# Radio Dichotomy in Quasars with H$\beta$ FWHM greater than $15,000$ km s-1

Avinanda Chakraborty1,2, Anirban Bhattacharjee3, Michael S. Brotherton4,
Ritaban Chatterjee2,1, Suchetana Chatterjee2,1, Miranda Gilbert3
1Department of Physics, Presidency University, 86/1 College Street, Kolkata
700073, India
2School of Astrophysics, Presidency University, 86/1 College Street, Kolkata
700073, India
3Department of Biology, Geology and Physical Sciences, Sul Ross State
University, East Highway 90 Alpine, TX 79832, USA
4Department of Physics and Astronomy, University of Wyoming, Laramie, WY
82071, USA

###### Abstract

It has been inferred from large unbiased samples that $10\%$-$15\%$ of all
quasars are radio-loud (RL). Using the quasar catalog from the Sloan Digital
Sky Survey, we show that the radio-loud fraction (RLF) for high broad line
(HBL) quasars, containing H$\beta$ FWHM greater than $15,000$ km s-1, is $\sim
57\%$. While there is no significant difference between the RL and radio-quiet
(RQ) populations in our sample in terms of their black hole mass, Eddington
ratio, and covering fraction (CF), optical continuum luminosity of the RL
quasars are higher. The similarity in the distribution of their CF indicates
that our analysis is unbiased in terms of the viewing angle of the HBL RL and
RQ quasars. Hence, we conclude that the accretion disc luminosity of the RL
quasars in our HBL sample is higher, which indicates a connection between a
brighter disc and a more prominent jet. By comparing them with the non-HBL
H$\beta$ broad emission line quasars, we find that the HBL sources have the
lowest Eddington ratios in addition to having a very high RLF. That is
consistent with the theories of jet formation, in which jets are launched from
low Eddington ratio accreting systems. We find that the [O III] narrow
emission line is stronger in the RL compared to RQ quasars in our HBL sample,
which is consistent with previous findings in the literature, and may be
caused by the interaction of the narrow line gas with the jet.

###### keywords:

Galaxies: active — quasars: general— techniques: spectroscopic

##  1 Introduction

Since the very first discovery of quasars (Schmidt, 1963), the radio bi-
modality of the quasar population has been discussed in the literature (e.g.,
White et al., 2000; Brotherton et al., 2001; Lacy et al., 2001; Cirasuolo et
al., 2003a, b; Doi et al., 2013; Tadhunter, 2016; Hardcastle & Croston, 2020).
While the principal difference between the radio-loud (RL) and radio-quiet
(RQ) quasars lies in the strength of their radio jets (e.g., Bridle et al.,
1994; Mullin et al., 2008), several studies have been undertaken to address
the source of this dichotomy (e.g., Kellermann et al., 1989; Miller et al.,
1990; White et al., 2000; Lacy et al., 2001; Ivezić et al., 2002; White et
al., 2007; Laor & Behar, 2008; Zamfir et al., 2008; Baloković et al., 2012;
Zhang et al., 2021). The dichotomy has been studied in the context of black
hole (BH) mass (e.g., McLure & Jarvis, 2004), accretion rate (e.g., Sikora et
al., 2007; Hamilton, 2010), host galaxy properties (e.g., Sikora et al., 2007;
Lagos et al., 2009; Kimball et al., 2011), as well as black hole (BH) spin
(e.g., Blandford & Znajek, 1977; Blandford, 1990; Garofalo et al., 2010). It
has been proposed that there may be multiple mechanisms responsible for
powering the radio emission, for example it may come from a combination of
small-scale jets as well as star formation regions (Cirasuolo et al., 2003a;
Sabater et al., 2019).

In the cosmological context, the role of galaxy mergers and host dark matter
halos in the RL-RQ dichotomy has also been discussed (e.g., Hughes &
Blandford, 2003; Shen et al., 2007; Hickox et al., 2009; Lagos et al., 2009;
Garofalo et al., 2010; Hamilton, 2010). Using host galaxy photometry and
optical spectroscopy of the nuclear region, several recent works revealed a
consistent mass excess of the central BHs powering RL quasars compared to RQ
quasars (e.g., Dunlop et al., 2003; Chiaberge et al., 2005; Jarvis & McLure,
2006). Other studies have reached similar conclusions (e.g., Magorrian et al.,
1998; Laor, 2000b; McLure & Dunlop, 2001) regarding the mass distributions of
RL and RQ quasars.

