# An extensive study of blazar broad emission line: ‘Changing-look’ blazars
and ‘Baldwin effect’

Hubing Xiao Shanghai Key Lab for Astrophysics, Shanghai Normal University,
Shanghai, 200234, China  Junhui Fan  Center for Astrophysics, Guangzhou
University, Guangzhou 510006, China  Key Laboratory for Astronomical
Observation and Technology of Guangzhou, Guangzhou, 510006, China  Astronomy
Science and Technology Research Laboratory of Department of Education of
Guangdong Province,
Guangzhou, 510006, China  Zhihao Ouyang  School of Physics and Materials
Science, Guangzhou University
Guangzhou, 510006, China  Liangjun Hu  Wuhu No.27 Middle School, Wuhu, 241001,
China  Guohai Chen  Center for Astrophysics, Guangzhou University, Guangzhou
510006, China  Key Laboratory for Astronomical Observation and Technology of
Guangzhou, Guangzhou, 510006, China  Astronomy Science and Technology Research
Laboratory of Department of Education of Guangdong Province,
Guangzhou, 510006, China  Liping Fu  Shanghai Key Lab for Astrophysics,
Shanghai Normal University, Shanghai, 200234, China  Shaohua Zhang  Shanghai
Key Lab for Astrophysics, Shanghai Normal University, Shanghai, 200234, China

###### Abstract

It is known that the blazar jet emissions are dominated by non-thermal
radiation while the accretion disk jets are normally dominated by thermal
emission. In this work, our aim is to study the connection between the two
types of emission by investigating the correlation between the blazar emission
line intensity property, which embodies the nature of accretion disk, and the
$\gamma$-ray flux property, which is the representative of jet emission. We
compiled a sample of 656 blazars with available emission line equivalent
widths ($EW$), the GeV $\gamma$-ray flux, and the SED information from the
literature. In this work, we found 55 previous BCUs are now identified as
FSRQs, and found 52 ‘Changing-look’ blazars based on their $EW$ and 45 of them
are newly confirmed. These ‘Changing-look’ blazars have a larger accretion
ratio (${\dot{M}}/{\dot{M}}_{\rm Edd}$) than BL Lac objects. In addition, we
suggest that the lower synchrotron peak blazars (LSPs) could be the source of
‘Changing-look’ blazars because 90.7% of the ‘Changing-look’ blazars in this
work are confirmed as LSPs. An anti-correlation between $EW$ and continuum
intensity, the so-called global ‘Baldwin effect’ (BEff) has been confirmed. We
suggest the steeper global BEff observed for blazar than for radio-quiet
active galactic nuclei (RQ-AGNs) is caused by the inverse Compton scattering
of broad-emission-line photons. This interpretation is further supported by
the positive correlation between the emission line $EW$ and intrinsic inverse
Compton luminosity.

##  1 Introduction

Blazars, one of the most extreme subclasses of active galactic nuclei (AGNs),
show extreme observational properties, such as rapid and strong multi-
wavelength variability, high and variable polarization, strong and variable
$\gamma$-ray emissions and apparent superluminal motion at radio frequencies
(Wills et al., 1992; Urry & Padovani, 1995; Fan, 2002; Fan et al., 2014, 2021;
Villata et al., 2006; Xiao et al., 2015; Gupta et al., 2016; Hagen-Thorn et
al., 2018; Lister et al., 2018; Xiao et al., 2019; Abdollahi et al., 2020).
These observational properties are characteristic of the relativistic jet
which points toward the observer. Blazar emit radiation, which is non-thermal
dominated, across the entire electromagnetic spectrum. A typical blazar
broadband spectral energy distribution (SED) displays a two-bump structure.
The lower energy bump peaks in the range of infrared to X-ray and is
attributed to the synchrotron radiation of relativistic electrons, while the
origin of the higher energy bump which peaks at X-ray to $\gamma$-ray
wavelengths is under debate. There are two models for the higher-energy bump,
the leptonic model states that the higher energy bump is attributed to the
inverse Compton (IC) scattering (Blandford & Königl, 1979; Sikora et al.,
1994; Sokolov & Marscher, 2005; H. E. S. S. Collaboration et al., 2015; Zheng
& Yang, 2016; Tan et al., 2020), while the hadronic model interprets that the
higher-energy bump is attributed to the proton synchrotron radiation and
secondary particle cascade (Mücke & Protheroe, 2001; Dimitrakoudis et al.,
2012; Zheng & Kang, 2013; Diltz et al., 2015; IceCube Collaboration et al.,
2018; Xue et al., 2021).

Both the optical spectrum and SED are used to make classifications for
blazars. Historically, blazars are divided into two classes, namely flat
spectrum radio quasars (FSRQs) and BL Lacertae objects (BL Lacs), according to
their optical spectra. The former is characterized by strong emission lines
with the rest-frame equivalent width ($EW$) of the strongest emission line
greater than $5\,\mathring{\rm A}$, while the latter one shows featureless
optical spectrum or weak emission lines with the $EW$ of the strongest
emission line less than $5\,\mathring{\rm A}$ (Stocke et al., 1990, 1991;
Stickel et al., 1991; Urry & Padovani, 1995; Scarpa & Falomo, 1997). The
separation value of $5\,\mathring{\rm A}$ was deduced from an examination of
the line strength of a representative sample of FSRQs from the Parkes
catalogue (Wilkes, 1986), using the same limit in the selection of the X-ray
selected BL Lacs (XBLs) from the Einstein Extended Medium Sensitivity Survey
(EMSS) (Stocke et al., 1990). This separation value was also applied in
Stickel et al. (1991) to select BL Lacs from $1\,{\rm Jy}$ catalogue of radio
sources (Kühr et al., 1981), and 34 BL Lacs were obtained. It is clear that
the separation value of $5\,\mathring{\rm A}$ is rather arbitrarily settled. A
Doppler boosted non-thermal continuum could swamp out spectral emission lines
(Blandford & Rees, 1978; Xiong & Zhang, 2014), and $EW$ greater than
$5\,\mathring{\rm A}$ may be the result of a particular low-state of jet
activity.

The classification types, which based on the optical spectrum of AGNs and that
of blazars were harmonious until the discovery of ‘Changing-look’ AGNs (Matt
et al., 2003; Bianchi et al., 2005). The shifting between Type I AGNs and Type
II AGNs, or the shifting between FSRQs and BL Lacs has brought great
challenges to the AGN unification model (Marchese et al., 2012; Shappee et
al., 2014; Isler et al., 2013; Mishra et al., 2021; Peña-Herazo et al., 2021).
There are explanations for the shifting, among them the sudden change of
accretion ratio arising the shift seems promising.

The SED features, e.g. synchrotron peak frequency ($\nu_{\rm s}$) and Compton
dominance are employed to make classifications for blazars in previous works.
A parabolic function is widely used to describe the blazar SEDs in the diagram
of ${\rm log}\,\nu F_{\nu}-{\rm log}\,\nu$ since it was proposed in Landau et
al. (1986). Nieppola et al. (2006) fitted SEDs in the form of ${\rm log}\,\nu
F_{\nu}-{\rm log}\,\nu$ with parabolic function for 308 blazars and classified
BL Lacs into low synchrotron peak BL Lacs (LBLs, ${\rm log}\,\nu_{\rm
s}<14.5$), intermediate synchrotron peak BL Lacs (IBLs, $14.5<{\rm
log}\,\nu_{\rm s}<16.5$), and high synchrotron peak BL Lacs (HBLs, ${\rm
log}\,\nu_{\rm s}>16.5$) based on the synchrotron peak frequency. Similarly,
Fan et al. (2016) calculated SEDs by fitting the multi-wavelength data with
parabolic function for a larger sample of 1392 blazars and classified blazars
into low synchrotron peak sources (LSPs, ${\rm log}\,\nu_{\rm s}\leq 14.0$),
intermediate synchrotron peak sources (ISPs, $14.0<{\rm log}\,\nu_{\rm s}\leq
15.3$), and high synchrotron peak sources (HSPs, ${\rm log}\,\nu_{\rm
s}>15.3$). Recently, Paliya et al. (2021) proposed that Compton dominance (CD)
can be considered as one such parameter to reveal the physics of the non-
thermal jets in beamed AGNs, suggesting broad-emission-line blazars being more
Compton-dominated sources, and separated blazars as HCD (high ${\rm CD}>1$)
and LCD (low ${\rm CD}<1$).

It is no doubt that the last decade has been a golden age for blazar research
in high energy bands due to the all-sky survey carried out by the Large Area
Telescope (Fermi-LAT, Atwood et al. 2009). The fourth catalogue of active
galactic nuclei (AGNs) detected by the Fermi-LAT (4LAC, Ajello et al. 2020)
between 2008 August 4 and 2016 August 2 contains 3511 sources in 4LAC_DR2,
among them 3437 blazars are included. The blazar $\gamma$-ray emission from a
non-thermal mechanism dominates the entire electromagnetic radiation
(Ghisellini et al., 2011, 2014; Xiong & Zhang, 2014).

