these new results highlight the shift in the overall location of the line-emitting R_BLR—in terms of the ionization parameter U and the local cloud density (n_H) recovered from the analysis towards lower values (by up to 2 dexes) compared to the R_BLR values estimated from the photoionization theory. This brings the modelled location in agreement with the reverberation mapping results, especially for the high-accreting NLS1s which show shorter time-lags/smaller emitting regions. A corollary result is that to retrieve such physical conditions, the BLR should “see” a different, filtered SED with only a fraction of the total ionizing photon flux. This analysis was performed on selected sources with readily available broad-band SEDs and archival spectroscopic measurements. In addition, we assumed source-specific metallicities that were derived using the UV diagnostic lines from earlier studies (see, e.g., Marziani et al., 2022). There is a need to extend this analysis to a larger number of reverberation-mapped sources. We, therefore, need synchronous multi-wavelength observations to build robust SEDs that can be used to confirm this scenario. Also, there is a need to bring together a global picture where a combined analysis of the UV and optical emitting regions can be put together—to allow us to gauge the salient differences in the low- and high-ionization line emitting regions.

Wang et al. (2014c) derived the analytical solutions (steady-state) for the structure of “slim” accretion discs from sub-Eddington accretion rates to extremely high, super-Eddington rates. They notice the appearance of a funnel-like structure very close to the SMBH at super-Eddington rates and attribute this feature to the puffing up of the inner accretion disk by radiation pressure, wherein the Shakura and Sunyaev (1973) prescription for a geometrically thin, optically thick accretion disk doesn’t hold (Abramowicz et al., 1988; Sadowski, 2011; Wang et al., 2014c). We show an illustration of this scenario in Figure 4. Such modifications to the disk structure strongly affect the overall anisotropic emission of ionizing photons from the disk in addition to just inclination effects that arise due to the axisymmetric nature of these systems. Therefore, with a rise in accretion rates, a continuum anisotropy needs to be accounted for. The anisotropy then leads to the shrinking in the position of the BLR—exposed to a lower continuum flux than the observer (Figure 4)—that brings the modelled location in agreement with the observed estimates from the reverberation mapping campaigns (see Panda, 2021b, for more details).

FIGURE 4

The left panel in Figure 5 shows the slim-disk SEDs (Jian-Min Wang, priv. comm.) for a representative BH mass of 10⁸ M_⊙, accreting at = 100 for a range of viewing angles¹⁰. The right panel shows the distribution of slim-disk SEDs as a function of for a representative BH mass of 10⁸ M_⊙ observed at a viewing angle (i = 40°). We also report the relative area under the SEDs shown in Figure 5. These values are tabulated in Tables 1, 2 corresponding to the left and right panels of Figure 5, respectively. We estimate the area under the SEDs accounting only for the fluxes corresponding to a frequency ≥1 Rydberg¹¹. We then compute the relative area (a) with respect to the SED case with the viewing angle, i = 10° (Table 1 and left panel of Figure 5); and (b) with respect to the SED case with the dimensionless accretion rate, = 1,000 (Table 2 and right panel of Figure 5). From the left panel of Figure 5, we can notice that going from the SED viewed at i = 10°–80°, keeping the BH mass and accretion rate constant, the extended region II receives only a very small fraction of the actual ionizing photon flux (only 0.23%), meaning almost all of the ionizing photons (99.77%) never make it to the BLR. This ∼2 dex reduction in the photon flux results in an equal reduction in the ionization parameter (U) which was confirmed already in Panda (2021b). On the other hand, changing the accretion rate, going from = 1 to 1,000 and keeping the BH mass and viewing angle constant (right panel of Figure 5), we find that an accretion rate = 1 relates to only a 0.44% of the total photon flux, also a factor ≳ 100 from the case = 1,000. There is however a fundamental difference: whereas changing  produces an almost self-similar shift of the SED, a change in the viewing angle produces the change by a factor ≳ 100 in the ionizing flux, while the optical flux changes by a factor  (Figure 5). This indirectly confirms the results of previous works that have been pointing to the main sequence drive being the Eddington ratio convolved with the effect of orientation (Marziani et al., 2001; Shen and Ho, 2014; Sun and Shen, 2015; Marziani et al., 2018b; Panda et al., 2019c). The shape of the SED thus plays an important role in explaining the trends in the quasar main sequence wherein the information of the fundamental BH parameters—BH mass, Eddington ratio, orientation and the BH spin—are embedded (see Panda, 2021a, for more details).

FIGURE 5

In Figure 6, we highlight a representative fit (in grey) to the high SED case (in blue) from Ferland et al. (2020) utilizing a two-component model, i.e., a slim accretion disk that represents the thermal component (in dashed red) along with a hot Comptonized component (in dashed green). Here, the slim accretion disk SED is modelled for a face-on viewing angle, i = 10° for a = 500 for a BH mass of 10⁸ M_⊙. The X-ray to optical-UV normalization is set by a spectral index (α_ox = −1.47) and the high-energy cutoff is assumed to be ∼2.5 keV with a slope (α_x = −0.79) to match the exponential drop in the observed SED. The assumed values for these parameters are considered from Jin et al. (2012a); Jin et al. (2012b) who were the first to carry out a broadband analysis on these composite SEDs. This instance of the slim accretion disk SED is a good fit for xA as shown in Figure 7 where the latter is represented by a composite spectrum made using the optical-UV spectral observations for xA sources from Marziani et al. (2013). In our forthcoming work, we will incorporate these slim disk SEDs into our photoionization modelling setup and recover the trends for the low- and high-ionization emission lines, their relative strengths (e.g., R_FeII) and the EWs, concerning these fundamental BH parameters, concentrating on the xA.

FIGURE 6

FIGURE 7

Table 2 shows that the fraction of ionizing photons at a very high accretion rate tends to saturate, with only a 10% increasing from the doubling of the accretion rate, from to 1,000. At such the Eddington ratio should converge toward a limiting value of . The product n_HU is also little affected by changes in SED in the cases shown in Figure 1. The factors and are expected to be stable,¹² although their actual dispersion and systematics for sources selected as radiating closer to the Eddington limit should be further investigated through dedicated observations. In other words, even if anisotropy effects in line widths are strong, anisotropy in continuum emission and differences in SEDs might not be so strong as to compromise an application to the cosmology of Eq. 1 that is—we stress it—generally valid for all AGNs but in practice exploitable for high accretors only. Preliminary applications to cosmology of Eq. 1 have been encouraging (Marziani and Sulentic, 2014a; Marziani and Sulentic, 2014b; Marziani et al., 2019; Czerny et al., 2021; Marziani et al., 2021b; Marziani et al., 2021a).

5 Concluding remarks

We have reviewed some basic aspects of luminous type-1 AGN (Section 1) to introduce highly accreting sources with special attention to their SEDs (Section 3), focusing on the most relevant aspects (such as anisotropy, Section 4.3) in the context of the possible application to the measurements of the cosmological parameters (Sections 3, 4). We have stressed that Population A includes NLS1s but that only a fraction of the sources can be considered highly accreting, as a defining criterion is R_FeII ≳ 1, and this criterion isn’t met by all Population A and NLS1 sources. On the converse, a criterion based on line width implies a dependence on black hole mass (and hence luminosity in flux-limited samples), viewing angle, Eddington ratio, and yields a selection that is, unavoidably, sample dependent (Marziani et al., 2018a; Panda et al., 2019c).

Looking at the bigger picture, we can construct the Hubble diagram with the luminosity distances using Eq. 5 corrected according to Eq. 4 and the corresponding redshifts for each source. The key here is to have the measurement of the time-delay (e.g., from Eq. 4) and the AGN monochromatic flux using single epoch spectra that allow us to estimate the luminosity distances regardless of accretion properties. Hence, very large samples of reverberation-mapped AGNs can be used as cosmological candles (Collier et al., 1999; Elvis and Karovska, 2002; Horne et al., 2003; Panda et al., 2019b; Khadka et al., 2022). This may further allow us to study the evolution of the cosmological parameters as a function of the redshift allowing for the reconciliation of the Hubble tension—the disparity between the measured value of the Hubble constant in the local and the early Universe (see Figure 8).