Some authors have shown that BH mass is correlated with radio luminosity
(e.g., Franceschini et al., 1998; McLure et al., 1999; Laor, 2000a; Lacy et
al., 2001; McLure & Jarvis, 2004) while some studies report the contrary
(e.g., Ho, 2002; Oshlack et al., 2002; Woo & Urry, 2002; Snellen et al.,
2003). In the context of this dichotomy, there have been proposals of radio-
intermediate sources, which essentially implies the distribution of radio
loudness to be quasi-continuous (Falcke et al., 1996; Brotherton et al., 2001;
Lacy et al., 2001; Gürkan et al., 2019). One of the key reasons for which it
has been particularly difficult to establish the bimodality, is the absence of
a universally accepted definition of radio loudness. A ratio of the radio to
optical luminosity has been used most often while the absolute value of the
radio luminosity has been assumed to be the characteristic parameter in some
other cases. However, for both of those parameters the RL-RQ boundary has not
been universal over various studies in the literature (e.g., Baloković et al.,
2012).

Figure 1: Blue dots denote the radio loud fraction (RLF) as a function of FWHM
of broad H$\beta$ emission line. The x-axis represents the central value of
the FWHM bin. The error-bars used here are binomial errors of the mean value
of the RLF. We see that the RLF increases with FWHM.

Radio emission in quasars is shown to be related with the optical emission
lines (e.g., Osterbrock, 1977). In radio galaxies, a correlation between
emission line luminosities and radio jet luminosities has been established
(e.g., Baum & Heckman, 1989; Saunders et al., 1989; Rawlings & Saunders, 1991;
Willott et al., 1999; Zirbel & Baum, 1995; Buttiglione et al., 2010; Sikora et
al., 2013). Therefore, emission line diagnostic has been proposed by some
authors as a distinct method to define RL quasars. However, studies reveal
that there can be significant scatter in the radiative vs kinetic emission
correlations (see e.g., Rawlings & Saunders, 1991; Punsly & Zhang, 2011; Mingo
et al., 2014; Mingo et al., 2016; Gürkan et al., 2015). It has been shown that
jets do produce emissions beyond the radio, and are capable of photoionizing
the surrounding interstellar medium, but through shock-ionization they can
also produce emission lines (e.g., Mingo et al., 2011).

Zamfir et al. (2008) showed that RL quasars are located at a specific region
on an H$\beta$ full width half maximum (FWHM) versus $R_{Fe}$ plane. We
followed Zamfir et al. (2008) to define $R_{Fe}=F_{Fe}/F_{H\beta}$, where
$F_{H\beta}$ is the optical continuum flux at 5100 Åand $F_{Fe}$ is the FeII
flux at 4861 Å , calculated using the optical continuum flux at 5100 Å and
H$\beta$ slope. Among the sample they used roughly 78% of RL quasars have
H$\beta$ FWHM $>$ 4000 km s-1. While some RQ quasars are located in that part
of the plane, the restricted location of RL quasars is statistically
significant.

Accreting BH systems have been found to reside preferentially on a plane in a
space comprising disc luminosity, jet power and BH mass (e.g., Maccarone et
al., 2003; Merloni et al., 2003; Terashima & Wilson, 2003). This is known as
the fundamental plane of BH activity. It shows that the disc activity, jet
emission, and BH mass are connected with each other. However, the exact nature
of the above connection is not well understood. In general relativistic
magnetohydrodynamic simulations of jet launching and collimation (e.g.,
Komissarov, 2007; McKinney et al., 2012) it is revealed that the conversion of
accreted material into collimated jets may depend on multiple variables or a
combination thereof, including, BH mass (e.g., Dunlop et al., 2003; Chiaberge
& Marconi, 2011), BH spin (e.g., Blandford & Znajek, 1977; Fernandes et al.,
2011), and disc activity (e.g., Lin et al., 2009; Fernandes et al., 2011).
During the accretion process, gas in the disc gets heated and strong
ultraviolet (UV) to optical continuum emission is radiated (Pringle, 1981;
Malkan & Sargent, 1982; Ma & Wang, 2013). Optical spectroscopic observations
of the continuum and line emission of RL and RQ quasars have shed light on the
connection between the accretion disc and jet of these systems (Kawakatu et
al., 2009). It has been shown that the radio loud fraction (RLF, hereafter) of
quasars decreases with decreasing optical luminosity (Jiang et al., 2007).