In this work, we investigate the optical emission line strength and
$\gamma$-ray intensity (including the inverse Compton intensity) to study the
thermal emission and its connection with the non-thermal emission in blazars.
In section 2, we define our sample, and our results will be presented in
section 3. The discussions will be given in section 4. Section 5 presents our
conclusions.

##  2 Sample

We collect blazar emission-line profiles from Paliya et al. (2021), in which
674 Fermi sources are included. According to 4LAC_DR2 (Ajello et al., 2020),
17 out of 674 blazars in Paliya et al. (2021) are not considered as blazars.
Besides, 4FGL J0014.1+1910 is excluded because it shows a noisy spectrum and
gives no $EW$ of emission lines. At last, we have a sample of 656 Fermi
blazars with emission line $EW$ and redshift from Paliya et al. (2021),
$\gamma$-ray intensity and SED features from 4LAC_DR2 (Ajello et al., 2020).
We list our sample and the parameters in Table 1. Among the 656 sources, 55
sources are classified as BCUs (blazar candidates of uncertain type), 51
sources are classified as BL Lacs, and 550 sources are classified as FSRQs
according to the classification types in 4LAC_DR2.

##  3 Results

###  3.1 The blazars classification

The redshift distributes from 0.027 to 4.314 with a mean value of $1.163\pm
0.665$ for the blazars in our sample. There are 47 sources with detected ${\rm
H{\alpha}}$ emission line, the average equivalent width of ${\rm H{\alpha}}$
is $\langle EW_{\rm H{\alpha}}\,(\mathring{\rm A})\rangle=258.53\pm 59.21$;
160 sources with detected $\rm H{\beta}$ emission line, and the $\langle
EW_{\rm H{\beta}}\,(\mathring{\rm A})\rangle=121.82\pm 13.61$; 482 sources
with detected MgII emission line, and the $\langle EW_{\rm
MgII}\,(\mathring{\rm A})\rangle=79.72\pm 9.50$; 193 sources with detected CIV
emission line, and the $\langle EW_{\rm CIV}\,(\mathring{\rm
A})\rangle=106.63\pm 9.41$;

We are able to determine the blazar type for these 55 BCUs based on their
optical spectra. All of the 55 BCUs show $EW$ of emission lines greater than 5
$\mathring{\rm A}$, thus we suggest these sources are FSRQs and list them in
Table 2. Based on the classic blazar type criterion of 5 $\mathring{\rm A}$,
we find 52 blazars that show type change, denote as ‘Changing-look’ blazars,
including 10 FSRQs change their types to BL Lacs (‘F$\to$B’) and 42 BL Lacs
change their types to FSRQs (‘B$\to$F’), and list them in Table 3.

###  3.2 Correlations between equivalent width and continuum luminosity

We collect the continuum luminosity ($L_{\rm\lambda}$) at $5100\,\mathring{\rm
A}$, at $3000\,\mathring{\rm A}$, and at $1350\,\mathring{\rm A}$ from Paliya
et al. (2021), in which these continuum luminosities were calculated via
empirical relations with emission line luminosities (Shen et al., 2011; Shaw
et al., 2012):

${\rm log}\,L_{\rm 3000}=(1.016\pm 0.003)\,{\rm log}\,L_{\rm MgII}+(1.22\pm
0.11)$ (1) ${\rm log}\,L_{\rm 1350}=(0.863\pm 0.009)\,{\rm log}\,L_{\rm
CIV}+(7.66\pm 0.41)$ (2) ${\rm log}\,L_{\rm 5100}=(0.802\pm 0.049)\,{\rm
log}\,L_{\rm H\beta}+(1.574\pm 0.060)$ (3)

Figure 1 shows the correlations between the emission line $EW$ and the line
continuum luminosity. The linear regression results are illustrated in Table
4, where the linear correlation is expressed as $y=(a\pm\Delta a)x+(b\pm\Delta
b)$, $N$ is the size of the considered sample, $r$ is a correlation
coefficient, and $p$ is a chance probability.

###  3.3 Correlations between equivalent width and GeV $\gamma$-ray parameters

Assuming the GeV $\gamma$-ray photons follow a power law function and is
expressed as

${\frac{dN}{dE}}=N_{\rm 0}E^{-\alpha_{\rm ph}},$ (4)

where $\alpha_{\rm{ph}}$ is the photon spectral index, and $N_{\rm 0}$ can be
expressed as $N_{\rm 0}=N_{(E_{\rm L}\sim E_{\rm U})}({\frac{1}{E_{\rm
L}}-\frac{1}{E_{\rm U}}}),$ if $\alpha_{\rm ph}=2$, otherwise $N_{\rm
0}=\frac{N_{(E_{\rm L}\sim E_{\rm U})}(1-\alpha_{\rm ph})}{(E_{\rm
U}^{1-\alpha_{\rm ph}}-E_{\rm L}^{1-\alpha_{\rm ph}})},$ where $N_{(E_{\rm
L}\sim E_{\rm U})}$ is the integral photons in units of ${\rm photons\cdot
cm^{-2}\cdot s^{-1}}$ in the energy range of $E_{\rm L}$ \- $E_{\rm U}$, where
$E_{\rm L}$ and $E_{\rm U}$ correspond to 1 GeV and 100 GeV respectively. The
integral flux, $F$, in units of ${\rm GeV\cdot cm^{-2}\cdot s^{-1}}$, can be
expressed in the form (Fan et al., 2013; Xiao et al., 2015)

$F=N_{(E_{\rm L}\sim E_{\rm U})}{\frac{E_{\rm U}\times E_{\rm L}}{E_{\rm
U}-E_{\rm L}}}\ln\frac{E_{\rm U}}{E_{\rm L}}$ (5)

for $\alpha_{\rm ph}=2$, otherwise

$F=N_{(E_{\rm L}\sim E_{\rm U})}{\frac{1-\alpha_{\rm ph}}{2-\alpha_{\rm
ph}}}{\frac{(E_{\rm U}^{2-\alpha_{\rm ph}}-E_{\rm L}^{2-\alpha_{\rm
ph}})}{(E_{\rm U}^{1-\alpha_{\rm ph}}-E_{\rm L}^{1-\alpha_{\rm ph}})}}.$ (6)

The $\gamma$-ray luminosity is calculated by

$L_{\rm\gamma}=4\pi d_{\rm L}^{2}(1+z)^{(\alpha_{\rm ph}-2)}F,$ (7)

where $d_{\rm L}=\frac{c}{H_{\rm 0}}\int^{1+z}_{1}\frac{1}{\sqrt{\Omega_{\rm
m}x^{3}+1-\Omega_{\rm m}}}dx$ is a luminosity distance and
$(1+z)^{(\alpha_{\rm ph}-2)}$ stands for a $K$-correction.

Figure 2 shows the correlations between the $EW$ of emission lines ($\rm
H{\alpha}$, $\rm H{\beta}$, $\rm Mg{II}$, and $\rm C{IV}$) and the GeV
$\gamma$-ray parameters ($\gamma$-ray photon spectral index $\alpha_{\rm ph}$
and $\gamma$-ray luminosity $L_{\gamma}$). The linear regression fitting
results are shown in Table 5.

##  4 Discussion

###  4.1 The completeness of this sample

In this work, we have compiled a sample of blazars with emission line features
from literature, these sources are all included in 4LAC_DR2. To evaluate the
completeness of our sample, we compare the redshift ($z$) and the $\gamma$-ray
luminosity ($L_{\rm\gamma}$) from our sample and from the whole Fermi blazar
sample. The redshift and $\gamma$-ray luminosity distributions for blazars in
our sample and for 1701 blazars with known redshift in 4LAC_DR2 are shown in
Figure 3. The sources in our sample (this work, TW) have larger redshift and
higher $\gamma$-ray luminosity, noting that most of the 4FGL_DR2 blazars with
$z<1$ and ${\rm log}\,L_{\rm\gamma}<46\ {\rm erg\cdot s^{-1}}$ are not
included in our sample. The redshift ranges from 0.000017 (4FGL J0654.0-4152)
to 4.313 (4FGL J1510.1+5702) with an average value of $\langle z^{\rm
4LAC}\rangle=0.80\pm 0.66$ for the 1701 4LAC_DR2 blazars; the redshift spans
from 0.027 (4FGL J2204.3+0438) to 4.314 (4FGL J1510.1+5702) with an average
value of $\langle z^{\rm TW}\rangle=1.16\pm 0.66$ for the 656 blazars in our
sample. The $\gamma$-ray luminosity ranges from 35.75 (4FGL J0654.0-4152) to
48.76 (4FGL J1833.6-2103) with an average value of $\langle{\rm
log}\,L_{\rm\gamma}^{\rm 4LAC}\rangle=45.49\pm 1.27$ for the 1701 4LAC_DR2
redshift-known blazars; the $\gamma$-ray luminosity ranges from 42.45 (4FGL
J2204.3+0438) to 48.60 (4FGL J1427.9-4206) with an average value of
$\langle{\rm log}\,L_{\rm\gamma}^{\rm TW}\rangle=46.22\pm 0.94$ for the 656
blazars in our sample. Anderson-Darling (A-D) test is applied to test if
$z^{\rm TW}$ and $z^{\rm 4LAC}$, ${\rm log}\,L_{\rm\gamma}^{\rm TW}$ and ${\rm
log}\,L_{\rm\gamma}^{\rm 4LAC}$ come from the same distributions. The A-D test
gives statistics 123.7 and 120.8 for redshift and $\gamma$-ray luminosity,
respectively. The values are both greater than the critical statistic of 6.5
for a significance level of 0.001 and this rejects the null hypothesis that
two distributions come from the same distribution. Thus, we can state that our
sample is not a good representative of the whole Fermi blazars, but a sample
of brighter and more distant Fermi blazars. This sample incompleteness is
caused by the selection criteria as we preferentially selected sources with
emission line features in the optical spectrum, leading us to select the
brighter and more distant Fermi blazars. We caution the readers that our
results are valid for this sample.