FIGURE 8

There will be an immense potential for the ideas and results presented in this work in the near future, serving as test-beds for the vast number of AGNs that will be explored with the ongoing and upcoming ground-based 10-m-class (e.g., Maunakea Spectroscopic Explorer, Marshall et al., 2019) and 40 m-class (e.g., The European Extremely Large Telescope, Evans et al., 2015) telescopes; and space-based missions such as the JWST (Gardner et al., 2006; Jakobsen et al., 2022) and the Nancy Grace Roman Space Telescope (Spergel et al., 2013). Increased availability of high-quality, multi-wavelength photometric, spectroscopic and interferometric measurements extending to higher redshifts is a necessity to help develop our ever-growing theoretical understanding of how these massive, energetic cosmic sources work and evolve as well as how they might be exploited for cosmic distance estimates.

References

• 1

  AbramowiczM. A.CzernyB.LasotaJ. P.SzuszkiewiczE. (1988). Slim accretion disks. Astrophysical J.332, 646–658. 10.1086/166683

  • CrossRef
  • Google Scholar

• 2

  AiY. L.YuanW.ZhouH.WangT. G.DongX. B.WangJ. G.et al (2013). A comparative study of optical/ultraviolet variability of narrow-line Seyfert 1 and broad-line Seyfert 1 active galactic nuclei. Astronomical J.145, 90. 10.1088/0004-6256/145/4/90

  • CrossRef
  • Google Scholar

• 3

  AiY. L.YuanW.ZhouH. Y.WangT. G.ZhangS. H. (2011). X-Ray properties of narrow-line Seyfert 1 galaxies with very small broadline widths. Astrophysical J.727, 31. 10.1088/0004-637X/727/1/31

  • CrossRef
  • Google Scholar

• 4

  AntonucciR. (2012). A panchromatic review of thermal and nonthermal active galactic nuclei. Astronomical Astrophysical Trans.27, 557–602.

  • Google Scholar

• 5

  AntonucciR. (1993). Unified models for active galactic nuclei and quasars. Annu. Rev. Astronomy Astrophysics31, 473–521. 10.1146/annurev.aa.31.090193.002353

  • CrossRef
  • Google Scholar

• 6

  ArnaudK. A.Branduardi-RaymontG.CulhaneJ. L.FabianA. C.HazardC.McGlynnT. A.et al (1985). EXOSAT observations of a strong soft X-ray excess in MKN 841. Mon. Notices R. Astronomical Soc.217, 105–113. 10.1093/mnras/217.1.105

  • CrossRef
  • Google Scholar

• 7

  BaldwinJ. A.FerlandG. J.KoristaK. T.HamannF.LaCluyzéA. (2004). The origin of Fe II emission in active galactic nuclei. Astrophysical J.615, 610–624. 10.1086/424683

  • CrossRef
  • Google Scholar

• 8

  BarthA. J.PancoastA.BennertV. N.BrewerB. J.CanalizoG.FilippenkoA. V.et al (2013). The lick AGN monitoring project 2011: Fe II reverberation from the outer broad-line region. Astrophysical J.769, 128. 10.1088/0004-637X/769/2/128

  • CrossRef
  • Google Scholar

• 9

  BenschK.del OlmoA.SulenticJ.PereaJ.MarzianiP. (2015). Measures of the soft X-ray excess as an eigenvector 1 parameter for active galactic nuclei. J. Astrophysics Astronomy36, 0–474. 10.1007/s12036-015-9355-8

  • CrossRef
  • Google Scholar

• 10

  BentzM. C.CackettE. M.CrenshawD. M.HorneK.StreetR.Ou-YangB. (2016). A reverberation-based black hole mass for MCG-06-30-15. Astrophysical J.830, 136. 10.3847/0004-637X/830/2/136

  • CrossRef
  • Google Scholar

• 11

  BentzM. C.DenneyK. D.GrierC. J.BarthA. J.PetersonB. M.VestergaardM.et al (2013). The low-luminosity end of the radius-luminosity relationship for active galactic nuclei. Astrophysical J.767, 149. 10.1088/0004-637X/767/2/149

  • CrossRef
  • Google Scholar

• 12

  BentzM. C.HorensteinD.BazhawC.Manne-NicholasE. R.Ou-YangB. J.AndersonM.et al (2014). The mass of the central black hole in the nearby Seyfert galaxy NGC 5273. Astrophysical J.796, 8. 10.1088/0004-637X/796/1/8

  • CrossRef
  • Google Scholar

• 13

  BentzM. C.PetersonB. M.NetzerH.PoggeR. W.VestergaardM. (2009). The radius-luminosity relationship for active galactic nuclei: The effect of host-galaxy starlight on luminosity measurements. II. The full sample of reverberation-mapped AGNs. Astrophysical J.697, 160–181. 10.1088/0004-637X/697/1/160

  • CrossRef
  • Google Scholar

• 14

  BertonM.FoschiniL.CaccianigaA.CiroiS.CongiuE.CraccoV.et al (2017). An orientation-based unification of young jetted AGN: The case of 3C 286. Front. Astronomy Space Sci.4, 8. 10.3389/fspas.2017.00008

  • CrossRef
  • Google Scholar

• 15

  BlandfordR. D.McKeeC. F. (1982). Reverberation mapping of the emission line regions of Seyfert galaxies and quasars. Astrophysical J.255, 419–439. 10.1086/159843

  • CrossRef
  • Google Scholar

• 16

  BoissayR.RicciC.PaltaniS. (2016). A hard X-ray view of the soft excess in AGN. Astronomy Astrophysics588, A70. 10.1051/0004-6361/201526982

  • CrossRef
  • Google Scholar

• 17

  BorosonT. A. (2002). Black hole mass and Eddington ratio as drivers for the observable properties of radio-loud and radio-quiet QSOs. Astrophysical J.565, 78–85. 10.1086/324486

  • CrossRef
  • Google Scholar

• 18

  BorosonT. A.GreenR. F. (1992). The emission-line properties of low-redshift quasi-stellar objects. Astrophysical J. Suppl.80, 109. 10.1086/191661

  • CrossRef
  • Google Scholar

• 19

  Buendia-RiosT. M.NegreteC. A.MarzianiP.DultzinD. (2023). Statistical analysis of Al III and C III] emission lines as virial black hole mass estimators in quasars. Astronomy Astrophysics669, A135. 10.1051/0004-6361/202244177

  • CrossRef
  • Google Scholar

• 20

  CaccianigaA.AntónS.BalloL.FoschiniL.MaccacaroT.Della CecaR.et al (2015). WISE colours and star formation in the host galaxies of radio-loud narrow-line Seyfert 1. Mon. Notices R. Astronomical Soc.451, 1795–1805. 10.1093/mnras/stv939

  • CrossRef
  • Google Scholar

• 21

  CarnianiS.MarconiA.MaiolinoR.BalmaverdeB.BrusaM.Cano-DíazM.et al (2015). Ionised outflows inz∼ 2.4 quasar host galaxies. Astronomy Astrophysics580, A102. 10.1051/0004-6361/201526557

  • CrossRef
  • Google Scholar

• 22

  CavaliereA.VittoriniV. (2000). The fall of the quasar population. Astrophysical J.543, 599–610. 10.1086/317155

  • CrossRef
  • Google Scholar

• 23

  CollierS.HorneK.WandersI.PetersonB. M. (1999). A new direct method for measuring the Hubble constant from reverberating accretion discs in active galaxies. Mon. Notices R. Astronomical Soc.302, L24–L28. 10.1046/j.1365-8711.1999.02250.x

  • CrossRef
  • Google Scholar

• 24

  CollinS.KawaguchiT.PetersonB. M.VestergaardM. (2006). Systematic effects in measurement of black hole masses by emission-line reverberation of active galactic nuclei: Eddington ratio and inclination. Astronomy Astrophysics456, 75–90. 10.1051/0004-6361:20064878

  • CrossRef
  • Google Scholar

• 25

  Collin-SouffrinS.DysonJ. E.McDowellJ. C.PerryJ. J. (1988). The environment of active galactic nuclei - I. A two-component broad emission line model. Mon. Notices R. Astronomical Soc.232, 539–550. 10.1093/mnras/232.3.539

  • CrossRef
  • Google Scholar

• 26

  CollinsonJ. S. (2016). Spectral and temporal studies of supermassive black holes. UK: Ph.D. thesis, Durham University.

