High Eddington quasars as discovery tools current state and challenges Swayamtrupta Panda1yand Paola Marziani2

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High Eddington quasars as discovery tools:

current state and challenges

Swayamtrupta Panda 1,†,*and Paola Marziani 2

Laborat

orio Nacional de Astrof

ısica - MCTI, R. dos Estados Unidos, 154 - Na

c¸ ˜

oes,

Itajub´

a - MG, 37504-364, Brazil

2INAF-Astronomical Observatory of Padova, Vicolo dell’Osservatorio, 5, 35122

Padova PD, Italy

Correspondence†:

Swayamtrupta Panda

spanda@lna.br

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 SED, and the physical conditions in the BLR. In this work, we critically review long-standing

issues pertaining to the SED 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 AGN 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.

Keywords: galaxies: active, quasars: emission lines; quasars: supermassive black holes; quasars: accretion, accretion disks;

quasars: reverberation mapping; cosmology

1 ACTIVE GALACTIC NUCLEI AS ACCRETING BLACK HOLES

Active galactic nuclei (AGNs) are among the brightest cosmic objects known to us (Weedman, 1976,

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

*CNPq Fellow

arXiv:2210.15041v3 [astro-ph.GA] 10 May 2023

Panda & Marziani High Eddington AGNs: current state and challenges

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

-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.)*

In this contribution, we shall restrict the attention mainly to the ionizing continuum in radio-quiet quasars.

In this case, various components of the SED arise due to different radiation mechanisms and at varying

distances, notably among them:

The characteristic ‘Big Blue Bump’ (Czerny and Elvis, 1987; Shields, 1978) that is formed by the

optical and ultraviolet radiation produced due to thermal emission from the accretion disk.

The X-ray emission well-ﬁt by a power-law, and produced when the UV photons from the disk undergo

inverse Compton scattering by hot electrons in a Compton-thin corona close to the SMBH (e.g.,

Zdziarski et al., 1990).

A spectral component observationally described as a “soft X-ray excess” (e.g., Arnaud et al., 1985).

The most widely-accepted interpretation of the excess detected in soft X-rays is of emission in a

Compton-thick corona connected with the innermost accretion disk (Walter and Fink 1993; Petrucci

et al. 2020 and references therein). The competing model – relativistically blurred photoionized disc

reﬂection (Ross and Fabian, 2005; Crummy et al., 2006) – is not anymore favoured as an explanation

for the soft X-ray excess itself, although blurred accretion disk reﬂection can occur independently from

the soft excess (Boissay et al., 2016). The soft excess helps bridge the absorption gap between the UV

downturn and the soft X-ray upturn (e.g., Elvis et al., 1994; Laor et al., 1997; Richards et al., 2006;

Kubota and Done, 2018), and changes the far-UV and soft-X-ray part of the spectrum, affecting the

line production, including Fe II emission in the BLR (Panda et al., 2019a).

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

The old description of AGN continuum as non-thermal, featureless was perhaps inspired by the earliest quasars studied that were mainly radio loud

and at any rate sub-Eddington accretors. The power-law function used to ﬁt the optical/UV continuum over a limited range in frequency is now considered to

represent the thermal continuum from an accretion disk, whose power

Pν∝v1

(Shakura and Sunyaev, 1973), not as associated with a featureless synchrotron

continuum. The synchrotron radiation from relativistic jets that accounts for most of the radio emission is only a fraction of the optical continuum in non-blazar

type-1 AGN or “thermal” radio-loud AGN (Antonucci, 2012, and references therein).

This is a provisional ﬁle, not the ﬁnal typeset article 2

Panda & Marziani High Eddington AGNs: current state and challenges

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

∼1

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.

A weak but statistically signiﬁcant correlation between hard-X photon index

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

values compared to uncertainties in individual

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

∼100

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

≈100

keV in the SEDs of Figure 1

although it was not actually measured. In recent years measurements by NuSTAR and

- ray observatories

such as SWIFT indicate a dispersion in the actual turnover, from 50 to 200 keV (Fabian et al., 2015;

Lubi

nski et al., 2016). It is currently debated whether the downturn energy may depend on the Eddington

ratio, although the trend between

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 

[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

m, 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

(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).

Frontiers 3

Panda & Marziani High Eddington AGNs: current state and challenges

Figure 1.

Templates of SEDs for high Eddington radiators. Grey; the landmark Mathews and Ferland

(1987) SED; dashed blue: the Marziani and Sulentic (2014a) SED for quasars radiating close to the

Eddington limit; blue and magenta: high and highest Eddington ratio templates from Ferland et al. (2020)).

The SEDs have been normalized at ≈0.18 Ryd.

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

This is a provisional ﬁle, not the ﬁnal typeset article 4

Panda & Marziani High Eddington AGNs: current state and challenges

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, AlIII

1860/SiIII]

1892, Fe II/H

) in order to estimate

these (density, ionization condition, and metallicity) parameters (Negrete et al., 2012, 2014;

Sniegowska

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

nH∼1013

−3

log U∼ −2.5,Z&20Z.†.

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 (Collin-Souffrin et al., 1988; Gaskell, 1982). The broad emission

spectrum in AGNs can be divided into two parts: the ﬁrst set of lines that include Ly

, C III], C IV,

He I, He II, and N Vpredominantly emitted by a highly ionized region that presumably has a relatively

low density (

−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]

1892. 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 Iand Ca II, emitted by a mildly

ionized medium having a much higher density (

−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 (Mart

ınez-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).

MBH

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: (i) 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, (ii) 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

†

There is a general consensus that type-1 AGN BLR gas has supersolar abundance with canonical estimates reaching over

times solar (Hamann and

Ferland, 1993, 1999). However, the highest values depend also on the lines employed as diagnostics: the Al and Si lines are strongly dependent on enrichment

by supernovæ ejecta, as stressed by the authors themselves (see e.g., Garnica et al., 2022)

Frontiers 5

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High Eddington quasars as discovery tools current state and challenges Swayamtrupta Panda1yand Paola Marziani2

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