In the preliminary work of Chakraborty & Bhattacharjee (2021), we used a
sample of RL and RQ quasars selected through the Sloan Digital Sky Survey
(SDSS) and performed a systematic study of their optical continuum and line
emission to search for clues in understanding the dichotomy in their radio
emission. In particular, we studied the BH properties of RL and RQ objects
selected through their H$\beta$ FWHM. Our results showed that the RLF
increases with FWHM, and hence to further understand this result we select a
sub-sample of extreme objects in the catalog with H$\beta$ FWHM greater than
15,000 km s-1 and study the bolometric luminosity, optical continuum
luminosity and fiducial virial BH mass (Vestergaard & Peterson, 2006)
distributions of these extreme objects. Gas in the broad line region (BLR) of
quasars is photoionized by the emission from the central continuum source at
ultraviolet frequencies, and gives rise to prominent recombination lines
observed in the quasar spectra (Sulentic et al., 2000). Due to their proximity
to the central SMBH the BLR gas has high speed ($10^{3}-10^{4}$ km s-1) which
results in the Doppler broadening of the emission lines. Thus our sample
consists of quasars with large values of the speed of the BLR gas.

---
Figure 2: Upper Panel: Orange solid and blue dashed lines denote the
distributions of black hole mass of the radio-quiet and radio-loud quasars,
respectively, with H$\beta$ FWHM greater than 15,000 km $s^{-1}$ in our
sample. Middle Panel:  Same as the top panel but now for Eddington ratios.
Bottom Panel:  Same as the other panels but for covering fraction. From the
figure we note that the distributions of black hole mass, Eddington ratio and
covering fraction of the two population are similar.

In the current work, we investigate whether the radio luminosity, bolometric
luminosity, BH mass, and radio loudness parameter of the RL quasars in our
sample of high broad line (HBL hereafter; with FWHM greater than $15,000$ km
s-1) quasars are related to each other while comparing them with their
relevant RQ counterparts. We construct the composite SDSS spectra of the
quasars in our sample and search for any difference between the RL and RQ
populations in terms of their mean spectrum. Finally, we compare our HBL
sample with the rest of the H$\beta$ broad emission line objects (non-HBL
hereafter) in terms of BH mass, Eddington ratio, bolometric luminosity,
H$\beta$ FWHM, and $R_{Fe}$ to search for possible clues of higher RLF.

Throughout this paper we use $\Omega_{\Lambda}$ = 0.7, $\Omega_{0}$ = 0.3 and
h = 0.7 as the cosmological parameters (Spergel et al., 2007). In §2, we give
a brief description of our sample and present our results in §3. In §4 we
discuss the results and summarize our conclusions.

##  2 Data Sets

In this section we discuss our data sets and sample selection.

###  2.1 Sloan Digital Sky Survey (SDSS)

We use RL and RQ quasars from Shen et al. (2011), which is the complete quasar
catalog obtained from SDSS DR7 (Schneider et al., 2010). The catalog consists
of $105,783$ spectroscopically confirmed quasars, that are brighter than
$Mi=-22.0$ obtained from a sky coverage of $\sim 9,380{deg}^{2}$. The catalog
consists of quasars that have reliable redshifts and have at least one
emission line with FWHM greater than $1,000$ km s-1. The flux limit for the
main spectroscopic sample is $i<19.1$. Therefore, the majority of quasars are
brighter than $i\approx 19$. We have chosen to use DR7 because it includes
line fits with multiple Gaussian functions for the broad H$\beta$ lines along
with compilation of equivalent widths, line luminosities, and iron emission
strengths with high accuracy.

We note that we have checked the SDSS DR10 (Pâris et al., 2014) and DR14
(Pâris, Isabelle et al., 2018) quasar samples. SDSS DR10 samples do not have
H$\beta$ emission-line properties and SDSS DR14 has very few objects which
satisfy our magnitude ($i<19.1$), redshift ($z$ $<$ 0.75) and FWHM ($>$ 15000
km$s^{-1}$) cuts. Since we are looking for H$\beta$ emission line objects,
this redshift limit is necessary. Thus DR7 allows us to construct a larger HBL
catalog when compared with DR10 and DR14, making the sample ideal for our
analysis.

---
Figure 3: Left Panel: The blue solid triangles represent the RL quasars in our
HBL sample on the radio luminosity versus optical continuum luminosity plane.
Right Panel: The blue solid triangles represent the same sources on the radio-
loudness versus black hole mass plane.