  • Google Scholar

• 27

  CrummyJ.FabianA. C.GalloL.RossR. R. (2006). An explanation for the soft X-ray excess in active galactic nuclei. Mon. Notices R. Astronomical Soc.365, 1067–1081. 10.1111/j.1365-2966.2005.09844.x

  • CrossRef
  • Google Scholar

• 28

  CzernyB.ElvisM. (1987). Constraints on quasar accretion disks from the optical/ultraviolet/soft X-ray big bump. Astrophysical J.321, 305–320. 10.1086/165630

  • CrossRef
  • Google Scholar

• 29

  CzernyB.HryniewiczK.MaityI.Schwarzenberg-CzernyA.ŻyckiP. T.BilickiM. (2013). Towards equation of state of dark energy from quasar monitoring: Reverberation strategy. Astronomy Astrophysics556, A97. 10.1051/0004-6361/201220832

  • CrossRef
  • Google Scholar

• 30

  CzernyB.LiY.-R.HryniewiczK.PandaS.WildyC.SniegowskaM.et al (2017). Failed radiatively accelerated dusty outflow model of the broad line region in active galactic nuclei. I. Analytical solution. Astrophysical J.846, 154. 10.3847/1538-4357/aa8810

  • CrossRef
  • Google Scholar

• 31

  CzernyB.Martínez-AldamaM. L.WojtkowskaG.ZajačekM.MarzianiP.DultzinD.et al (2021). Dark energy constraintsfrom quasar observations. Acta Phys. Pol. A139, 389–393. 10.12693/APhysPolA.139.389

  • CrossRef
  • Google Scholar

• 32

  CzernyB.OlejakA.RałowskiM.KozłowskiS.Martinez AldamaM. L.ZajacekM.et al (2019). Time delay measurement of Mg II line in CTS C30.10 with SALT. Astrophysical J.880, 46. 10.3847/1538-4357/ab2913

  • CrossRef
  • Google Scholar

• 33

  DavidsonK.NetzerH. (1979). The emission lines of quasars and similar objects. Rev. Mod. Phys.51, 715–766. 10.1103/RevModPhys.51.715

  • CrossRef
  • Google Scholar

• 34

  DavidsonK. (1977). On photoionization analyses of emission spectra of quasars. Astrophysical J.218, 20–32. 10.1086/155653

  • CrossRef
  • Google Scholar

• 35

  DecarliR.DottiM.TrevesA. (2011). Geometry and inclination of the broad-line region in blazars. Mon. Notices R. Astronomical Soc.413, 39–46. 10.1111/j.1365-2966.2010.18102.x

  • CrossRef
  • Google Scholar

• 36

  D’OnofrioM.MarzianiP.ChiosiC. (2021). Past, present and future of the scaling relations of galaxies and active galactic nuclei. Front. Astronomy Space Sci.8, 157. 10.3389/fspas.2021.694554

  • CrossRef
  • Google Scholar

• 37

  DuP.HuC.LuK.-X.HuangY.-K.ChengC.QiuJ.et al (2015). Supermassive black holes with high accretion rates in active galactic nuclei. IV. Hβ time lags and implications for super-eddington accretion. Astrophysical J.806, 22. 10.1088/0004-637X/806/1/22

  • CrossRef
  • Google Scholar

• 38

  DuP.HuC.LuK.-X.WangF.QiuJ.LiY.-R.et al (2014). Supermassive black holes with high accretion rates in active galactic nuclei. I. First results from a new reverberation mapping campaign. Astrophysical J.782, 45. 10.1088/0004-637X/782/1/45

  • CrossRef
  • Google Scholar

• 39

  DuP.LuK.-X.ZhangZ.-X.HuangY.-K.WangK.HuC.et al (2016a). Supermassive black holes with high accretion rates in active galactic nuclei. V. A new size-luminosity scaling relation for the broad-line region. Astrophysical J.825, 126. 10.3847/0004-637X/825/2/126

  • CrossRef
  • Google Scholar

• 40

  DuP.WangJ.-M.HuC.HoL. C.LiY.-R.BaiJ.-M. (2016b). The fundamental plane of the broad-line region in active galactic nuclei. Astrophysical J. Lett.818, L14. 10.3847/2041-8205/818/1/L14

  • CrossRef
  • Google Scholar

• 41

  DuP.WangJ.-M. (2019). The radius-luminosity relationship depends on optical spectra in active galactic nuclei. Astrophysical J.886, 42. 10.3847/1538-4357/ab4908

  • CrossRef
  • Google Scholar

• 42

  DuP.ZhangZ.-X.WangK.HuangY.-K.ZhangY.LuK.-X.et al (2018). Supermassive black holes with high accretion rates in active galactic nuclei IX 10 new observations of reverberation mapping and shortened Hβ lags. Astrophysical J.856, 6. 10.3847/1538-4357/aaae6b

  • CrossRef
  • Google Scholar

• 43

  DultzinD.MarzianiP.de DiegoJ. A.NegreteC. A.Del OlmoA.Martínez-AldamaM. L.et al (2020). Extreme quasars as distance indicators in cosmology. Front. Astronomy Space Sci.6, 80. 10.3389/fspas.2019.00080

  • CrossRef
  • Google Scholar

• 44

  ElvisM.KarovskaM. (2002). Quasar parallax: A method for determining direct geometrical distances to quasars. Astrophysical J. Lett.581, L67–L70. 10.1086/346015

  • CrossRef
  • Google Scholar

• 45

  ElvisM.WilkesB. J.McDowellJ. C.GreenR. F.BechtoldJ.WillnerS. P.et al (1994). Atlas of quasar energy distributions. Astrophysical J. Suppl.95, 1–68. 10.1086/192093

  • CrossRef
  • Google Scholar

• 46

  EvansC.PuechM.AfonsoJ.AlmainiO.AmramP.AusselH.et al (2015). The science case for multi-object spectroscopy on the European ELT. arXiv e-prints , arXiv:1501.04726.

  • Google Scholar

• 47

  FaberS. M.JacksonR. E. (1976). Velocity dispersions and mass-to-light ratios for elliptical galaxies. Astrophysical J.204, 668–683. 10.1086/154215

  • CrossRef
  • Google Scholar

• 48

  FabianA. C.LohfinkA.KaraE.ParkerM. L.VasudevanR.ReynoldsC. S. (2015). Properties of AGN coronae in the NuSTAR era. Mon. Notices R. Astronomical Soc.451, 4375–4383. 10.1093/mnras/stv1218

  • CrossRef
  • Google Scholar

• 49

  FausnaughM. M.GrierC. J.BentzM. C.DenneyK. D.De RosaG.PetersonB. M.et al (2017). Reverberation mapping of optical emission lines in five active galaxies. Astrophysical J.840, 97. 10.3847/1538-4357/aa6d52

  • CrossRef
  • Google Scholar

• 50

  FerlandG. J.DoneC.JinC.LandtH.WardM. J. (2020). State-of-the-art AGN SEDs for photoionization models: BLR predictions confront the observations. Mon. Notices R. Astronomical Soc.494, 5917–5922. 10.1093/mnras/staa1207

  • CrossRef
  • Google Scholar

• 51

  Fraix-BurnetD.MarzianiP.D’OnofrioM.DultzinD. (2017). The phylogeny of quasars and the ontogeny of their central black holes. Front. Astronomy Space Sci.4, 1. 10.3389/fspas.2017.00001

  • CrossRef
  • Google Scholar

• 52

  GanciV.MarzianiP.D’OnofrioM.del OlmoA.BonE.BonN.et al (2019). Radio loudness along the quasar main sequence. Astronomy Astrophysics630, A110. 10.1051/0004-6361/201936270

  • CrossRef
  • Google Scholar

• 53

  GardnerJ. P.MatherJ. C.ClampinM.DoyonR.GreenhouseM. A.HammelH. B.et al (2006). The james webb space telescope. Space Sci. Rev.123, 485–606. 10.1007/s11214-006-8315-7

  • CrossRef
  • Google Scholar

• 54

  GarnicaK.NegreteC. A.MarzianiP.DultzinD.ŚniegowskaM.PandaS. (2022). High metal content of highly accreting quasars: Analysis of an extended sample. Astronomy Astrophysics667, A105. 10.1051/0004-6361/202142837

  • CrossRef
  • Google Scholar

• 55

  GaskellC. M. (1982). A redshift difference between high and low ionization emission-line regions in QSO’s-evidence for radial motions. Astrophysical J.263, 79–86. 10.1086/160481

  • CrossRef
  • Google Scholar

• 56

  GiannuzzoM. E.MignoliM.StirpeG. M.ComastriA. (1998). A search for variability in Narrow Line Seyfert 1 Galaxies. II. New data from the Loiano monitoring programme. Astronomy Astrophysics330, 894–900.