###  4.2 ‘Changing-look’ blazars

####  4.2.1 Comparing with ‘Changing-look’ blazars in previous works

AGNs are divided into ‘Type I’ and ‘Type II’ based on their optical spectra.
The former displays a blue continuum from an accretion disk and broad emission
lines created by photoionization, the latter shows only narrow lines and no
continuum variability (Khachikian & Weedman, 1974; Peterson et al., 2004). The
common understanding of these two categories is that the line of sight to the
central engine is unobscured for Type I AGNs and obscured for Type II AGNs
(Antonucci, 1993; Urry & Padovani, 1995). For the extreme AGNs, blazars are
usually grouped into FSRQs and BL Lacs based on the $EW$. However, these
standard unification pictures for the difference between these classes meet
challenges after the discovery of ‘Changing-look’ AGNs or blazars. The
shifting between Type I AGNs and Type II AGNs was observed and reported (Matt
et al., 2003; Bianchi et al., 2005; Marchese et al., 2012; Shappee et al.,
2014), and the shifting was also reported between FSRQs and BL Lacs (Isler et
al., 2013, 2015; Mishra et al., 2021).

There are works exploring ‘Changing-look’ blazars. Peña-Herazo et al. (2021)
carried out a sample of 26 ‘Changing-look’ blazars by searching the available
optical spectra in the Large Sky Area Multi-object Fiber Spectroscopic
Telescope (LAMOST) Data Release 5 (DR5) archive (Yao et al., 2019). Mishra et
al. (2021) presented multi-wavelength photometric and spectroscopic monitoring
observations of the blazar B2 1420+32, focusing on its outbursts in 2018–2020,
and suggested that this source had transitioned between BL Lac and FSRQ states
multiple times.

Cross-checking the sample in Peña-Herazo et al. (2021) and Mishra et al. 2021
with our sample, it is found that seven sources are confirmed as ‘Changing-
look’ blazars in common. Three of them, 4FGL J1001.1+2911 (5BZB J1001+2911),
4FGL J1402.6+1600 (5BZB J1402+1559), and 4FGL J1503.5+4759 (TXS 1501+481) are
indicated as ‘Changing-look’ blazars in both our work and in Peña-Herazo et
al. (2021). The rest of the four sources, 4FGL J1043.2+2408 (5BZQ J1043+2408),
4FGL J1106.0.2+2813 (5BZQ J1106+2812), 4FGL J1321.1+2216 (5BZQ J1321+2216),
and 4FGL J1422.3+3223 (B2 J1420+32) that are listed in Table 6, are classified
as ‘Changing-look’ blazars in Peña-Herazo et al. (2021) and Mishra et al.
(2021), but they are not classified as ‘Changing-look’ blazars in the present
work because of different time domains of observed spectra. 4FGL J1043.2+2408,
also known as 5BZQ J1043+2408, is contained in SDSS_DR16 (Ahumada et al.,
2020) with a spectrum taken in 2013 March (MJD 56358) and was considered as an
FSRQ due to a broad emission line of $\rm Mg{II}$; 4FGL J1106.0+2813, also
known as 5BZQ J1106+2812, was considered as an FSRQ in this work, Paliya et
al. (2021) and 4LAC_DR2 because its spectrum was taken from Shaw et al.
(2012), in which this source was classified as FSRQ; 4FGL J1321.1+2216, also
known as 5BZQ J1321+2216, is contained in SDSS_DR16 (Ahumada et al., 2020)
with a spectrum taken in 2012 May (MJD 56070), based on which we consider this
source to be an FSRQ as also noted as FSRQ in 4LAC_DR2. However, these three
sources show clear evidence of ‘Changing-look’, from FSRQs to BL Lacs, when
analyzing their latest spectra from LAMOST_DR5, in which the spectra were
taken in the duration of 2015 September to 2017 June. 4FGL J1422.3+3223, known
as B2 1420+32, was classified as an FSRQ by both Paliya et al. (2021) and
4LAC_DR2 based on the spectrum from SDSS_DR16, which contains the spectrum
data through 2018 August. The new classification of BL Lac (Mishra et al.,
2021) makes this source be a ‘Changing-look’ blazars based on the
spectroscopic study of outbursts in 2018–2020.

In total, we managed to obtain a sample of 56 ‘Changing-look’ blazars, in
which 52 (Table 3) are found in this work and 4 (Table 6) are collected from
Peña-Herazo et al. (2021) and Mishra et al. (2021). Among our 52 ‘Changing-
look’ blazars, there are 45 newly confirmed sources.

####  4.2.2 The accretion ratio of ‘Changing-look’ blazars

The ‘Changing-look’ AGNs/blazars were supposed to originate from obscuration
of the quasar core by dusty clouds moving in the torus, but this explanation
was basically dismissed because the expected high linear optical polarization
was not widely observed (Hutsemékers et al., 2019). An alternative explanation
is that the ‘Changing-look’ is arisen by a sudden change in accretion rate,
broad emission lines emerge when the accretion rate increase and broad
emission lines disappear when the accretion rate suddenly decreases.

The blazar classification has been studied in previous studies (Ghisellini et
al., 2011; Sbarrato et al., 2012; Xiong & Zhang, 2014), based on the
normalized BLR luminosity ($L_{\rm BLR}/L_{\rm Edd}$) and the normalized
$\gamma$-ray luminosity ($L_{\rm\gamma}/L_{\rm Edd}$, in Eddington units).
$L_{\rm BLR}=\xi L_{\rm Disk}$ and $L_{\rm Disk}=\eta\dot{M}c^{2}$, where
$\xi$ is photoionization coefficient, $\eta$ is energy accretion efficiency,
$\dot{M}$ is an accretion rate; $L_{\rm Edd}=\dot{M}_{\rm Edd}c^{2}$, where
$\dot{M}_{\rm Edd}$ is an Eddington accretion rate. Then one can get
$\frac{L_{\rm BLR}}{L_{\rm Edd}}=\xi\eta\frac{\dot{M}}{\dot{M}_{\rm Edd}}$ by
substituting $L_{\rm BLR}$ and $L_{\rm Edd}$. Ghisellini et al. (2011) assumed
$\dot{M}/\dot{M}_{\rm Edd}=0.1$, together with the assumed $\xi=\eta=0.1$, and
suggested a separation value of $L_{\rm BLR}/L_{\rm Edd}=1\times 10^{-3}$ to
separate FSRQs from BL Lacs, and FSRQs have higher $L_{\rm BLR}/L_{\rm Edd}$
than BL Lacs. Later, Sbarrato et al. (2012) suggested a separation value of
$5\times 10^{-4}$ by using $\dot{M}/\dot{M}_{\rm Edd}=0.05$. Xiao et al.
(2022) suggested that $\xi=\eta=0.1$ may not appropriate for the blazars in
our sample, and proposed the use of $\xi=0.11$ and $\eta=0.05$. In this case,
we can obtain $\left(L_{\rm BLR}/L_{\rm Edd}\right)^{\rm TW}=5.5\times
10^{-4}$ refer to $\dot{M}/\dot{M}_{\rm Edd}=0.1$, or $\left(L_{\rm
BLR}/L_{\rm Edd}\right)^{\rm TW}=2.8\times 10^{-4}$ refer to
$\dot{M}/\dot{M}_{\rm Edd}=0.05$.