###  2.2 Faint Images of the Radio Sky at Twenty-cm (FIRST)

Faint Images of the Radio Sky at Twenty-Centimetres (FIRST) is a radio survey
(White et al., 2007) that scanned over 10,000 square degrees of the North and
South Galactic Caps at 1.4 GHz. It produces images with $1.8^{\prime\prime}$
pixels with an rms of 0.15 mJy bm-1, resolution $5^{\prime\prime}$ and a
threshold flux density of 1.0 mJy bm-1. FIRST was designed to overlap with the
SDSS survey with 40% identification rate for optical counterparts at SDSS
limiting magnitude of $\sim 23$.

Shen et al. (2011) use a specific criterion to cross-match the DR7 quasars
with the FIRST survey. To include radio properties, following Jiang et al.
(2007) $R=f_{6cm}/f_{2500}$ has been estimated, where $f_{6cm}$ and $f_{2500}$
are the rest-frame flux densities at 6 cm and 2500Å, respectively. SDSS
sources which have single counterparts within $5^{\prime\prime}$ (core
dominated), or have multiple counterparts within $30^{\prime\prime}$ (lobe
dominated) in FIRST are classified as RL quasars (Shen et al., 2011). Out of
all 105,783 quasars in SDSS, 99,182 have a FIRST match. Among those, 9,399 are
identified as RL and 88,979 as RQ quasars. We have taken a redshift ($z$ $<$
0.75) limited sample to consider only the quasars which have broad H$\beta$
emission lines in their spectra (Richards & Kratzer, 2014). Our final sample
contains 298 RL and 1,910 RQ quasars. Among RL quasars, 56 have HBL (FWHM $>$
15000 km$s^{-1}$) while the corresponding number in RQ quasars is 41. We note
that the redshift distributions of RL and RQ quasars in our sample are similar
(Chakraborty & Bhattacharjee, 2021, CB21 hereafter).

---
Figure 4: Top Left Panel:Spectrum of a RL quasar with H$\beta$ FWHM greater
than 15,000 km $s^{-1}$. $F_{\lambda}$ is in the units of $10^{-17}$ erg cm-2
s-1 Å-1. Top Right Panel:Same as the top left panel but for a RQ quasar.
Bottom Left Panel:Combined spectra of RL and RQ quasars with H$\beta$ FWHM
greater than 15000 km $s^{-1}$. $F_{\lambda}$ is in the units of
$10^{-17}erg/cm^{2}/s/$Å. Bottom Right Panel: Normalised combined spectra of
RL and RQ quasars with H$\beta$ FWHM greater than 15,000 km $s^{-1}$, where
$F_{\lambda}$ is in the units of $10^{-16}$ erg cm-2 s-1 Å-1. From the
combined spectra we can see that the RL quasars have higher optical continuum
flux and stronger [O III] line. See section §3.2.

##  3 Results

Here we present our results for the HBL as well as the entire H$\beta$ broad
emission line sample of Shen et al. (2011) for RL and RQ quasars.

###  3.1 Radio Loud Fraction (RLF) and the HBL Sample

Figure 1 shows a plot of RLF versus FWHM of broad H$\beta$ emission line of
SDSS quasars. It clearly exhibits an increasing trend of RLF with the RLF
reaching $\sim$57% in the highest FWHM bin. We note that we see a similar
trend when we use the MgII sample (Chakraborty et al., 2021). Additionally we
found the correlation coefficient (Figure 1) to be 0.97 with a $p$-value
0.0002, implying only a 0.02% chance of finding such high value of the
correlation coefficient by chance if the data were not correlated. While
relative uncertainties in RLF are large in the three bins containing FWHM
$>10^{4}$ km s-1 it is evident that the RLF value for the highest FWHM bin is
1-$\sigma$ above that containing FWHM value between $10^{4}$ and $1.25\times
10^{4}$ km s-1. That indicates the increase in RLF value at higher FWHM is
significant. To investigate this phenomenon in more detail, particularly at
the very high end of the H$\beta$ FWHM values, we analyse the properties of
the HBL RL and RQ quasars.

In CB21, we showed that the distributions of the fiducial virial BH mass,
optical continuum luminosity, and the bolometric luminosity of our HBL sample
are slightly different from the parent catalog of Shen et al. (2011). CB21
noted that the HBL RL quasars have higher optical continuum luminosity at 5100
Å in the rest frame of the quasars as well as higher bolometric luminosities,
compared to the RQ ones, while in the parent sample there is no difference
between the luminosities and BH mass between RL and RQ quasars. There is a
possibility that the optical continuum luminosity may be contaminated by light
from the host galaxy. CB21 used a correction provided by Shen et al. (2011),
who utilized an empirical fitting formula of the average host contamination by
stacking spectra of quasars for that purpose.