  • Google Scholar

• 57

  GiustiniM.ProgaD. (2019). A global view of the inner accretion and ejection flow around super massive black holes. Radiation-driven accretion disk winds in a physical context. Astronomy Astrophysics630, A94. 10.1051/0004-6361/201833810

  • CrossRef
  • Google Scholar

• 58

  GoodrichR. W. (1989). Spectropolarimetry of “narrow-line” Seyfert 1 galaxies. Astrophysical J.342, 224. 10.1086/167586

  • CrossRef
  • Google Scholar

• 59

  GreensteinJ. L.SchmidtM. (1964). The quasi-stellar radio sources 3C 48 and 3C 273. Astrophysical J.140, 1. 10.1086/147889

  • CrossRef
  • Google Scholar

• 60

  GrierC. J.TrumpJ. R.ShenY.HorneK.KinemuchiK.McGreerI. D.et al (2017). The sloan digital sky survey reverberation mapping project: Hα and Hβ reverberation measurements from first-year spectroscopy and photometry. Astrophysical J.851, 21. 10.3847/1538-4357/aa98dc

  • CrossRef
  • Google Scholar

• 61

  GrupeD. (2004). A complete sample of soft X-ray-selected AGNs. II. Statistical analysis. Astronomical J.127, 1799–1810. 10.1086/382516

  • CrossRef
  • Google Scholar

• 62

  GrupeD.KomossaS.LeighlyK. M.PageK. L. (2010). The simultaneous optical-to-X-ray spectral energy distribution of soft X-ray selected active galactic nuclei observed by swift. Astrophysical J. Suppl.187, 64–106. 10.1088/0067-0049/187/1/64

  • CrossRef
  • Google Scholar

• 63

  HaasM.ChiniR.RamollaM.Pozo NuñezF.WesthuesC.WatermannR.et al (2011). Photometric AGN reverberation mapping - an efficient tool for BLR sizes, black hole masses, and host-subtracted AGN luminosities. Astronomy Astrophysics535, A73. 10.1051/0004-6361/201117325

  • CrossRef
  • Google Scholar

• 64

  HamannF.FerlandG. (1999). Elemental abundances in quasistellar objects: Star formation and galactic nuclear evolution at high redshifts. Annu. Rev. Astronomy Astrophysics37, 487–531. 10.1146/annurev.astro.37.1.487

  • CrossRef
  • Google Scholar

• 65

  HamannF.FerlandG. (1993). The chemical evolution of QSOs and the implications for cosmology and galaxy formation. Astrophysical J.418, 11. 10.1086/173366

  • CrossRef
  • Google Scholar

• 66

  HarrisonC. (2014). Observational constraints on the influence of active galactic nuclei on the evolution of galaxies. Ph.D. thesis, Durham University.

  • Google Scholar

• 67

  HopkinsP. F.HernquistL.CoxT. J.Di MatteoT.RobertsonB.SpringelV. (2006). A unified, merger-driven model of the origin of starbursts, quasars, the cosmic X-ray background, supermassive black holes, and galaxy spheroids. Astrophysical J. Suppl.163, 1–49. 10.1086/499298

  • CrossRef
  • Google Scholar

• 68

  HorneK.KoristaK. T.GoadM. R. (2003). Quasar tomography: Unification of echo mapping and photoionization models. Mon. Notices R. Astronomical Soc.339, 367–386. 10.1046/j.1365-8711.2003.06036.x

  • CrossRef
  • Google Scholar

• 69

  HorneK.PetersonB. M.CollierS. J.NetzerH. (2004). Observational requirements for high-fidelity reverberation mapping. Publ. Astronomical Soc. Pac.116, 465–476. 10.1086/420755

  • CrossRef
  • Google Scholar

• 70

  HuC.DuP.LuK.-X.LiY.-R.WangF.QiuJ.et al (2015). Supermassive black holes with high accretion rates in active galactic nuclei. III. Detection of Fe II reverberation in nine narrow-line Seyfert 1 galaxies. Astrophysical J.804, 138. 10.1088/0004-637X/804/2/138

  • CrossRef
  • Google Scholar

• 71

  JakobsenP.FerruitP.Alves de OliveiraC.ArribasS.BagnascoG.BarhoR.et al (2022). The near-infrared spectrograph (NIRSpec) on the james webb space telescope. I. Overview of the instrument and its capabilities. Astronomy Astrophysics661, A80. 10.1051/0004-6361/202142663

  • CrossRef
  • Google Scholar

• 72

  JinC.WardM.DoneC. (2012a). A combined optical and X-ray study of unobscured type 1 active galactic nuclei - II. Relation between X-ray emission and optical spectra. Mon. Notices R. Astronomical Soc.422, 3268–3284. 10.1111/j.1365-2966.2012.20847.x

  • CrossRef
  • Google Scholar

• 73

  JinC.WardM.DoneC.GelbordJ. (2012b). A combined optical and X-ray study of unobscured type 1 active galactic nuclei - I. Optical spectra and spectral energy distribution modelling. Mon. Notices R. Astronomical Soc.420, 1825–1847. 10.1111/j.1365-2966.2011.19805.x

  • CrossRef
  • Google Scholar

• 74

  KamrajN.BrightmanM.HarrisonF. A.SternD.GarcíaJ. A.BalokovićM.et al (2022). X-ray coronal properties of swift/BAT-selected Seyfert 1 active galactic nuclei. Astrophysical J.927, 42. 10.3847/1538-4357/ac45f6

  • CrossRef
  • Google Scholar

• 75

  KaspiS.SmithP. S.NetzerH.MaozD.JannuziB. T.GiveonU. (2000). Reverberation measurements for 17 quasars and the size-mass-luminosity relations in active galactic nuclei. Astrophysical J.533, 631–649. 10.1086/308704

  • CrossRef
  • Google Scholar

• 76

  KhadkaN.Martínez-AldamaM. L.ZajačekM.CzernyB.RatraB. (2022). Do reverberation-measured Hβ quasars provide a useful test of cosmology?Mon. Notices R. Astronomical Soc.513, 1985–2005. 10.1093/mnras/stac914

  • CrossRef
  • Google Scholar

• 77

  KhadkaN.YuZ.ZajačekM.Martinez-AldamaM. L.CzernyB.RatraB. (2021). Standardizing reverberation-measured Mg II time-lag quasars, by using the radius-luminosity relation, and constraining cosmological model parameters. Mon. Notices R. Astronomical Soc.508, 4722–4737. 10.1093/mnras/stab2807

  • CrossRef
  • Google Scholar

• 78

  KirkpatrickA.PopeA.SajinaA.RoebuckE.YanL.ArmusL.et al (2015). The role of star formation and an AGN in dust heating of z = 0.3-2.8 galaxies. I. Evolution with redshift and luminosity. Astrophysical J.814, 9. 10.1088/0004-637X/814/1/9

  • CrossRef
  • Google Scholar

• 79

  KubotaA.DoneC. (2018). A physical model of the broad-band continuum of AGN and its implications for the UV/X relation and optical variability. Mon. Notices R. Astronomical Soc.480, 1247–1262. 10.1093/mnras/sty1890

  • CrossRef
  • Google Scholar

• 80

  KwanJ.KrolikJ. H. (1981). The formation of emission lines in quasars and Seyfert nuclei. Astrophysical J.250, 478–507. 10.1086/159395

  • CrossRef
  • Google Scholar

• 81

  LaorA.FioreF.ElvisM.WilkesB. J.McDowellJ. C. (1997). The soft X-ray properties of a complete sample of optically selected quasars. II. Final results. Astrophysical J.477, 93–113. 10.1086/303696