We have collected $L_{\rm BLR}/L_{\rm Edd}$ from Xiao et al. (2022) for the 52
‘Changing-look’ blazars in this work and those 4 in Peña-Herazo et al. (2021)
and Mishra et al. (2021), listed them for the 56 changing-look blazars in
column (4) of Table 3 and in column (7) of Table 6. We notice that 14 (taking
100%) ‘F$\to$B’ and 38 (taking 90.5%) ‘B$\to$F’ blazars show $L_{\rm
BLR}/L_{\rm Edd}$ greater than $2.8\times 10^{-4}$, which means 92.9%
‘Changing-look’ blazars in our sample have values of $\dot{M}/\dot{M}_{\rm
Edd}$ larger than 0.05; and that 12 (taking 85.7%) ‘F$\to$B’ and 36 (taking
85.7%) ‘B$\to$F’ blazars show $L_{\rm BLR}/L_{\rm Edd}$ greater than
$5.5\times 10^{-4}$, which means 85.7% ‘Changing-look’ blazars in our sample
have values of $\dot{M}/\dot{M}_{\rm Edd}$ larger than 0.1, see in Table 7. It
is that the ‘Changing-look’ blazars mostly lie above the dividing lines
proposed by Ghisellini et al. (2011) and Sbarrato et al. (2012). Pei et al.
(2022) studied the correlation $L_{\rm Disk}/L_{\rm Edd}\,{\rm
vs}\,L_{\rm\gamma}/L_{\rm Edd}$, assuming $L_{\rm Disk}=10L_{\rm BLR}$, and it
was clearly shown in Figure 5 of their work that the FSRQs and the ‘Changing-
look’ blazars have larger accretion ratio than the BL Lacs. They have proposed
an ‘appareling zone’, $2.0\times 10^{-4}\leq L_{\rm BLR}/L_{\rm Edd}\leq
8.5\times 10^{-3}$, to select ‘Changing-look’ blazars candidates, and we found
that there are 46 (taking 82.1%) ‘Changing-look’ blazars in our sample that
lie in this zone. Thus, the ‘Changing-look’ blazars have a larger accretion
ratio than the normal BL Lacs, which are believed to have a lower accretion
ratio. And our result supports the explanation that the ‘Changing-look’
originated from the sudden change in accretion rate.

####  4.2.3 SED classification of ‘Changing-look’ blazars

Blazars are also divided into LSPs, ISPs, HSPs based on their synchrotron peak
locations. We notice that among the 56 ‘Changing-look’ blazars, there are 54
sources with available SED classification from 4LAC_R2, and 2 ‘B$\to$F’
sources, 4FGL J0127.9+4857 and 4FGL J0823.3+2224, without SED classification.
For the 54 blazars with SED classification, all 14 ‘F$\to$B’ blazars are
associated to LSPs, 35 ‘B$\to$F’ blazars are associated to LSPs, 4 ‘B$\to$F’
blazars are associated to ISPs, 1 ‘B$\to$F’ blazars are associated to HSPs. It
makes 90.7% of ‘Changing-look’ blazars are LSPs, and only a small fraction of
ISPs or HSPs are associated to ‘Changing-look’ blazars. In this case, the LSPs
could be a bank of ‘Changing-look’ blazars, especially for the lower
synchrotron peaked BL Lacs (LBLs).

###  4.3 The correlations

####  4.3.1 The correlation between $EW$ and the GeV $\gamma$-ray parameters

Figure 2 illustrates the correlation between the equivalent width of emission
lines and the $\gamma$-ray photon index, and the observed $\gamma$-ray
luminosity. We note that there are positive correlations (with $p$ values less
than 0.05), according to the fitting results in Table 5, between ${\rm
log}\,EW$ and $\alpha_{\rm ph}$ for ${\rm H{\alpha}}$, ${\rm H{\beta}}$, and
${\rm Mg{II}}$, meanwhile, a positive trend (with $p$ values greater than
0.05) between ${\rm log}\,EW$ and $\alpha_{\rm ph}$ for ${\rm C}{\rm IV}$ is
found. The positive correlation and trend suggest that the stronger the
emission line the softer the GeV spectrum. There are anti-correlations between
${\rm log}\,EW$ and ${\rm log}\,L_{\rm\gamma}$ for ${\rm Mg}{\rm II}$ and
${\rm C}{\rm IV}$, but this anti-correlation is not found for either ${\rm
H}{\rm\alpha}$ or ${\rm H}{\rm\beta}$.

####  4.3.2 The Baldwin effect

The correlation between the equivalent width and continuum luminosity is
illustrated in Figure 1 and the corresponding regression results are tabulated
in Table 4. A trend of anti-correlation between ${\rm
log}\,EW\,{\rm(H{\beta})}$ and ${\rm log}\,L_{\rm 5100}$ shows up; and solid
anti-correlations between ${\rm log}\,EW\,{\rm(Mg{II})}$ and ${\rm
log}\,L_{\rm 3000}$, between ${\rm log}\,EW\,{\rm(C{IV})}$ and ${\rm
log}\,L_{\rm 1350}$ are found. Our results of the anti-correlations are
consistent with the results reported in many other previous works (Dietrich et
al., 2002; Shields, 2007; Kovačević et al., 2010; Shemmer & Lieber, 2015;
Patiño-Álvarez et al., 2016; Rakić et al., 2017).

The anti-correlation between the broad-line equivalent width and the continuum
luminosity of single-epoch observations of a large number of AGNs are known as
the global ‘Baldwin effect’ (hereafter, BEff) (Baldwin, 1977; Carswell &
Smith, 1978). The BEff indicates that the line flux is increasing more slowly
than the local continuum (or is constant) because $EW$ is the ratio of line
flux to the local continuum flux. The BEff is well established for broad
emission lines in the UV/optical regions (Shields, 2007; Dietrich et al.,
2002), even for narrow lines (Dietrich et al., 2002; Kovačević et al., 2010),
and it is found that it steepens with increasing ionization potential (Zheng &
Malkan, 1993). Various mechanisms have been proposed as possible
interpretations of the BEff, such as a luminosity-dependent ionization
continuum and the BLR covering factor (Mushotzky & Ferland, 1984; Zheng &
Malkan, 1993), the geometrical effect of an inclination-dependent anisotropic
continuum (Netzer, 1985), different variability patterns in the thermal and
non-thermal components of the continuum (Kinney et al., 1990; Patiño-Álvarez
et al., 2016), or processes that involve different Eddington ratio or black
hole mass (Xu et al., 2008; Bian et al., 2012), etc. The most widely accepted
explanation is that the ionization continuum softens as the luminosity
increases (Zheng & Malkan, 1993), so that high-luminosity AGNs decrease the
fraction of ionizing photons for broad emission line formation.

In this work, we found a slope of $-0.08\pm 0.06$ for the correlation between
$EW$ of ${\rm H{\beta}}$ emission line and continuum luminosity at
$5100\,\mathring{\rm A}$, however, a chance probability of 0.23 suggests the
correlation is not evident. Rakić et al. (2017) obtained a slope of -0.0467 of
$EW\,{\rm(H{\beta})}$ against $L_{\rm 5100}$ in a logarithmic diagram, and
claimed that no evidence of significant BEff for ${\rm H{\beta}}$ emerged.
Both of the results, in this work and in Rakić et al. (2017), agree with
previous findings, which concluded that no BEff was present in the broad
Balmer lines (Dietrich et al., 2002; Kovačević et al., 2010)). But, the BEff
is found for ${\rm Mg{II}}$ and ${\rm C{IV}}$ emission lines with slopes of
$-0.24\pm 0.03$ and of $-0.25\pm 0.05$ (see Table 4) in this work. Our results
are consistent with the results of BEff for ${\rm Mg{II}}$ and ${\rm C{IV}}$
emission lines in Patiño-Álvarez et al. (2016), in which they found the BEff
for these two lines and reported slopes of 0.20 and 0.21 were derived,
respectively.

It is found that there is a difference in BEff anti-correlation between
blazars and radio-quiet (RQ) AGNs, the former shows a steeper anti-correlation
than the latter one Patiño-Álvarez et al. (2016). We suggest the steepen of
BEff for blazars results from the IC scattering of broad emission line photons
from BLR for two reasons. On one hand, consider the case that the continuum
luminosity is identical for blazars and RQs, the $EW$ for blazars should be
smaller than it is for RQs because a significant number of broad-emission-line
photons are fed to the IC process, and scattered to $\gamma$-ray band,
resulting in a high Compton dominance (Abdo et al., 2010; Ghisellini et al.,
2011; Paliya et al., 2021) and a weaker emission line; On the other hand, when
the continuum luminosity gets stronger, the bolometric luminosity should
increase. The bolometric luminosity is dominated by the IC emission (or the
$\gamma$-ray luminosity, Ghisellini & Tavecchio, 2010; Ghisellini et al.,
2014; Xiong & Zhang, 2014; Xiao et al., 2022), therefore, increased bolometric
luminosity yields more external photons (e.g. broad-emission-line photons) to
feed the IC process. Then the emission lines of blazars (mainly FSRQs) get
weaker and the difference on $EW$ become larger with the increase of continuum
luminosity compare to the RQs. Consequently, a steepening of BEff is formed
for blazars.

In this scenario, it is natural that strong emission lines should be able to
contribute more seed photons to the IC process, therefore, we can expect a
positive correlation between $EW$ and IC luminosity.