In Figure 2 top panel, we show the distribution of BH mass for the RL and RQ
quasars in our HBL sample. While computing the BH mass, unlike CB21, we
include a correction due to $R_{Fe}$ (Du & Wang, 2019). The $R_{Fe}$ corrected
mass is a more robust and accurate estimate of mass compared to the ones
obtained in DR7. The following equation defines the modified BH mass.

${\rm M=f_{BLR}\frac{\Delta V^{2}R_{BLR}}{G}}$

Where $f_{BLR}$ is the virial factor related to the geometry and kinematics of
the BLR (e.g., Onken et al., 2004; Ho & Kim, 2014; Woo et al., 2015) which is
0.4 for FWHM$\geq$ 10,000 km$s^{-1}$, $\Delta V$ is the FWHM and RBLR is the
size of the BLR region, which is RHβ in our case. The radius of the H$\beta$
emitting region in light-days can be written as

$\rm{Log({R}_{H\beta})=\alpha+\beta Logl_{44}+\gamma R_{Fe}},$

Where $\alpha$ = 1.65 $\pm$ 0.06, $\beta$ = 0.45 $\pm$ 0.03, $\gamma$ = - 0.35
$\pm$ 0.08 and $\rm{l_{44}=L_{5100\AA}/10^{44}}$ erg/s (Du & Wang, 2019).

Our results with the $R_{Fe}$ correction do not show any significant
difference in the distribution of BH mass between the HBL RL and RQ quasar
samples (we find a $p$-value for Wilcoxon rank sum test 0.0726, which implies
that the null hypothesis that the two distributions are different may be
rejected at the 95% confidence level). We compute the Eddington ratios (Figure
2 middle panel) of our HBL sample, using our estimates of the $R_{Fe}$
corrected BH mass and bolometric luminosity. From the figure we see that there
is almost no significant difference between mean values of the Eddington
ratios of RL and RQ quasars ($p$-value 0.2317, which again implies that the
null hypothesis that the two distributions are different may be rejected).

---
Figure 5: Top panel: Red filled circles and blue filled triangles denote the
RQ and RL sources in our HBL sample respectively on the bolometric luminosity
versus H$\beta$ FWHM plane. Middle panel:  Same as the top panel but shows the
variation of the BH mass versus H$\beta$ FWHM. We note that our black hole
masses are computed using the $R_{Fe}$ correction. Bottom panel Same as the
other two panels but on the BH mass versus bolometric luminosity plane. The
related correlation matrices are shown in Tables 1 and 2

It is known that, in a torus outside the BLR, the continuum and line emission
are further modified by dust. UV and optical light are absorbed and scattered
by dust and part of the energy is re-emitted at infrared wavelengths, causing
anisotropic obscuration. This obscuration is often quantified by the covering
fraction (CF). We estimate the covering fraction from the ratio of the
integrated flux at the mid-IR (MIR) to optical wavelengths, which is
quantified as the fraction of the optical flux coming from the accretion disk,
that gets reprocessed into the infrared by the dusty torus (e.g., Netzer,
2019). We have taken $r-W1$ as the MIR flux, where $r$ is the SDSS $r$-band
flux, corrected for galactic extinction and $W1$ is the $k$-corrected
normalised (at $z=2$) flux at the WISE W1 band (Shen & Ho, 2014). The
distributions of CF are shown in the bottom panel of Figure 2. For the optical
flux, $k-$corrected normalised (at $z=2$) optical continuum flux has been used
in our analysis.

For our HBL sample we see that there is no difference in the mean values of CF
distribution of RL and RQ quasars ($p$-value 0.6890 with usual significance).
In the local universe the number ratios of obscured to unobscured active
galactic nuclei (AGNs) provide useful constraints on the average covering
fraction of the torus (Reyes et al., 2008; Lu et al., 2010; Ma & Wang, 2013).
If, for instance, RL quasars have smaller CF than RQ, we will be observing
them at larger angles. In that case the FWHM of RL quasars may be higher due
to the differences in angular distributions. Our analysis of the CF reveals
that for our HBL sample, orientation is not a critical factor.

