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Long-term Optical and γ-Ray Variability of the Blazar PKS 1222+216
Savithri H. Ezhikode
1,2
, Amit Shukla
3
, Gulab C. Dewangan
1
, Pramod K. Pawar
1
, Sushmita Agarwal
3
,
Blesson Mathew
2
, and Akhil Krishna R.
2
1
Inter-University Centre for Astronomy and Astrophysics, Post Bag 4, Ganeshkhind, Pune, 411007, India; savithri.ezhikode@christuniversity.in
2
Department of Physics and Electronics, CHRIST (Deemed to be University), Hosur Road, Bengaluru, 560029, India
3
Department of Astronomy, Astrophysics and Space Engineering, Indian Institute of Technology Indore, Khandwa Road, Simrol, Indore, 453552, India
Received 2021 December 31; revised 2022 September 24; accepted 2022 September 26; published 2022 November 8
Abstract
The γ-ray emission from flat-spectrum radio quasars (FSRQs)is thought to be dominated by the inverse Compton
scattering of the external sources of photon fields, e.g., accretion disk, broad-line region (BLR), and torus. FSRQs
show strong optical emission lines and hence can be a useful probe of the variability in BLR output, which is the
reprocessed disk emission. We study the connection between the optical continuum, Hγline, and γ-ray emissions
from the FSRQ PKS 1222+216, using long-term (∼2011–2018)optical spectroscopic data from Steward
Observatory and γ-ray observations from Fermi Large Area Telescope (LAT). We measured the continuum (F
C,opt
)
and Hγ(F
Hγ
)fluxes by performing a systematic analysis of the 6029–6452 Åoptical spectra. We observed stronger
variability in F
C,opt
than F
Hγ
, an inverse correlation between the Hγequivalent width and F
C,opt
, and a redder-
when-brighter trend. Using discrete cross-correlation analysis, we found a positive correlation (DCF ∼0.5)
between the F
γ‐ray>100 MeV
and F
C,opt
(6024–6092 Å)light curves with a time lag consistent with zero at the 2σ
level. We found no correlation between the F
γ‐ray>100 MeV
and F
Hγ
light curves, probably dismissing the disk
contribution to the optical and γ-ray variability. The observed strong variability in the Fermi-LAT flux and
F
γ‐ray>100 MeV
−F
C,opt
correlation could be due to the changes in the particle acceleration at various epochs. We
derived the optical-to-γ-ray spectral energy distributions during the γ-ray flaring and quiescent epochs that show a
dominant disk component with no variability. Our study suggests that the γ-ray emission zone is likely located at
the edge of the BLR or in the radiation field of the torus.
Unified Astronomy Thesaurus concepts: Blazars (164);Flat-spectrum radio quasars (2163);High energy
astrophysics (739);Active galactic nuclei (16);Jets (870)
1. Introduction
Blazars are sources with highly collimated jets that point very
close to the line of sight of the observer (Urry & Padovani 1995).
The radiation from blazars ranges from radio to high-energy γ-
rays, and their nonthermal emission is dominated by the output
from the jet. Blazars are highly variable sources, and they show
flaring activities at multiple wave bands that may be correlated or
uncorrelated (e.g., Liodakis et al. 2018,2019; Meyer et al. 2019).
The spectral energy distribution (SED)of blazars has a double-
hump structure (Sambruna et al. 1996; Urry 1996).Thelow-
energy hump typically peaks around infrared to X-ray regions and
is attributed to synchrotron emission from electrons present in the
jet. The high-energy component arising from the X-ray to γ-ray
bands is believed to be produced by either the synchrotron self
Compton (SSC)process or the external Comptonization (EC)
process (e.g., Ghisellini & Madau 1996;Dermeretal.1997;
Mukherjee et al. 1997; Bottcher 1999; Arbeiter et al. 2002).Inthe
EC scenario, the external photon field could be the accretion disk,
broad-line region (BLR), and dusty torus.