  • CrossRef
  • Google Scholar

• 82

  LaorA.NetzerH. (1989). Massive thin accretion discs. - I. Calculated spectra. Mon. Notices R. Astronomical Soc.238, 897–916. 10.1093/mnras/238.3.897

  • CrossRef
  • Google Scholar

• 83

  LaurentiM.PiconcelliE.ZappacostaL.TombesiF.VignaliC.BianchiS.et al (2022). X-ray spectroscopic survey of highly accreting AGN. Astronomy Astrophysics657, A57. 10.1051/0004-6361/202141829

  • CrossRef
  • Google Scholar

• 84

  LeighlyK. M. (1999). A comprehensive spectral and variability study of narrow-line Seyfert 1 galaxies observed by ASCA. II. Spectral analysis and correlations. Astrophysical J. Suppl.125, 317–348. 10.1086/313287

  • CrossRef
  • Google Scholar

• 85

  LeighlyK. M. (2004). Hubble space telescope STIS ultraviolet spectral evidence of outflow in extreme narrow-line Seyfert 1 galaxies. II. Modeling and interpretation. Astrophysical J.611, 125–152. 10.1086/422089

  • CrossRef
  • Google Scholar

• 86

  LeighlyK. M.MooreJ. R. (2004). Hubble space telescope STIS ultraviolet spectral evidence of outflow in extreme narrow-line Seyfert 1 galaxies. I. Data and analysis. Astrophysical J.611, 107–124. 10.1086/422088

  • CrossRef
  • Google Scholar

• 87

  LiY.-R.SongshengY.-Y.QiuJ.HuC.DuP.LuK.-X.et al (2018). Supermassive black holes with high accretion rates in active galactic nuclei. VIII. Structure of the broad-line region and mass of the central black hole in mrk 142. Astrophysical J.869, 137. 10.3847/1538-4357/aaee6b

  • CrossRef
  • Google Scholar

• 88

  LiY.-R.WangJ.-M.BaiJ.-M. (2016). A non-parametric approach to constrain the transfer function in reverberation mapping. Astrophysical J.831, 206. 10.3847/0004-637X/831/2/206

  • CrossRef
  • Google Scholar

• 89

  LiuH.LuoB.BrandtW. N.BrothertonM. S.GallagherS. C.NiQ.et al (2021). On the observational difference between the accretion disk-corona connections among super- and sub-eddington accreting active galactic nuclei. Astrophysical J.910, 103. 10.3847/1538-4357/abe37f

  • CrossRef
  • Google Scholar

• 90

  LuK.-X.DuP.HuC.LiY.-R.ZhangZ.-X.WangK.et al (2016). Reverberation mapping of the broad-line region in NGC 5548: Evidence for radiation pressure?Astrophysical J.827, 118. 10.3847/0004-637X/827/2/118

  • CrossRef
  • Google Scholar

• 91

  LubińskiP.BeckmannV.GibaudL.PaltaniS.PapadakisI. E.RicciC.et al (2016). A comprehensive analysis of the hard X-ray spectra of bright Seyfert galaxies. Mon. Notices R. Astronomical Soc.458, 2454–2475. 10.1093/mnras/stw454

  • CrossRef
  • Google Scholar

• 92

  MalkanM. A.SargentW. L. W. (1982). The ultraviolet excess of Seyfert 1 galaxies and quasars. Astrophysical J.254, 22–37. 10.1086/159701

  • CrossRef
  • Google Scholar

• 93

  MalkanM. A. (1983). The ultraviolet excess of luminous quasars. II. Evidence for massive accretion disks. Astrophysical J.268, 582–590. 10.1086/160981

  • CrossRef
  • Google Scholar

• 94

  MarinF. (2014). A compendium of AGN inclinations with corresponding UV/optical continuum polarization measurements. Mon. Notices R. Astronomical Soc.441, 551–564. 10.1093/mnras/stu593

  • CrossRef
  • Google Scholar

• 95

  MarinelloM.Rodríguez-ArdilaA.Garcia-RissmannA.SigutT. A. A.PradhanA. K. (2016). The Fe II emission in active galactic nuclei: Excitation mechanisms and location of the emitting region. Astrophysical J.820, 116. 10.3847/0004-637X/820/2/116

  • CrossRef
  • Google Scholar

• 96

  MarshallJ.BoltonA.BullockJ.BurgasserA.ChambersK.DePoyD.et al (2019). The maunakea spectroscopic explorer. Bull. Am. Astronomical Soc.51, 126.

  • Google Scholar

• 97

  Martínez-AldamaM. L.CzernyB.KawkaD.KarasV.PandaS.ZajačekM.et al (2019). Can reverberation-measured quasars Be used for cosmology?Astrophysical J.883, 170. 10.3847/1538-4357/ab3728

  • CrossRef
  • Google Scholar

• 98

  Martínez-AldamaM. L.DultzinD.MarzianiP.SulenticJ. W.BressanA.ChenY.et al (2015). O I and Ca II observations in intermediate redshift quasars. Astrophysical J. Suppl.217, 3. 10.1088/0067-0049/217/1/3

  • CrossRef
  • Google Scholar

• 99

  Martínez-AldamaM. L.PandaS.CzernyB.MarinelloM.MarzianiP.DultzinD. (2021). The CaFe project: Optical Fe II and near-infrared Ca II triplet emission in active galaxies. II. The driver(s) of the Ca II and Fe II and its potential use as a chemical clock. Astrophysical J.918, 29. 10.3847/1538-4357/ac03b6

  • CrossRef
  • Google Scholar

• 100

  MarzianiP.del OlmoA.D’OnofrioM.DultzinD.NegreteC. A.Martínez-AldamaM. L.et al (2018a). “Narrow-line Seyfert 1s: What is wrong in a name?,” in Revisiting narrow-line Seyfert 1 galaxies and their place in the universe, 2. 10.22323/1.328.0002

  • CrossRef
  • Google Scholar

• 101

  MarzianiP.del OlmoA.Martínez-CarballoM. A.Martínez-AldamaM. L.StirpeG. M.NegreteC. A.et al (2019). Black hole mass estimates in quasars. A comparative analysis of high- and low-ionization lines. Astronomy Astrophysics627, A88. 10.1051/0004-6361/201935265

  • CrossRef
  • Google Scholar

• 102

  MarzianiP.DultzinD.del OlmoA.D’OnofrioM.de DiegoJ. A.StirpeG. M.et al (2021a). “The quasar main sequence and its potential for cosmology,” in Nuclear activity in galaxies across cosmic time. Editors PovićM.MarzianiP.MasegosaJ.NetzerH.NeguS. H.Tessema.S. B., 356, 66–71. 10.1017/S1743921320002598

  • CrossRef
  • Google Scholar

• 103

  MarzianiP.DultzinD.SulenticJ. W.Del OlmoA.NegreteC. A.Martínez-AldamaM. L.et al (2018b). A main sequence for quasars. Front. Astronomy Space Sci.5, 6. 10.3389/fspas.2018.00006

  • CrossRef
  • Google Scholar

• 104

  MarzianiP.Dultzin-HacyanD.SulenticJ. W. (2006). “Accretion onto supermassive black holes in quasars: Learning from optical/UV observations,” in New developments in black hole research. Editor KreitlerP. V. (New York: Nova Press), 123.