###  3.2 Comparison of Properties of the RL and RQ quasars

We have shown the dependencies of the different properties of RL quasars in
the HBL sample in the left and right panels of Figure 3. The variables we use
are, $L_{\rm rad}$, $L_{5100\AA}$, BH mass and $R$, which imply radio
luminosity at 6 cm, optical continuum luminosity at $5100$ Å, black hole mass
and radio loudness (ratio of radio luminosity to optical continuum
luminosity), respectively. Errors associated with $L_{\rm rad}$ and $R$ are
$\sim$ 1% and BH mass are $\sim$ 35% to 42%. Optical luminosity errors are
quoted from Shen et al. (2011) and BH mass, $L_{\rm rad}$ and $R$ errors are
calculated accordingly. We note that RQ quasars may generate weak but non-
negligible radio emission through nuclear as well as star formation processes.
(e.g., Kimball et al., 2011; Heckman & Best, 2014; Zakamska et al., 2016;
Symeonidis, 2017). They may contain low-power jets although it may not
contribute significantly to the overall energy budget (e.g., Harrison et al.,
2014; Jarvis et al., 2019). However, we follow Richards & Kratzer (2014), who
use the FIRST completeness limit. To maintain the limiting radio luminosity,
only the RL quasars have been considered in our analysis.

For our HBL sample we see a correlation between $L_{rad}$ and L5100Å such that
a linear regression relation may be obtained given by $L_{rad}=(1.36\pm 0.30)$
L${}_{5100\AA}-(28.89\pm 13.58)$ with a correlation coefficient of 0.62.
However, we note that if a pair of luminosities are correlated that could be
driven by their dependency on redshift. To test the reliability of the
correlations obtained, we calculate partial correlation coefficient (Akritas &
Siebert, 1996) to check the dependence of the radio luminosity and optical
continuum luminosity at $5100$ Å on redshift. Pearson’s partial correlation
coefficient (PPCC) eliminates the common dependence on a third variable and
measures the correlation between two quantities. We obtain PPCC $=0.26$ for
radio and optical continuum luminosity, which is significantly lower than the
correlation coefficient between them implying the correlation to be much
weaker when the redshift dependency is subtracted. On the other hand, we do
not find any significant dependence of $R$ on the BH mass. We have verified
that for a given continuum luminosity the radio luminosity is higher for the
HBL quasars compared to the non-HBL ones.

We compute the composite spectra of RL and RQ quasars of our HBL sample. We
first obtain the individual spectra from SDSS (Figure 4, upper panels), de-
redshifted them and combine the rest frame RL and RQ spectra separately. We
compare them on the bottom left panel of Figure 4. We find that the average
continuum flux is higher for RL quasars. From the normalised composite spectra
(Figure 4, bottom right) we observe that one of the major differences between
the combined spectra of RL and RQ quasars is in the strength of the [O III]
narrow line (equivalent width for RL is $42.9\pm 0.1$ and for RQ is $41.7\pm
0.2$) while the H$\beta$ broad lines are identical for both the samples. So
the difference 1.2 and error associated with the difference is 22% (less than
1-$\sigma$). We checked that these effects are physical effects and not driven
by the most massive sources.

Table 1: Correlation Matrix for RL quasars of our HBL Sample | $L_{Bol}$ | BH mass | FWHM
---|---|---|---
$L_{Bol}$ | 1.0000 | 0.6855 | 0.0829
BH mass | 0.6855 | 1.0000 | 0.7757
FWHM | 0.0829 | 0.7757 | 1.0000
Table 2: Correlation Matrix for RQ quasars of our HBL Sample | $L_{Bol}$ | BH mass | FWHM
---|---|---|---
$L_{Bol}$ | 1.0000 | 0.5352 | -0.1464
BH mass | 0.5352 | 1.0000 | 0.7486
FWHM | -0.1464 | 0.7486 | 1.0000

###  3.3 Correlation of Various Properties of the HBL Quasars

We perform correlation studies between bolometric luminosity, FWHM, and BH
mass of our HBL sample where the errors associated with bolometric luminosity
and FWHM are $\sim$ 1% and $\sim$ 20% respectively (Shen et al., 2011). The
results are summarized in Tables 1 and 2. In our sample we observe, as shown
in Figure 5, that bolometric luminosity is not correlated with FWHM ($p$-value
for RL and RQ are 0.15 and 0.39 respectively) but BH mass is correlated with
FWHM ($p$-value 0.0001 for both RL and RQ) as well as bolometric luminosity
($p$-value for RL and RQ are 0.0001 and 0.0004). Both FWHM and bolometric
luminosity increase with BH mass for both our RL and RQ quasar samples. We
have found that the four most massive and the most luminous sources (upper
right corner : Figure 5, bottom panel) are RL quasars. As evident from the
tables, the correlation between bolometric luminosity and BH mass is slightly
stronger in RL quasars compared to the RQ ones.