The relationship between the accretion disk and jet power is
one of the major unresolved issues in active galactic nuclei
(AGNs). The thermal disk emission from AGNs mostly emerges
in the optical/UV bands. As the optical/UV continuum in blazars
is contributed by both the disk and jet emissions, it is difficult to
get a direct measurement of the disk emission from the observed
optical/UV luminosity. However, the radiation from the disk can
photoionize the gas in broad-line and narrow-line regions, prod-
ucing emission lines in the optical spectra. Then, any variability
found in the optical emission lines could be an indicator of the
changes in the accretion power. Therefore, the correlation between
the variable BLR luminosity or flux with the γ-ray flux can
provide an indirect way to find the connection between the
accretion disk and jet emissions. Many studies have been carried
out in a similar line to find this correlation in blazars (e.g., Du
et al. 2013;Sbarratoetal.2014).
A broad classification of blazars is based on the optical spectra
where objects with strong optical emission lines (rest-frame
equivalent width, EW >5Å)are known as flat-spectrum radio
quasars (FSRQs), and those with relatively featureless optical
spectra are called BL Lacertae (BL Lac)objects (Urry &
Padovani 1995). FSRQs are more luminous, on average, than
BL Lac objects, and the γ-ray emission from these objects is
better explained by the EC model. We study an FSRQ PKS 1222
+216 to probe the possible correlation between the optical and
γ-ray emission components. The cross-correlation analysis of
optical and γ-ray fluxes may indicate the possible location of γ-
ray emission in the source.
PKS 1222+216 or 4C+21.35 (R.A. =12
h
24
m
54 4, decl. =
+21
d
22
m
46
s
;redshift=0.432)is one of the bright FSRQs in
which very-high-energy emission was observed. It was first
discovered in the γ-rays with the Energetic Gamma Ray
Experiment Telescope (EGRET; Hartman et al. 1999).Later,
the source was also detected with Fermi Large Area Telescope
(LAT; Abdo et al. 2010)and MAGIC (Aleksic et al. 2011)when
it was in a very high state. The MAGIC observations showed
The Astrophysical Journal, 939:76 (11pp), 2022 November 10 https://doi.org/10.3847/1538-4357/ac9627
© 2022. The Author(s). Published by the American Astronomical Society.
Original content from this work may be used under the terms
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1
rapid variability in the VHE emission, and the GeV spectrum of
the source was found to be hard (Aleksic et al. 2011). The source
was also observed in the radio (e.g., Planck Collaboration et al.
2016), infrared (e.g., Malmrose et al. 2011), optical (e.g., Smith
et al. 2011;Farinaetal.2012), UV, and X-ray (e.g., Paliya et al.
2018)wave bands. Epochs of flaring activities have been reported
in the source. Detailed studies on these active and quiescent states
have been carried out using Fermi-LAT observations (Kushwaha
et al. 2014b; Meyer et al. 2019). Quite a few studies are available
on the multiwavelength SED analysis of this source (Tavecchio
et al. 2011; Paliya et al. 2018; Bhattacharya et al. 2021; Roy et al.
2021).
PKS 1222+216 is one among the blazars monitored with
Fermi-LAT for a long timescale. The optical properties of the
source were studied using the coordinated monitoring observa-
tions with Steward Observatory (SO; Smith et al. 2011; Farina
et al. 2012). The optical spectrophotometric study of the source
during the first two years of Fermi-LAT observations did not
reveal any connection between the variable optical and γ-ray
flux (Smith et al. 2011). They also did not find any variability in
the broad optical emission lines. Farina et al. (2012)found
similar optical spectral properties in the source where they do not
find any notable variations in the broad-line emissions. From the
Hβline width obtained from SO observations, they also found a
black hole mass of (5.0–11.4)×10
8
M
e
. The multiwavelength
SED studies (from radio to γ-rays)of various quiescent states
suggest relevant disk–jet connection in the source (e.g., Roy
et al. 2021). Though studies were carried out to understand the
temporal correlation between the disk, BLR, and jet emission
from the source at various flaring states, no attempts have been
made to probe the correlations on longer timescales. This work
investigates the relationship between these emission components
via the cross-correlation studies of long-term optical continuum/
line and γ-ray light curves of the source.