  • Google Scholar

• 105

  MarzianiP.Martínez CarballoM. A.SulenticJ. W.Del OlmoA.StirpeG. M.DultzinD. (2016). The most powerful quasar outflows as revealed by the Civ λ1549 resonance line. Astrophysics Space Sci.361, 29. 10.1007/s10509-015-2611-1

  • CrossRef
  • Google Scholar

• 106

  MarzianiP.OlmoA. d.NegreteC. A.DultzinD.PiconcelliE.VietriG.et al (2022). The intermediate-ionization lines as virial broadening estimators for population A quasars. Astrophysical J. Suppl.261, 30. 10.3847/1538-4365/ac6fd6

  • CrossRef
  • Google Scholar

• 107

  MarzianiP.SniegowskaM.PandaS.CzernyB.NegreteC. A.DultzinD.et al (2021b). The main sequence view of quasars accreting at high rates: Influence of star formation. Res. Notes Am. Astronomical Soc.5, 25. 10.3847/2515-5172/abe46a

  • CrossRef
  • Google Scholar

• 108

  MarzianiP.SulenticJ. W. (2012). Estimating black hole masses in quasars using broad optical and UV emission lines. NARev56, 49–63. 10.1016/j.newar.2011.09.001

  • CrossRef
  • Google Scholar

• 109

  MarzianiP.SulenticJ. W. (2014a). Highly accreting quasars: Sample definition and possible cosmological implications. Mon. Notices R. Astronomical Soc.442, 1211–1229. 10.1093/mnras/stu951

  • CrossRef
  • Google Scholar

• 110

  MarzianiP.SulenticJ. W.NegreteC. A.DultzinD.ZamfirS.BachevR. (2010). Broad-line region physical conditions along the quasar eigenvector 1 sequence. Mon. Notices R. Astronomical Soc.409, 1033–1048. 10.1111/j.1365-2966.2010.17357.x

  • CrossRef
  • Google Scholar

• 111

  MarzianiP.SulenticJ. W.Plauchu-FraynI.del OlmoA. (2013). Low-ionization outflows in high Eddington ratio quasars. Astrophysical J.764, 150. 10.1088/0004-637x/764/2/150

  • CrossRef
  • Google Scholar

• 112

  MarzianiP.SulenticJ. W. (2014b). Quasars and their emission lines as cosmological probes. Adv. Space Res.54, 1331–1340. 10.1016/j.asr.2013.10.007

  • CrossRef
  • Google Scholar

• 113

  MarzianiP.SulenticJ. W.ZwitterT.Dultzin-HacyanD.CalvaniM. (2001). Searching for the physical drivers of the eigenvector 1 correlation space. ApJ558, 553–560. 10.1086/322286

  • CrossRef
  • Google Scholar

• 114

  MathewsW. G.FerlandG. J. (1987). What heats the hot phase in active nuclei?Astrophysical J.323, 456–467. 10.1086/165843

  • CrossRef
  • Google Scholar

• 115

  MathurS. (2000). Narrow-line Seyfert 1 galaxies and the evolution of galaxies and active galaxies. Mon. Notices R. Astronomical Soc.314, L17–L20. 10.1046/j.1365-8711.2000.03530.x

  • CrossRef
  • Google Scholar

• 116

  MatsuokaY.KawaraK.OyabuS. (2008). Low-ionization emission regions in quasars: Gas properties probed with broad O I and Ca II lines. Astrophysical J.673, 62–68. 10.1086/524193

  • CrossRef
  • Google Scholar

• 117

  McHardyI. M.KoerdingE.KniggeC.UttleyP.FenderR. P. (2006). Active galactic nuclei as scaled-up Galactic black holes. Nature444, 730–732. 10.1038/nature05389

  • CrossRef
  • Google Scholar

• 118

  McLureR. J.JarvisM. J. (2002). Measuring the black hole masses of high-redshift quasars. Mon. Notices R. Astronomical Soc.337, 109–116. 10.1046/j.1365-8711.2002.05871.x

  • CrossRef
  • Google Scholar

• 119

  MineshigeS.KawaguchiT.TakeuchiM.HayashidaK. (2000). Slim-disk model for soft X-ray excess and variability of narrow-line Seyfert 1 galaxies. Publ. Astronomical Soc. Jpn.52, 499–508.

  • Google Scholar

• 120

  MolinaM.MaliziaA.BassaniL.UrsiniF.BazzanoA.UbertiniP. (2019). Swift/XRT-NuSTAR spectra of type 1 AGN: Confirming INTEGRAL results on the high-energy cut-off. Mon. Notices R. Astronomical Soc.484, 2735–2746. 10.1093/mnras/stz156

  • CrossRef
  • Google Scholar

• 121

  NegreteA.DultzinD.MarzianiP.SulenticJ. (2012). BLR physical conditions in extreme population A quasars: A method to estimate central black hole mass at high redshift. Astrophysical J.757, 62. 10.1088/0004-637x/757/1/62

  • CrossRef
  • Google Scholar

• 122

  NegreteC. A.DultzinD.MarzianiP.EsparzaD.SulenticJ. W.del OlmoA.et al (2018). Highly accreting quasars: The SDSS low-redshift catalog. Astronomy Astrophysics620, A118. 10.1051/0004-6361/201833285

  • CrossRef
  • Google Scholar

• 123

  NegreteC. A.DultzinD.MarzianiP.SulenticJ. W. (2014). A photoionization method for estimating BLR “size” in quasars. Adv. Space Res.54, 1355–1361. 10.1016/j.asr.2013.11.037

  • CrossRef
  • Google Scholar

• 124

  NetzerH. (2019). Bolometric correction factors for active galactic nuclei. Mon. Notices R. Astronomical Soc.488, 5185–5191. 10.1093/mnras/stz2016

  • CrossRef
  • Google Scholar

• 125

  NetzerH. (2015). Revisiting the unified model of active galactic nuclei. Annu. Rev. Astronomy Astrophysics53, 365–408. 10.1146/annurev-astro-082214-122302

  • CrossRef
  • Google Scholar

• 126

  OjhaV.ChandH.DewanganG. C.RakshitS. (2020). A comparison of X-ray photon indices among the narrow- and broad-line Seyfert 1 galaxies. Astrophysical J.896, 95. 10.3847/1538-4357/ab94ac

  • CrossRef
  • Google Scholar

• 127

  OsterbrockD. E.FerlandG. J. (2006). Astrophysics of gaseous nebulae and active galactic nuclei.

  • Google Scholar

• 128

  OsterbrockD. E.PoggeR. W. (1985). The spectra of narrow-line Seyfert 1 galaxies. Astrophysical J.297, 166–176. 10.1086/163513

  • CrossRef
  • Google Scholar

• 129

  PadovaniP.AlexanderD. M.AssefR. J.De MarcoB.GiommiP.HickoxR. C.et al (2017). Active galactic nuclei: what’s in a name?Astronomy Astrophysics Rev.25, 2. 10.1007/s00159-017-0102-9

  • CrossRef
  • Google Scholar

• 130

  PadovaniP.RafanelliP. (1988). Mass-luminosity relationships and accretion rates for Seyfert 1 galaxies and quasars. Astronomy Astrophysics205, 53–70.

  • Google Scholar

• 131

  PanagiotouC.WalterR. (2020). NuSTAR view of Swift/BAT AGN: The R-Γ correlation. Astronomy Astrophysics640, A31. 10.1051/0004-6361/201937390

  • CrossRef
  • Google Scholar

• 132

  PancoastA.BrewerB. J.TreuT.ParkD.BarthA. J.BentzM. C.et al (2014). Modelling reverberation mapping data - II. Dynamical modelling of the Lick AGN Monitoring Project 2008 data set. Mon. Notices R. Astronomical Soc.445, 3073–3091. 10.1093/mnras/stu1419

  • CrossRef
  • Google Scholar

• 133

  PandaS.CzernyB.AdhikariT. P.HryniewiczK.WildyC.KuraszkiewiczJ.et al (2018). Modeling of the quasar main sequence in the optical plane. Astrophysical J.866, 115. 10.3847/1538-4357/aae209

  • CrossRef
  • Google Scholar

• 134

  PandaS.CzernyB.DoneC.KubotaA. (2019a). CLOUDY view of the warm corona. Astrophysical J.875, 133. 10.3847/1538-4357/ab11cb

  • CrossRef
  • Google Scholar

• 135

  PandaS.CzernyB.WildyC. (2017). The physical driver of the optical eigenvector 1 in quasar main sequence. Front. Astronomy Space Sci.4, 33. 10.3389/fspas.2017.00033

  • CrossRef
  • Google Scholar

• 136

  PandaS.Martínez-AldamaM. L.ZajačekM. (2019b). Current and future applications of Reverberation-mapped quasars in Cosmology. Front. Astronomy Space Sci.6, 75. 10.3389/fspas.2019.00075

  • CrossRef
  • Google Scholar

• 137

  PandaS.MarzianiP.CzernyB. (2019c). The quasar main sequence explained by the combination of Eddington ratio, metallicity, and orientation. Astrophysical J.882, 79. 10.3847/1538-4357/ab3292

  • CrossRef
  • Google Scholar

• 138

  PandaS. (2022). Parameterizing the AGN radius–luminosity relation from the eigenvector 1 viewpoint. Front. Astronomy Space Sci.9, 850409. 10.3389/fspas.2022.850409

  • CrossRef
  • Google Scholar

• 139

  PandaS. (2021a). Physical Conditions in the broad-line Regions of active galaxies. Ph.D. Thesis, polish Academy of Sciences. Institute of Physics.