---
Figure 6: Top Panel: Yellow filled circles and green filled triangles show the
variation of Eddington ratio with BH mass of quasars with H$\beta$ FWHM
greater and less than 15,000 km $s^{-1}$, i.e., non-HBL and HBL, respectively.
Middle Panel: Same as the top panel but here the horizontal axis denotes the
optical continuum luminosity. Bottom Panel:  Same as the above two panels but
shows the variation of H$\beta$ FWHM versus $R_{Fe}$. As we know that the
Eddington ratio is inversely related to the BH mass, we note that our HBL
sample contains the quasars with the highest BH mass and the lowest Eddington
ratios. In addition, it demonstrates that the bolometric luminosity is not
correlated with the Eddington ratio for the total sample. Furthermore, we can
see $R_{Fe}$ is inversely related to FWHM for the total sample and the quasars
in our HBL sample has the lowest $R_{Fe}$ values. See §3 and 4 for more
discussions.

###  3.4 Comparison of the HBL and Non-HBL Sample

We compare our HBL sample with quasars from the non-HBL sample (Figure 6). The
top, middle, and bottom panels show the distributions of mass versus Eddington
ratio, Eddington ratio versus optical continuum luminosity, and the FWHM
versus $R_{Fe}$, relations respectively, of both the samples of interest.
Errors associated with Eddington ratio and $R_{Fe}$ are $\sim$ 40% and $\sim$
1.5% respectively. We find that quasars in our HBL sample has the highest BH
mass and lowest Eddington ratios and optical continuum luminosities compared
to the non-HBL sample. We also observe that our HBL sample has the lowest
$R_{Fe}$. For the entire sample we found $R_{Fe}$ to be decreasing with FWHM
consistent with previous studies (Zamfir et al., 2008; Shen & Ho, 2014).

##  4 Summary and Discussion

In Figure 1, we show that the radio loud fraction increases with the FWHM of
broad H$\beta$ emission line for z $<0.75$ quasars identified in the SDSS. To
further investigate this effect, we selected a subsample of extreme objects
(FWHM $>$ 15,000 km $s^{-1}$) to study their properties in detail. For our
sample, number of RL quasars is 56 among a total of 97, which implies an RLF
of 57%, much larger than that of the larger sample of all SDSS quasars matched
with FIRST.On the other hand, the BH mass of the quasars in our subsample is,
on average, higher than that of the non-HBL SDSS quasars while the optical
continuum luminosities are lower on average. Consequently, the Eddington ratio
of the quasars in our subsample $\sim 10^{-2}-10^{-1}$, is significantly lower
than the average of that of the non-HBL quasars. Thus, for equivalent BH mass
range, quasars in our sample tend to have lower luminosity and hence a lower
accretion rate.

It has been observed in stellar mass BH accreting systems that the X-ray
spectrum becomes harder when the luminosity goes below a threshold, the value
of which is in the range $0.01-0.1$ $L_{edd}$, where $L_{edd}$ is the
Eddington luminosity. It has been thoroughly discussed in the literature that
this hard state may be caused by a switch of the nature of the accretion flow
from the standard geometrically thin optically thick radiatively efficient
disk to a geometrically thick optically thin advection-dominated accretion
flow (ADAF) (Narayan & Yi, 1994, 1995; Abramowicz et al., 1995; Kato et al.,
1997; Narayan, 2005). In the ADAF, unlike a standard disk, heat produced in
the accretion through viscous dissipation is not emitted. Instead the energy
is confined to the accreted material and moves toward the BH along with the
flow. As a result the accretion luminosity is low (Narayan & Yi, 1995;
Nakamura et al., 1997; Narayan, 2005). ADAF has been used to explain
observations of various classes of AGN as well. ADAF has the additional
property that the accreted material is not strongly bound to the BH. Hence,
there is a higher probability of producing disk wind and jet in AGN containing
an ADAF (Meier, 2001; Narayan, 2005). Although our findings show some
characteristics of ADAF, we note that ADAF models are unlikely to produce the
strong optical emission observed in quasars. Studies of jet systems led to
magnetically arrested accretion (e.g., Tchekhovskoy et al., 2011). We refer
the reader to Hardcastle & Croston (2020) for discussion on accretion
processes in accreting systems with radio jets. We propose that our subsample
of quasars with high FWHM of emission lines may contain magnetically arrested
accrection flows, which is consistent with having a very large fraction being
radio-loud.