We collected the flux of the IC peak ($F_{\rm IC}$) from Paliya et al. (2021)
to calculate the IC peak luminosity ($L_{\rm IC}=4\pi d_{\rm L}^{2}F_{\rm
IC}$, in the observer frame). However, the observed IC luminosity is boosted
by a Doppler beaming effect (Dermer, 1995; Paliya et al., 2015, 2021) and
gives the intrinsic IC luminosity (in the source rest frame) as $L^{\rm
in}_{\rm IC}=L_{\rm IC}/\delta^{4}$, where $\delta$ is the Doppler factor.
Doppler factors are available in different literature and are given in
discrepancy. In this work, we employ the method that was proposed by Zhang et
al. (2020), in which they proposed to use $\gamma$-ray luminosity and broad
line region luminosity ($L_{\rm BLR}$) to calculate $\delta$ of $\gamma$-ray
emission, to calculate $\delta$ for our sources. And the data of $L_{\rm BLR}$
is calculated based on the work Paliya et al. (2021) and Xiao et al. (2022).

The results between $EW$ and intrinsic inverse Compton luminosity are shown in
Table 8 and in Figure 4. The results illustrate that the $EW$ is positively
correlated with intrinsic inverse Compton luminosity, suggesting that sources
with the stronger intrinsic inverse Compton luminosity tend to have stronger
emission lines. Moreover, these results have confirmed our prediction, a
positive correlation between $EW$ and inverse Compton luminosity, and have
proven that the steepening of BEff for blazars (mostly FSRQs) is, indeed,
caused by the IC scattering of broad-emission-line photons.

##  5 Conclusion

In this work, we aim to study the emission line property and its connection
with the non-thermal emission of blazars, we collected the equivalent width of
emission lines, $\gamma$-ray emission and SED information for a sample of 656
Fermi blazars. We have studied the blazar classification according to the $EW$
and the correlation between the $EW$ and $\gamma$-ray luminosity, and the
global ‘Baldwin effect’.

Our main results are as follows: (1) There are, out of the 656 Fermi blazars,
55 previously classified as BCUs are now classified as FSRQs; (2) We find 52
‘Changing-look’ blazars through the study of the $EW$, among them there is 10
FSRQs change to BL Lacs and 42 BL Lacs change to FSRQs. Besides, 45 of them
are newly confirmed as ‘Changing-look’ blazars; (3) The accretion rate
($\dot{M}/\dot{M}_{\rm Edd}$) of the 52 ‘Changing-look’ blazars are
calculated. We notice that there are 92.9% ‘Changing-look’ blazars in our
sample with $\dot{M}/\dot{M}_{\rm Edd}>0.05$ and 85.7% ‘Changing-look’ blazars
in our sample with $\dot{M}/\dot{M}_{\rm Edd}>0.1$, suggesting the ‘Changing-
look’ blazars have larger accretion ratio. Besides, we notice that 90.7% of
the ‘Changing-look’ blazars in this work are LSPs and suggest the LSPs are a
bank of ‘Changing-look’ blazars; (4) The global ‘Baldwin effect’ is confirmed
for blazars (mostly FSRQs) in this work, the results indicate a steeper anti-
correlation of equivalent width (${\rm log}\,EW$) against continuum luminosity
(${\rm log}\,L_{\rm 3000}$ and ${\rm log}\,L_{\rm 1350}$) than that of the
radio-quiet (RQ) AGNs; (5) We propose that the steepening of global BEff is
caused by the inverse Compton scattering of broad-emission-line photons and
predict a positive correlation between the equivalent width (${\rm log}\,EW$)
and the inverse Compton luminosity (${\rm log}\,L_{\rm IC}$). This prediction
is indeed correct and the $EW$ of H$\alpha$, H$\beta$, MgII and CIV are
positively correlated with the intrinsic inverse Compton peak luminosity.

We thank the support from our laboratory, the key laboratory for astrophysics
of Shanghai, we thank Dr. Vaidehi S. Paliya for his kindly help and for
sharing data. Meanwhile, L. P, Fu acknowledges the support from the National
Natural Science Foundation of China (NSFC) grants 11933002, STCSM grants
18590780100, 19590780100, SMEC Innovation Program 2019-01-07-00-02-E00032 and
Shuguang Program 19SG41. S. H, Zhang acknowledges the support from by Natural
Science Foundation of Shanghai (20ZR1473600). J. H, Fan acknowledges the
support by the NSFC (NSFC U2031201, NSFC 11733001).