  • Google Scholar

• 140

  PandaS. (2021b). The CaFe project: Optical Fe II and near-infrared Ca II triplet emission in active galaxies: Simulated EWs and the co-dependence of cloud size and metal content. Astronomy Astrophysics650, A154. 10.1051/0004-6361/202140393

  • CrossRef
  • Google Scholar

• 141

  PeiL.BarthA. J.AlderingG. S.BrileyM. M.CarrollC. J.CarsonD. J.et al (2014). Reverberation mapping of the KEPLER field AGN KA1858+4850. Astrophysical J.795, 38. 10.1088/0004-637X/795/1/38

  • CrossRef
  • Google Scholar

• 142

  PetersonB. M. (1988). Emission-line variability in Seyfert galaxies. Publ. Astronomical Soc. Pac.100, 18. 10.1086/132130

  • CrossRef
  • Google Scholar

• 143

  PetersonB. M.FerrareseL.GilbertK. M.KaspiS.MalkanM. A.MaozD.et al (2004). Central masses and broad-line region sizes of active galactic nuclei. II. A homogeneous analysis of a large reverberation-mapping database. Astrophysical J.613, 682–699. 10.1086/423269

  • CrossRef
  • Google Scholar

• 144

  PetersonB. M. (1993). Reverberation mapping of active galactic nuclei. Publ. Astronomical Soc. Pac.105, 247. 10.1086/133140

  • CrossRef
  • Google Scholar

• 145

  PetrucciP. O.GronkiewiczD.RozanskaA.BelmontR.BianchiS.CzernyB.et al (2020). Radiation spectra of warm and optically thick coronae in AGNs. Astronomy Astrophysics634, A85. 10.1051/0004-6361/201937011

  • CrossRef
  • Google Scholar

• 146

  PhillipsM. M. (1978). Permitted fe II emission in Seyfert 1 galaxies and QSOs I. Observations. Astrophysical J. Suppl.38, 187. 10.1086/190553

  • CrossRef
  • Google Scholar

• 147

  RakshitS.StalinC. S.KotilainenJ. (2020). Spectral properties of quasars from sloan digital sky survey data release 14: The catalog. Astrophysical J. Suppl.249, 17. 10.3847/1538-4365/ab99c5

  • CrossRef
  • Google Scholar

• 148

  RicciC.HoL. C.FabianA. C.TrakhtenbrotB.KossM. J.UedaY.et al (2018). BAT AGN Spectroscopic Survey - XII. The relation between coronal properties of active galactic nuclei and the Eddington ratio. Mon. Notices R. Astronomical Soc.480, 1819–1830. 10.1093/mnras/sty1879

  • CrossRef
  • Google Scholar

• 149

  RichardsG. T.LacyM.Storrie-LombardiL. J.HallP. B.GallagherS. C.HinesD. C.et al (2006). Spectral energy distributions and multiwavelength selection of type 1 quasars. Astrophysical J. Suppl.166, 470–497. 10.1086/506525

  • CrossRef
  • Google Scholar

• 150

  RossR. R.FabianA. C. (2005). A comprehensive range of X-ray ionized-reflection models. Mon. Notices R. Astronomical Soc.358, 211–216. 10.1111/j.1365-2966.2005.08797.x

  • CrossRef
  • Google Scholar

• 151

  RunnoeJ. C.BrothertonM. S.ShangZ.WillsB. J.DiPompeoM. A. (2013). The orientation dependence of quasar single-epoch black hole mass scaling relationships. Mon. Notices R. Astronomical Soc.429, 135–149. 10.1093/mnras/sts322

  • CrossRef
  • Google Scholar

• 152

  SadowskiA. (2011). Slim accretion disks around black holes. arXiv e-prints , arXiv:1108.0396.

  • Google Scholar

• 153

  SchmidtM. (1963). 3C 273: A star-like object with large red-shift. Nature197, 1040. 10.1038/1971040a0

  • CrossRef
  • Google Scholar

• 154

  SchmidtM.GreenR. F. (1983). Quasar evolution derived from the Palomar bright quasar survey and other complete quasar surveys. Astrophysical J.269, 352–374. 10.1086/161048

  • CrossRef
  • Google Scholar

• 155

  ShakuraN. I.SunyaevR. A. (1973). Black holes in binary systems. Observational appearance. Astronomy Astrophysics24, 337–355.

  • Google Scholar

• 156

  ShenY.HallP. B.HorneK.ZhuG.McGreerI.SimmT.et al (2019). The sloan digital sky survey reverberation mapping project: Sample characterization. Astrophysical J. Suppl.241, 34. 10.3847/1538-4365/ab074f

  • CrossRef
  • Google Scholar

• 157

  ShenY.HoL. C. (2014). The diversity of quasars unified by accretion and orientation. Nature513, 210–213. 10.1038/nature13712

  • CrossRef
  • Google Scholar

• 158

  ShenY.RichardsG. T.StraussM. A.HallP. B.SchneiderD. P.SneddenS.et al (2011). A catalog of quasar properties from sloan digital sky survey data release 7. Astrophysical J. Suppl.194, 45. 10.1088/0067-0049/194/2/45

  • CrossRef
  • Google Scholar

• 159

  ShenY. (2013). The mass of quasars. Bull. Astronomical Soc. India41, 61–115.

  • Google Scholar

• 160

  ShieldsG. A. (1978). Thermal continuum from accretion disks in quasars. Nature272, 706–708. 10.1038/272706a0

  • CrossRef
  • Google Scholar

• 161

  SigutT. A. A.PradhanA. K.NaharS. N. (2004). Theoretical Fe I-iii emission-line strengths from active galactic nuclei with broad-line regions. Astrophysical J.611, 81–92. 10.1086/422027

  • CrossRef
  • Google Scholar

• 162

  SigutT. A. A.PradhanA. K. (2003). Predicted Fe II emission-line strengths from active galactic nuclei. Astrophysical J. Suppl.145, 15–37. 10.1086/345498

  • CrossRef
  • Google Scholar

• 163

  ŚniegowskaM.MarzianiP.CzernyB.PandaS.Martínez-AldamaM. L.del OlmoA.et al (2021). High metal content of highly accreting quasars. Astrophysical J.910, 115. 10.3847/1538-4357/abe1c8

  • CrossRef
  • Google Scholar

• 164

  SpergelD.GehrelsN.BreckinridgeJ.DonahueM.DresslerA.GaudiB. S.et al (2013). Wide-field InfraRed survey telescope-astrophysics focused telescope assets WFIRST-AFTA final report. arXiv e-prints , arXiv:1305.5422.

  • Google Scholar

• 165

  SulenticJ.MarzianiP. (2015). Quasars in the 4D eigenvector 1 context: A stroll down memory lane. Front. Astronomy Space Sci.2, 6. 10.3389/fspas.2015.00006

  • CrossRef
  • Google Scholar

• 166

  SulenticJ. W.BachevR.MarzianiP.NegreteC. A.DultzinD. (2007). C IV λ1549 as an eigenvector 1 parameter for active galactic nuclei. Astrophysical J.666, 757–777. 10.1086/519916

  • CrossRef
  • Google Scholar

• 167

  SulenticJ. W.MarzianiP.del OlmoA.DultzinD.PereaJ.Alenka NegreteC. (2014). GTC spectra of z ≈ 2.3 quasars: Comparison with local luminosity analogs. Astronomy Astrophysics570, A96. 10.1051/0004-6361/201423975

  • CrossRef
  • Google Scholar

• 168

  SulenticJ. W.MarzianiP.Dultzin-HacyanD. (2000a). Phenomenology of broad emission lines in active galactic nuclei. Annu. Rev. Astronomy Astrophysics38, 521–571. 10.1146/annurev.astro.38.1.521

  • CrossRef
  • Google Scholar

• 169

  SulenticJ. W.ZamfirS.MarzianiP.DultzinD. (2008). “Our search for an H-R diagram of quasars,” in Revista mexicana de Astronomia y astrofisica conference series. Vol. 32 of revista mexicana de Astronomia y astrofisica conference series, 51–58.