In our subsample of high broad line quasars, there is no significant
difference in mass among the two classes, namely, radio-loud and radio-quiet.
However, the RL quasars have larger bolometric luminosity than the RQ ones as
computed by Shen et al. (2011). Furthermore, we computed the combined spectrum
of SDSS quasars with FWHM $>$ 15,000 km $s^{-1}$. It shows the mean optical
continuum luminosity to be higher for RL quasars, in comparison to RQ quasars.
Given that the difference in the bolometric or optical continuum luminosity is
mainly due to that of the accretion disk, the above implies some connection
between a stronger jet and a brighter disk. Accretion disc-jet connection in
AGN has often been studied using X-ray and radio observations (e.g., Naik &
Rao, 2000; Marscher et al., 2002; Chatterjee et al., 2009, 2011; Chatterjee et
al., 2018). In AGN, X-rays are not produced in the disk itself but in a
distribution of energetic particles located near the disk termed corona, which
is assumed to be energetically and dynamically connected to the disk. On the
other hand, optical emission is directly produced in the disk and hence may be
better suitable for probing its connection to the jet. Inoue et al. (2017)
have found a strong correlation between the disc and the jet, using bolometric
luminosity (as inferred from the optical luminosity of the disc) and 1.4 GHz
radio luminosity as an indicator of the jet emission. We note that our
findings are consistent with this result although our sample size is not very
large.

We found that the emission line properties of the composite spectra of the RL
and RQ quasars of our HBL sample are similar. The major difference between the
two classes has been observed in the strength of the [O III] narrow line. The
composite spectrum of the HBL RL quasars exhibits a stronger [O III] line
compared to that of the RQ ones. This is consistent with previous findings
that RL quasars have a larger narrow line region and stronger narrow emission
lines than their RQ counterparts (e.g., Heckman et al., 1991; Brotherton et
al., 2001). This may be caused by the interaction of the radio jet with the
narrow line gas, as inferred from observations indicating an alignment between
the radio jet axis and the narrow emission line region (e.g., Cimatti et al.,
1993). On the other hand, Schulze et al. (2017) have shown an enhancement in
[O III] line strength by a factor of at least 1.5 in a radio-loud compared to
a radio-quiet sample, both of which have similar redshifts, black hole mass,
and accretion rate as indicated by the optical continuum luminosity. They
conclude that the enhanced [O III] line strength is due to higher extreme UV
luminosity at 35 eV for the RL objects. They suggest this is caused by the
radio-loud quasar population having systematically higher black hole spin and
consequently their radiative efficiency being larger, consistent with the
paradigm (Wilson & Colbert, 1995; Sikora et al., 2007; Tchekhovskoy et al.,
2010) that the radio loud objects have higher black hole spin.

In this work we employ a sample of high broad line quasars and search for
hints in the observed higher radio loud fraction in these objects compared to
the general population of quasars. Our study suggests that a magentically
arrested flow like accretion and a stronger connection between the disc and
the jet may result in this higher radio function. We propose that this might
provide a clue to the broader question of understanding radio dichotomy in the
quasar population.

## Acknowledgement

AC thanks Jaya Maithil from the University of Wyoming, Swamtrupta Panda from
the Laboratório Nacional de Astrofísica - MCTIC and Preeti Kharb of National
Centre for Radio Astrophysics, for their valuable inputs. The authors thank
the referee for several important feedback which greatly helped in improving
the draft.

SC acknowledges support from the Department of Science and Technology through
the SERB-ECR grant, SERB-CRG grant and from the Department of Atomic Energy
for the BRNS grant. RC and SC are grateful to the Inter University Center for
Astronomy and Astrophysics (IUCAA) for providing infrastructural and financial
support along with local hospitality through the IUCAA-Associateship program.
RC thanks Presidency University for support under the Faculty Research and
Professional Development (FRPDF) Grant, and ISRO for support under the
AstroSat archival data utilization program. RC acknowledges financial support
from BRNS through a project grant (sanction no: 57/14/10/2019-BRNS) and thanks
the project coordinator Pratik Majumdar for support regarding the BRNS
project.

## Data availability

The data underlying this article will be shared on reasonable request to the
corresponding author.

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