Table 1: Optical, $\gamma$-ray, and SED parameters of 656 4LAC blazars 4FGL name | z | $EW_{\rm H{\alpha}}$ | $EW_{\rm H{\beta}}$ | $EW_{\rm MgII}$ | $EW_{\rm CIV}$ | ${\rm log}L_{\rm BLR}$ | ${\rm log}F_{\rm IC}$ | ${\rm f}_{\gamma}$ | $\alpha_{\rm ph}$ | Class
---|---|---|---|---|---|---|---|---|---|---
(1) | (2) | (3) | (4) | (5) | (6) | (7) | (8) | (9) | (10) | (11)
J0001.5+2113 | 0.439 |  344.8$\pm$12.91 |  31.29$\pm$3.91 |  37.2$\pm$2.74 |  | 43.74 | -10.48 | 1.36E-09 | 2.66 | F
J0004.3+4614 | 1.81 |  |  |  |  246.4$\pm$17.19 | 45.07 | -11.7 | 2.41E-10 | 2.58 | F
J0004.4-4737 | 0.88 |  |  |  22.54$\pm$5.12 |  | 44.10 | -11.24 | 4.36E-10 | 2.37 | F
J0006.3-0620 | 0.347 |  117.39$\pm$26.82 |  8.33$\pm$5.75 |  |  | 43.60 | -12.04 | 1.40E-10 | 2.13 | B
J0010.6+2043 | 0.598 |  |  172.38$\pm$18.94 |  112.67$\pm$4.28 |  | 44.34 | -11.97 | 1.73E-10 | 2.32 | F
Table 2: The new classification for BCUs in our sample 4FGL name | z | Class | New classification
---|---|---|---
(1) | (2) | (3) | (4)
J0014.3-0500 | 0.791 | U | F
J0030.6-0212 | 1.804 | U | F
J0036.9+1832 | 1.595 | U | F
J0040.9+3203 | 0.632 | U | F
J0143.5-3156 | 0.374 | U | F
J0204.8+1513 | 0.407 | U | F
J0223.5-0928 | 1.005 | U | F
J0226.3-1845 | 1.67 | U | F
J0327.5-1805 | 0.73 | U | F
J0430.2-0356 | 0.628 | U | F
J0516.8-0509 | 1.417 | U | F
J0621.2-4648 | 1.212 | U | F
J0622.9+3326 | 1.062 | U | F
J0658.1-5840 | 0.421 | U | F
J0725.8-0054 | 0.128 | U | F
J0728.0+6735 | 0.844 | U | F
J0749.3+4453 | 0.559 | U | F
J0821.1+1007 | 0.954 | U | F
J0904.0+2724 | 1.721 | U | F
J0904.9-5734 | 0.697 | U | F
J0941.7+4125 | 0.816 | U | F
J0943.7+6137 | 0.791 | U | F
J0949.7+5819 | 1.424 | U | F
J1017.8+0715 | 1.54 | U | F
J1018.9+1043 | 0.66 | U | F
J1047.9+0055 | 0.252 | U | F
J1054.2+3926 | 2.635 | U | F
J1124.4+2308 | 0.795 | U | F
J1129.2-0529 | 0.922 | U | F
J1131.4-0504 | 0.263 | U | F
J1139.0+4033 | 2.361 | U | F
J1159.2-2227 | 0.565 | U | F
J1205.8+3321 | 1.007 | U | F
J1243.0+3950 | 1.22 | U | F
J1248.9+4840 | 1.856 | U | F
J1319.5-0045 | 0.891 | U | F
J1323.0+2941 | 1.142 | U | F
J1329.4-0530 | 0.576 | U | F
J1412.9+5018 | 1.53 | U | F
J1418.4+3543 | 0.825 | U | F
J1454.0+4927 | 2.106 | U | F
J1615.6+2130 | 1.627 | U | F
J1627.3+4758 | 2.32 | U | F
J1720.2+3824 | 0.454 | U | F
J1821.6+6819 | 1.69 | U | F
J2136.2-0642 | 0.941 | U | F
J2140.5-6731 | 2.009 | U | F
J2211.2-1325 | 0.392 | U | F
J2253.3+3233 | 0.257 | U | F
J2311.7+2604 | 1.748 | U | F
J2313.9-4501 | 2.877 | U | F
J2318.2+1915 | 2.163 | U | F
J2326.2+0113 | 1.6 | U | F
J2339.6+0242 | 2.661 | U | F
J2352.9+3031 | 0.876 | U | F
Table 3: The ‘Changing-look’ blazars in our sample 4FGL name | z | Change | $L_{\rm BLR}/L_{\rm Edd}$ | SED classification
---|---|---|---|---
(1) | (2) | (3) | (4) | (5)
J0102.8+5824 | 0.644 |  F $\to$ B | 0.00034 | LSP
J0337.8-1157 | 3.442 |  F $\to$ B | 0.00776 | LSP
J0347.0+4844 | 2.039 |  F $\to$ B | 0.00035 | LSP
J0521.3-1734 | 0.347 |  F $\to$ B | 0.00377 | LSP
J0539.6+1432 | 2.71 |  F $\to$ B | 0.01659 | LSP
J0539.9-2839 | 3.104 |  F $\to$ B | 0.00380 | LSP
J0601.1-7035 | 2.4 |  F $\to$ B | 0.00071 | LSP
J1816.9-4942 | 1.7 |  F $\to$ B | 0.01213 | LSP
J2015.5+3710 | 0.855 |  F $\to$ B | 0.00734 | LSP
J2121.0+1901 | 2.04 |  F $\to$ B | 0.00222 | LSP
J0006.3-0620 | 0.347 |  B $\to$ F | 0.00097 | LSP
J0127.9+4857 | 0.065 |  B $\to$ F | 0.00350 |
J0203.7+3042 | 0.761 |  B $\to$ F | 0.02678 | LSP
J0209.9+7229 | 0.895 |  B $\to$ F | 0.00230 | LSP
J0238.6+1637 | 0.94 |  B $\to$ F | 0.00240 | LSP
J0334.2-4008 | 1.359 |  B $\to$ F | 0.04245 | LSP
J0407.5+0741 | 1.139 |  B $\to$ F | 0.00097 | LSP
J0428.6-3756 | 1.11 |  B $\to$ F | 0.00598 | LSP
J0433.6+2905 | 0.91 |  B $\to$ F | 0.00228 | LSP
J0438.9-4521 | 2.027 |  B $\to$ F | 0.00065 | LSP
J0516.7-6207 | 1.3 |  B $\to$ F | 0.00021 | LSP
J0538.8-4405 | 0.896 |  B $\to$ F | 0.00561 | LSP
J0629.3-1959 | 1.718 |  B $\to$ F | 0.00172 | LSP
J0654.7+4246 | 0.129 |  B $\to$ F | 0.00108 | LSP
J0710.9+4733 | 1.292 |  B $\to$ F | 0.00194 | LSP
J0814.4+2941 | 0.374 |  B $\to$ F | 0.00092 | HSP
J0823.3+2224 | 0.951 |  B $\to$ F | 0.00575 |
J0831.8+0429 | 0.174 |  B $\to$ F | 0.00064 | LSP
J0832.4+4912 | 0.548 |  B $\to$ F | 0.00260 | LSP
J1001.1+2911 | 0.556 |  B $\to$ F | 0.00125 | LSP
J1031.1+7442 | 0.123 |  B $\to$ F | 0.00515 | ISP
J1058.0+4305 | 1.308 |  B $\to$ F | 0.00355 | LSP
J1058.4+0133 | 0.892 |  B $\to$ F | 0.00177 | LSP
J1058.6-8003 | 0.581 |  B $\to$ F | 0.00038 | LSP
J1147.0-3812 | 1.053 |  B $\to$ F | 0.00168 | LSP
J1250.6+0217 | 0.954 |  B $\to$ F | 0.00918 | LSP
J1331.2-1325 | 0.251 |  B $\to$ F | 0.00212 | LSP
J1402.6+1600 | 0.245 |  B $\to$ F | 0.00014 | ISP
J1412.1+7427 | 0.436 |  B $\to$ F | 0.00091 | ISP
J1503.5+4759 | 0.345 |  B $\to$ F | 0.00398 | LSP
J1647.5+4950 | 0.047 |  B $\to$ F | 0.00148 | LSP
J1751.5+0938 | 0.322 |  B $\to$ F | 0.00703 | LSP
J1800.6+7828 | 0.691 |  B $\to$ F | 0.00186 | LSP
J1806.8+6949 | 0.05 |  B $\to$ F | 0.00741 | LSP
J1954.6-1122 | 0.683 |  B $\to$ F | 0.00347 | LSP
J2134.2-0154 | 1.283 |  B $\to$ F | 0.00087 | LSP
J2152.5+1737 | 0.872 |  B $\to$ F | 0.00046 | LSP