  • Google Scholar

• 170

  SulenticJ. W.ZwitterT.MarzianiP.Dultzin-HacyanD. (2000b). Eigenvector 1: An optimal correlation space for active galactic nuclei. Astrophysical J. Lett.536, L5–L9. 10.1086/312717

  • CrossRef
  • Google Scholar

• 171

  SunJ.ShenY. (2015). Dissecting the quasar main sequence: Insight from host galaxy properties. Astrophysical J. Lett.804, L15. 10.1088/2041-8205/804/1/L15

  • CrossRef
  • Google Scholar

• 172

  TakeuchiS.OhsugaK.MineshigeS. (2013). Clumpy outflows from supercritical accretion flow. Publ. Astronomical Soc. Jpn.65, 88. 10.1093/pasj/65.4.88

  • CrossRef
  • Google Scholar

• 173

  TortosaA.BianchiS.MarinucciA.MattG.PetrucciP. O. (2018). A NuSTAR census of coronal parameters in Seyfert galaxies. Astronomy Astrophysics614, A37. 10.1051/0004-6361/201732382

  • CrossRef
  • Google Scholar

• 174

  TrakhtenbrotB.RicciC.KossM. J.SchawinskiK.MushotzkyR.UedaY.et al (2017). BAT AGN spectroscopic survey (BASS) - VI. The ?x-L/L_Edd relation. Mon. Notices R. Astronomical Soc.470, 800–814. 10.1093/mnras/stx1117

  • CrossRef
  • Google Scholar

• 175

  UlrichM.-H.MaraschiL.UrryC. M. (1997). Variability of active galactic nuclei. Annu. Rev. Astronomy Astrophysics35, 445–502. 10.1146/annurev.astro.35.1.445

  • CrossRef
  • Google Scholar

• 176

  UrryC. M.PadovaniP. (1995). Unified schemes for radio-loud active galactic nuclei. Publ. Astronomical Soc. Pac.107, 803. 10.1086/133630

  • CrossRef
  • Google Scholar

• 177

  VernerE. M.VernerD. A.KoristaK. T.FergusonJ. W.HamannF.FerlandG. J. (1999). Numerical simulations of Fe II emission spectra. Astrophysical J. Suppl.120, 101–112. 10.1086/313171

  • CrossRef
  • Google Scholar

• 178

  VestergaardM.PetersonB. M. (2006). Determining central black hole masses in distant active galaxies and quasars. II. Improved optical and UV scaling relationships. Astrophysical J.641, 689–709. 10.1086/500572

  • CrossRef
  • Google Scholar

• 179

  VietriG.PiconcelliE.BischettiM.DurasF.MartocchiaS.BongiornoA.et al (2018). The WISSH quasars project. IV. Broad line region versus kiloparsec-scale winds. Astronomy Astrophysics617, A81. 10.1051/0004-6361/201732335

  • CrossRef
  • Google Scholar

• 180

  WalterR.FinkH. H. (1993). The ultraviolet to soft X-ray bump of Seyfert 1 type active galactic nuclei. Astronomy Astrophysics274, 105.

  • Google Scholar

• 181

  WandelA.PetersonB. M.MalkanM. A. (1999). Central masses and broad-line region sizes of active galactic nuclei. I. Comparing the photoionization and reverberation techniques. Astrophysical J.526, 579–591. 10.1086/308017

  • CrossRef
  • Google Scholar

• 182

  WangA.AnT.ChengX.HoL. C.KellermannK. I.BaanW. A.et al (2022). VLBI observations of a sample of palomar-green quasars I: Parsec-scale morphology. Mon. Notices R. Astronomical Soc.518, 39–53. 10.1093/mnras/stac3091

  • CrossRef
  • Google Scholar

• 183

  WangJ.-M.DuP.HuC.NetzerH.BaiJ.-M.LuK.-X.et al (2014a). Supermassive black holes with high accretion rates in active galactic nuclei. II. The most luminous standard candles in the universe. Astrophysical J.793, 108. 10.1088/0004-637X/793/2/108

  • CrossRef
  • Google Scholar

• 184

  WangJ.-M.DuP.LiY.-R.HoL. C.HuC.BaiJ.-M. (2014b). A new approach to constrain black hole spins in active galaxies using optical reverberation mapping. Astrophysical J. Lett.792, L13. 10.1088/2041-8205/792/1/L13

  • CrossRef
  • Google Scholar

• 185

  WangJ.-M.DuP.Valls-GabaudD.HuC.NetzerH. (2013). Super-eddington accreting massive black holes as long-lived cosmological standards. Phys. Rev. Lett.110, 081301. 10.1103/PhysRevLett.110.081301

  • CrossRef
  • Google Scholar

• 186

  WangJ.-M.QiuJ.DuP.HoL. C. (2014c). Self-shadowing effects of slim accretion disks in active galactic nuclei: The diverse appearance of the broad-line region. Astrophysical J.797, 65. 10.1088/0004-637X/797/1/65

  • CrossRef
  • Google Scholar

• 187

  WataraiK.-Y.FukueJ.TakeuchiM.MineshigeS. (2000). Galactic black-hole candidates shining at the Eddington luminosity. Publ. Astronomical Soc. Jpn.52, 133.

  • Google Scholar

• 188

  WatsonD.DenneyK. D.VestergaardM.DavisT. M. (2011). A new cosmological distance measure using active galactic nuclei. Astrophysical J. Lett.740, L49. 10.1088/2041-8205/740/2/L49

  • CrossRef
  • Google Scholar

• 189

  WeedmanD. W. (1976). Luminosities of Seyfert galaxies and QSOs. Astrophysical J.208, 30–36. 10.1086/154577

  • CrossRef
  • Google Scholar

• 190

  WeedmanD. W. (1977). Seyfert galaxies. Annu. Rev. Astronomy Astrophysics15, 69–95. 10.1146/annurev.aa.15.090177.000441

  • CrossRef
  • Google Scholar

• 191

  WillsB. J.NetzerH.WillsD. (1985). Broad emission features in QSOs and active galactic nuclei. II - new observations and theory of Fe II and H I emission. Astrophysical J.288, 94–116. 10.1086/162767

  • CrossRef
  • Google Scholar

• 192

  YangG.BoquienM.BrandtW. N.BuatV.BurgarellaD.CieslaL.et al (2022). Fitting AGN/galaxy X-ray-to-radio SEDs with CIGALE and improvement of the code. Astrophysical J.927, 192. 10.3847/1538-4357/ac4971

  • CrossRef
  • Google Scholar

• 193

  ZajačekM.CzernyB.Martinez-AldamaM. L.RałowskiM.OlejakA.PrzyłuskiR.et al (2021). Time delay of Mg II emission response for the luminous quasar HE 0435-4312: Toward application of the high-accretor radius-luminosity relation in cosmology. Astrophysical J.912, 10. 10.3847/1538-4357/abe9b2

  • CrossRef
  • Google Scholar

• 194

  ZappacostaL.PiconcelliE.GiustiniM.VietriG.DurasF.MiniuttiG.et al (2020). The WISSH quasars project. VII. The impact of extreme radiative field in the accretion disc and X-ray corona interplay. Astronomy Astrophysics635, L5. 10.1051/0004-6361/201937292

  • CrossRef
  • Google Scholar

• 195

  ZdziarskiA. A.GhiselliniG.GeorgeI. M.SvenssonR.FabianA. C.DoneC. (1990). Electron-positron pairs, Compton reflection, and the X-ray spectra of active galactic nuclei. Astrophysical J. Lett.363, L1. 10.1086/185851

  • CrossRef
  • Google Scholar

• 196

  ZhangZ.-X.DuP.SmithP. S.ZhaoY.HuC.XiaoM.et al (2019). Kinematics of the broad-line region of 3C 273 from a 10 yr reverberation mapping campaign. Astrophysical J.876, 49. 10.3847/1538-4357/ab1099

  • CrossRef
  • Google Scholar