J2202.7+4216 | 0.069 |  B $\to$ F | 0.00020 | LSP
J2204.3+0438 | 0.027 |  B $\to$ F | 0.00020 | ISP
J2216.9+2421 | 1.033 |  B $\to$ F | 0.00289 | LSP
J2315.6-5018 | 0.811 |  B $\to$ F | 0.00143 | LSP
J2357.4-0152 | 0.816 |  B $\to$ F | 0.00277 | LSP
Table 4: The correlation between $EW$ and continuum luminosity y vs x |  $(a\pm\Delta a)$ |  $(b\pm\Delta b)$ |  $N$ |  $r$ |  $p$
---|---|---|---|---|---
${\rm log}\,EW\ {\rm(H{\beta})}\ vs\ {\rm log}\,L_{\rm 5100}$ |  -0.08 $\pm$ 0.06 |  5.23 $\pm$ 2.84 |  156 |  -0.1 |  0.23
${\rm log}\,EW\ {\rm(Mg{II})}\ vs\ {\rm log}\,L_{\rm 3000}$ |  -0.24 $\pm$ 0.03 |  12.74 $\pm$ 1.29 |  481 |  -0.36 |  $1.4\times 10^{-16}$
${\rm log}\,EW\ {\rm(C{IV})}\ vs\ {\rm log}\,L_{\rm 1350}$ |  -0.25 $\pm$ 0.05 |  13.26 $\pm$ 2.41 |  191 |  -0.33 |  $3.9\times 10^{-6}$
Table 5: The correlation between $EW$ and GeV $\gamma$-ray parameters y vs x |  $(a\pm\Delta a)$ |  $(b\pm\Delta b)$ |  $N$ |  $r$ |  $p$
---|---|---|---|---|---
${\rm log}\,EW\ {\rm(H{\alpha})}\ vs\ \alpha_{\rm ph}$ |  1.17 $\pm$ 0.35 |  -0.83 $\pm$ 0.85 |  47 |  0.45 |  0.002
${\rm log}\,EW\ {\rm(H{\alpha})}\ vs\ {\rm log}\,L_{\rm\gamma}$ |  0.12 $\pm$ 0.14 |  -3.31 $\pm$ 6.39 |  47 |  0.12 |  0.41
${\rm log}\,EW\ {\rm(H{\beta})}\ vs\ \alpha_{\rm ph}$ |  0.72 $\pm$ 0.20 |  0.06 $\pm$ 0.50 |  156 |  0.28 |  $5\times 10^{-4}$
${\rm log}\,EW\ {\rm(H{\beta})}\ vs\ {\rm log}\,L_{\rm\gamma}$ |  -0.01 $\pm$ 0.05 |  2.51 $\pm$ 2.22 |  156 |  -0.02 |  0.76
${\rm log}\,EW\ {\rm(Mg{II})}\ vs\ \alpha_{\rm ph}$ |  0.43 $\pm$ 0.10 |  0.65 $\pm$ 0.24 |  481 |  0.20 |  $1.6\times 10^{-5}$
${\rm log}\,EW\ {\rm(Mg{II})}\ vs\ {\rm log}\,L_{\rm\gamma}$ |  -0.17 $\pm$ 0.03 |  9.38 $\pm$ 1.17 |  481 |  -0.29 |  $1.5\times 10^{-10}$
${\rm log}\,EW\ {\rm(C{IV})}\ vs\ \alpha_{\rm ph}$ |  0.17 $\pm$ 0.17 |  1.38 $\pm$ 0.43 |  191 |  0.07 |  0.33
${\rm log}\,EW\ {\rm(C{IV})}\ vs\ {\rm log}\,L_{\rm\gamma}$ |  -0.19 $\pm$ 0.06 |  10.50 $\pm$ 2.82 |  191 |  -0.22 |  0.002
Figure 1: The correlations between emission line ($\rm H{\beta}$, $\rm Mg{II}$, and $\rm C{IV}$) EW and corresponding optical continuum luminosity. ‘F’ stands for FSRQs, ‘UF’ stands for the new confirmed FSRQs from BCUs in this work, ‘BF’ stands for the ‘changing look’ blazar that changes from BL Lac to FSRQs, ‘B’ stands for BL Lacs, ‘FB’ stands for the ‘changing look’ blazar that change from FSRQs to BL Lacs. Figure 2: The correlations between emission line ($\rm H{\alpha}$, $\rm H{\beta}$, $\rm Mg{II}$, and $\rm C{IV}$) $EW$ and GeV $\gamma$-ray parameters, photon spectral index $\alpha_{\rm ph}$ (left column) and $\gamma$-ray luminosity ${\rm log}L_{\gamma}$ (right column). ‘F’ stands for FSRQs, ‘UF’ stands for the new confirmed FSRQs from BCUs in this work, ‘BF’ stands for the ‘changing look’ blazar that change from BL Lac to FSRQs, ‘B’ stands for BL Lacs, ‘FB’ stands for the ‘changing look’ blazar that change from FSRQs to BL Lacs. Figure 3: The redshift and $\gamma$-ray luminosity distributions of the sources in our sample and in 4LAC_DR2. The blue bar stands for 4LAC sources, the orange bar stands for the sources in our sample. Figure 4: The correlations between emission line ($\rm H{\alpha}$, $\rm H{\beta}$, $\rm Mg{II}$, and $\rm C{IV}$) $EW$ and the intrinsic IC luminosity (${\rm log}L_{\rm IC}^{\rm in}$). ‘F’ stands for FSRQs, ‘UF’ stands for the new confirmed FSRQs from BCUs in this work, ‘BF’ stands for the ‘changing look’ blazar that change from BL Lac to FSRQs, ‘B’ stands for BL Lacs, ‘FB’ stands for the ‘changing look’ blazar that change from FSRQs to BL Lacs. Table 6: The overlapped ‘Changing-look’ blazars with previous works 4FGL name |  Other name |  P21 |  M21 |  TW |  Change |  $L_{\rm BLR}/L_{\rm Edd}$
---|---|---|---|---|---|---
(1) |  (2) |  (3) |  (4) |  (5) |  (6) |  (7)
J1043.2+2408 |  5BZQ J1043+2408 |  Y |  |  N |  F$\to$B |  0.00132
J1106.0+2813 |  5BZQ J1106+2812 |  Y |  |  N |  F$\to$B |  0.00554
J1321.1+2216 |  5BZQ J1321+2216 |  Y |  |  N |  F$\to$B |  0.01110
J1422.3+3223 |  B2 1420+32 |  |  Y |  N |  F$\to$B |  0.01010
Table 7: The $L_{\rm BLR}/L_{\rm Edd}$ for ‘Changing-look’ blazars in this work Change type | $>5.5\times 10^{-4}$ | $<5.5\times 10^{-4}$ | $>2.8\times 10^{-4}$ | $<2.8\times 10^{-4}$
---|---|---|---|---
F $\to$ B | 12 | 2 | 14 | 0
B $\to$ F | 36 | 6 | 38 | 4
Table 8: The correlations between EW and the intrinsic inverse Compton peak luminosity y vs x |  $(a\pm\Delta a)$ |  $(b\pm\Delta b)$ |  $N$ |  $r$ |  $p$
---|---|---|---|---|---
${\rm log}EW\ {\rm(H{\alpha})}\ vs\ {\rm log}L_{\rm IC}^{\rm in}$ |  0.22 $\pm$ 0.04 |  -7.01 $\pm$ 1.60 |  47 |  0.64 |  $1.1\times 10^{-6}$
${\rm log}EW\ {\rm(H{\beta})}\ vs\ {\rm log}L_{\rm IC}^{\rm in}$ |  0.13 $\pm$ 0.02 |  -3.66 $\pm$ 0.89 |  156 |  0.44 |  $6.0\times 10^{-9}$
${\rm log}EW\ {\rm(Mg{II})}\ vs\ {\rm log}L_{\rm IC}^{\rm in}$ |  0.07 $\pm$ 0.01 |  -1.53 $\pm$ 0.52 |  481 |  0.27 |  $9.1\times 10^{-10}$
${\rm log}EW\ {\rm(C{IV})}\ vs\ {\rm log}L_{\rm IC}^{\rm in}$ |  0.08 $\pm$ 0.02 |  -1.58 $\pm$ 1.03 |  191 |  0.23 |  0.001

## References

  * Abdo et al. (2010) Abdo, A. A., Ackermann, M., Agudo, I., et al. 2010, ApJ, 716, 30, doi: 10.1088/0004-637X/716/1/30
  * Abdollahi et al. (2020) Abdollahi, S., Acero, F., Ackermann, M., et al. 2020, ApJS, 247, 33, doi: 10.3847/1538-4365/ab6bcb
  * Ahumada et al. (2020) Ahumada, R., Prieto, C. A., Almeida, A., et al. 2020, ApJS, 249, 3, doi: 10.3847/1538-4365/ab929e
  * Ajello et al. (2020) Ajello, M., Angioni, R., Axelsson, M., et al. 2020, ApJ, 892, 105, doi: 10.3847/1538-4357/ab791e
  * Antonucci (1993) Antonucci, R. 1993, ARA&A, 31, 473, doi: 10.1146/annurev.aa.31.090193.002353
  * Atwood et al. (2009) Atwood, W. B., Abdo, A. A., Ackermann, M., et al. 2009, ApJ, 697, 1071, doi: 10.1088/0004-637X/697/2/1071
  * Baldwin (1977) Baldwin, J. A. 1977, ApJ, 214, 679, doi: 10.1086/155294
  * Bian et al. (2012) Bian, W.-H., Fang, L.-L., Huang, K.-L., & Wang, J.-M. 2012, MNRAS, 427, 2881, doi: 10.1111/j.1365-2966.2012.22123.x
  * Bianchi et al. (2005) Bianchi, S., Guainazzi, M., Matt, G., et al. 2005, A&A, 442, 185, doi: 10.1051/0004-6361:20053389
  * Blandford & Königl (1979) Blandford, R. D., & Königl, A. 1979, ApJ, 232, 34, doi: 10.1086/157262
  * Blandford & Rees (1978) Blandford, R. D., & Rees, M. J. 1978, Pittsburgh Conference on BL Lac Objects, 328
  * Carswell & Smith (1978) Carswell, R. F., & Smith, M. G. 1978, MNRAS, 185, 381, doi: 10.1093/mnras/185.2.381
  * Dermer (1995) Dermer, C. D. 1995, ApJ, 446, L63, doi: 10.1086/187931
  * Dietrich et al. (2002) Dietrich, M., Hamann, F., Shields, J. C., et al. 2002, ApJ, 581, 912, doi: 10.1086/344410
  * Diltz et al. (2015) Diltz, C., Böttcher, M., & Fossati, G. 2015, ApJ, 802, 133, doi: 10.1088/0004-637X/802/2/133
  * Dimitrakoudis et al. (2012) Dimitrakoudis, S., Mastichiadis, A., Protheroe, R. J., & Reimer, A. 2012, A&A, 546, A120, doi: 10.1051/0004-6361/201219770
  * Fan (2002) Fan, J.-H. 2002, PASJ, 54, L55, doi: 10.1093/pasj/54.4.L55
  * Fan et al. (2014) Fan, J.-H., Bastieri, D., Yang, J.-H., et al. 2014, Research in Astronomy and Astrophysics, 14, 1135, doi: 10.1088/1674-4527/14/9/004
  * Fan et al. (2013) Fan, J.-H., Yang, J.-H., Liu, Y., & Zhang, J.-Y. 2013, Research in Astronomy and Astrophysics, 13, 259, doi: 10.1088/1674-4527/13/3/002
  * Fan et al. (2016) Fan, J. H., Yang, J. H., Liu, Y., et al. 2016, ApJS, 226, 20, doi: 10.3847/0067-0049/226/2/20
  * Fan et al. (2021) Fan, J. H., Kurtanidze, S. O., Liu, Y., et al. 2021, ApJS, 253, 10, doi: 10.3847/1538-4365/abd32d
  * Ghisellini & Tavecchio (2010) Ghisellini, G., & Tavecchio, F. 2010, MNRAS, 409, L79, doi: 10.1111/j.1745-3933.2010.00952.x
  * Ghisellini et al. (2011) Ghisellini, G., Tavecchio, F., Foschini, L., & Ghirlanda, G. 2011, MNRAS, 414, 2674, doi: 10.1111/j.1365-2966.2011.18578.x
  * Ghisellini et al. (2014) Ghisellini, G., Tavecchio, F., Maraschi, L., Celotti, A., & Sbarrato, T. 2014, Nature, 515, 376, doi: 10.1038/nature13856
  * Gupta et al. (2016) Gupta, A. C., Agarwal, A., Bhagwan, J., et al. 2016, MNRAS, 458, 1127, doi: 10.1093/mnras/stw377
  * H. E. S. S. Collaboration et al. (2015) H. E. S. S. Collaboration, Abramowski, A., Aharonian, F., et al. 2015, A&A, 573, A31, doi: 10.1051/0004-6361/201321436
  * Hagen-Thorn et al. (2018) Hagen-Thorn, V. A., Larionov, V. M., Morozova, D. A., et al. 2018, Astronomy Reports, 62, 103, doi: 10.1134/S106377291802004X
  * Hutsemékers et al. (2019) Hutsemékers, D., Agís González, B., Marin, F., et al. 2019, A&A, 625, A54, doi: 10.1051/0004-6361/201834633
  * IceCube Collaboration et al. (2018) IceCube Collaboration, Aartsen, M. G., Ackermann, M., et al. 2018, Science, 361, eaat1378, doi: 10.1126/science.aat1378
  * Isler et al. (2013) Isler, J. C., Urry, C. M., Coppi, P., et al. 2013, ApJ, 779, 100, doi: 10.1088/0004-637X/779/2/100
  * Isler et al. (2015) Isler, J. C., Urry, C. M., Bailyn, C., et al. 2015, ApJ, 804, 7, doi: 10.1088/0004-637X/804/1/7
  * Khachikian & Weedman (1974) Khachikian, E. Y., & Weedman, D. W. 1974, ApJ, 192, 581, doi: 10.1086/153093
  * Kinney et al. (1990) Kinney, A. L., Rivolo, A. R., & Koratkar, A. P. 1990, ApJ, 357, 338, doi: 10.1086/168924
  * Kovačević et al. (2010) Kovačević, J., Popović, L. Č., & Dimitrijević, M. S. 2010, ApJS, 189, 15, doi: 10.1088/0067-0049/189/1/15
  * Kühr et al. (1981) Kühr, H., Witzel, A., Pauliny-Toth, I. I. K., & Nauber, U. 1981, A&AS, 45, 367
  * Landau et al. (1986) Landau, R., Golisch, B., Jones, T. J., et al. 1986, ApJ, 308, 78, doi: 10.1086/164480
  * Lister et al. (2018) Lister, M. L., Aller, M. F., Aller, H. D., et al. 2018, ApJS, 234, 12, doi: 10.3847/1538-4365/aa9c44
  * Marchese et al. (2012) Marchese, E., Braito, V., Della Ceca, R., Caccianiga, A., & Severgnini, P. 2012, MNRAS, 421, 1803, doi: 10.1111/j.1365-2966.2012.20445.x
  * Matt et al. (2003) Matt, G., Guainazzi, M., & Maiolino, R. 2003, MNRAS, 342, 422, doi: 10.1046/j.1365-8711.2003.06539.x
  * Mishra et al. (2021) Mishra, H. D., Dai, X., Chen, P., et al. 2021, ApJ, 913, 146, doi: 10.3847/1538-4357/abf63d
  * Mücke & Protheroe (2001) Mücke, A., & Protheroe, R. J. 2001, Astroparticle Physics, 15, 121, doi: 10.1016/S0927-6505(00)00141-9
  * Mushotzky & Ferland (1984) Mushotzky, R., & Ferland, G. J. 1984, ApJ, 278, 558, doi: 10.1086/161822
  * Netzer (1985) Netzer, H. 1985, MNRAS, 216, 63, doi: 10.1093/mnras/216.1.63
  * Nieppola et al. (2006) Nieppola, E., Tornikoski, M., & Valtaoja, E. 2006, A&A, 445, 441, doi: 10.1051/0004-6361:20053316
  * Paliya et al. (2021) Paliya, V. S., Domínguez, A., Ajello, M., Olmo-García, A., & Hartmann, D. 2021, ApJS, 253, 46, doi: 10.3847/1538-4365/abe135
  * Paliya et al. (2015) Paliya, V. S., Sahayanathan, S., & Stalin, C. S. 2015, ApJ, 803, 15, doi: 10.1088/0004-637X/803/1/15
  * Patiño-Álvarez et al. (2016) Patiño-Álvarez, V., Torrealba, J., Chavushyan, V., et al. 2016, Frontiers in Astronomy and Space Sciences, 3, 19, doi: 10.3389/fspas.2016.00019
  * Peña-Herazo et al. (2021) Peña-Herazo, H. A., Massaro, F., Gu, M., et al. 2021, AJ, 161, 196, doi: 10.3847/1538-3881/abe41d
  * Pei et al. (2022) Pei, Z., Fan, J., Yang, J., Huang, D., & Li, Z. 2022, ApJ, 925, 97, doi: 10.3847/1538-4357/ac3aeb
  * Peterson et al. (2004) Peterson, B. M., Ferrarese, L., Gilbert, K. M., et al. 2004, ApJ, 613, 682, doi: 10.1086/423269
  * Rakić et al. (2017) Rakić, N., La Mura, G., Ilić, D., et al. 2017, A&A, 603, A49, doi: 10.1051/0004-6361/201630085
  * Sbarrato et al. (2012) Sbarrato, T., Ghisellini, G., Maraschi, L., & Colpi, M. 2012, MNRAS, 421, 1764, doi: 10.1111/j.1365-2966.2012.20442.x
  * Scarpa & Falomo (1997) Scarpa, R., & Falomo, R. 1997, A&A, 325, 109
  * Shappee et al. (2014) Shappee, B. J., Prieto, J. L., Grupe, D., et al. 2014, ApJ, 788, 48, doi: 10.1088/0004-637X/788/1/48
  * Shaw et al. (2012) Shaw, M. S., Romani, R. W., Cotter, G., et al. 2012, ApJ, 748, 49, doi: 10.1088/0004-637X/748/1/49
  * Shemmer & Lieber (2015) Shemmer, O., & Lieber, S. 2015, ApJ, 805, 124, doi: 10.1088/0004-637X/805/2/124
  * Shen et al. (2011) Shen, Y., Richards, G. T., Strauss, M. A., et al. 2011, ApJS, 194, 45, doi: 10.1088/0067-0049/194/2/45
  * Shields (2007) Shields, J. C. 2007, in Astronomical Society of the Pacific Conference Series, Vol. 373, The Central Engine of Active Galactic Nuclei, ed. L. C. Ho & J. W. Wang, 355.  https://arxiv.org/abs/astro-ph/0612613
  * Sikora et al. (1994) Sikora, M., Begelman, M. C., & Rees, M. J. 1994, ApJ, 421, 153, doi: 10.1086/173633
  * Sokolov & Marscher (2005) Sokolov, A., & Marscher, A. P. 2005, ApJ, 629, 52, doi: 10.1086/431321
  * Stickel et al. (1991) Stickel, M., Padovani, P., Urry, C. M., Fried, J. W., & Kuehr, H. 1991, ApJ, 374, 431, doi: 10.1086/170133
  * Stocke et al. (1990) Stocke, J. T., Morris, S. L., Gioia, I., et al. 1990, ApJ, 348, 141, doi: 10.1086/168221
  * Stocke et al. (1991) Stocke, J. T., Morris, S. L., Gioia, I. M., et al. 1991, ApJS, 76, 813, doi: 10.1086/191582
  * Tan et al. (2020) Tan, C., Xue, R., Du, L.-M., et al. 2020, ApJS, 248, 27, doi: 10.3847/1538-4365/ab8cc6
  * Urry & Padovani (1995) Urry, C. M., & Padovani, P. 1995, PASP, 107, 803, doi: 10.1086/133630
  * Villata et al. (2006) Villata, M., Raiteri, C. M., Balonek, T. J., et al. 2006, A&A, 453, 817, doi: 10.1051/0004-6361:20064817
  * Wilkes (1986) Wilkes, B. J. 1986, MNRAS, 218, 331, doi: 10.1093/mnras/218.2.331
  * Wills et al. (1992) Wills, B. J., Wills, D., Breger, M., Antonucci, R. R. J., & Barvainis, R. 1992, ApJ, 398, 454, doi: 10.1086/171869
  * Xiao et al. (2022) Xiao, H., Ouyang, Z., Zhang, L., et al. 2022, ApJ, 925, 40, doi: 10.3847/1538-4357/ac36da
  * Xiao et al. (2019) Xiao, H., Fan, J., Yang, J., et al. 2019, Science China Physics, Mechanics, and Astronomy, 62, 129811, doi: 10.1007/s11433-018-9371-x
  * Xiao et al. (2015) Xiao, H. B., Pei, Z. Y., Xie, H. J., et al. 2015, Ap&SS, 359, 39, doi: 10.1007/s10509-015-2433-1
  * Xiong & Zhang (2014) Xiong, D. R., & Zhang, X. 2014, MNRAS, 441, 3375, doi: 10.1093/mnras/stu755
  * Xu et al. (2008) Xu, Y., Bian, W.-H., Yuan, Q.-R., & Huang, K.-L. 2008, MNRAS, 389, 1703, doi: 10.1111/j.1365-2966.2008.13545.x
  * Xue et al. (2021) Xue, R., Liu, R.-Y., Wang, Z.-R., Ding, N., & Wang, X.-Y. 2021, ApJ, 906, 51, doi: 10.3847/1538-4357/abc886
  * Yao et al. (2019) Yao, S., Wu, X.-B., Ai, Y. L., et al. 2019, ApJS, 240, 6, doi: 10.3847/1538-4365/aaef88
  * Zhang et al. (2020) Zhang, L., Chen, S., Xiao, H., Cai, J., & Fan, J. 2020, ApJ, 897, 10, doi: 10.3847/1538-4357/ab9180
  * Zheng & Malkan (1993) Zheng, W., & Malkan, M. A. 1993, ApJ, 415, 517, doi: 10.1086/173182
  * Zheng & Kang (2013) Zheng, Y. G., & Kang, T. 2013, ApJ, 764, 113, doi: 10.1088/0004-637X/764/2/113
  * Zheng & Yang (2016) Zheng, Y. G., & Yang, C. Y. 2016, MNRAS, 457, 3535, doi: 10.1093/mnras/stw078