The paper is organized as follows. In Section 2, we give the
details of the observations used in the work. In Section 3,we
explain the analysis of optical spectra. Section 4gives the
details of cross-correlation analysis and Section 5provides a
description of the SED analysis. In Section 6we summarize the
results. Section 7provides a detailed summary and discussion
of the study.
2. Observations
PKS 1222+216 has been continuously monitored with Fermi-
LAT from the start of the mission. In support of these observations,
an optical monitoring program has been carried out using the
ground-based Steward Observatory of the University of Arizona.
In this work, to explore the multiwavelength variability properties
of the blazar PKS 1222+216, we use the optical and γ-ray data on
the source from these coordinated observations.
2.1. Fermi-LAT
The Fermi-LAT (Atwood et al. 2009)is a pair-conversion γ-
ray telescope. Fermi-LAT scans the whole sky in a 3 hr period
due to its large field of view of 2.4 sr. The pass8 Fermi-LAT γ-
ray data
4
(>100 MeV)of PKS 1222+216 have been analyzed
using Science Tools version v10r0p5 by the Fermi-LAT
collaboration and the user-contributed Enrico software
(Sanchez & Deil 2013). A circular region of 15°radius around
the PKS 1222+216 was chosen for the analysis. A zenith angle
cut of 90°, the GTMKTIME cut of DATA_QUAL == 1&&
LAT_CONFIG == 1 together with the LAT event class ==
128 and the LAT event type == 3 were used. Spectral analysis
of the resulting data set was carried out by including
gll_iem_v06.fits and the isotropic diffuse model iso_
P8R2_SOURCE_V6_v06.txt. A log-parabola model was used
to fit the energy spectrum of PKS 1222+216, and its flux and
spectrum were determined using an unbinned gtlike algorithm
based on the NewMinuit optimizer. Fermi-LAT light curves
above 100 MeV were obtained through ScienceTools and
Fermipy.
2.2. Steward Observatory
Steward Observatory uses the 2.3 m Bok Telescope on Kitt
Peak and 1.54 m Kuiper Telescope on Mount Bigelow for the
optical monitoring of blazars. The observations were performed
with the high-throughput, moderate-resolution spectropolari-
meter (SPOL; Smith et al. 2009). The monitoring program
provides the spectroscopic, photometric, and polarization data
on each blazar observed. Here, we use the publicly available
flux spectra of PKS 1222+216 from cycle 3 (C-3)to cycle 10
(C-10), corresponding to Fermi cycles obtained from SPOL
observations. These observations span roughly eight years,
from 2011 to 2018. The optical spectra cover the wavelength
range of 4000–7550 Åwith a dispersion of 4 Åpixel
−1
. The
spectral resolution varies from 16 to 24 Ådepending on the slit
width (2″–12 7)used for the observation. The observations
used in this work were taken with apertures of slit widths of
41, 5 1 and 7 6. The flux density spectra of the source
corrected for airmass and instrument sensitivity are available at
the website.
5
We select those observations where the flux
Figure 1. Optical spectra of PKS 1222+216 from SO SPOL observations.
Observed and de-reddened (Cardelli et al. 1989)mean spectra are plotted as
dashed–dotted red and solid blue lines, respectively, along with the de-
reddened spectra at high- (black solid line)and low-flux (solid gray line)states.
The prominent emission lines, Hβ(λ
rest
=4861.3 Å),Hγ(λ
rest
=4340.48 Å),
Hδ(λ
rest
=4101.75 Å), and Mg II (λ
rest
=2795.50 and 2802.7 Å), in the
observed frame are also identified in the plot. The shaded region shows the
spectral region local to Hγemission line that we used for the analysis.
4
https://fermi.gsfc.nasa.gov/ssc/data/
5
http://james.as.arizona.edu/~psmith/Fermi/DATA/Objects/
pks1222.html
2
The Astrophysical Journal, 939:76 (11pp), 2022 November 10 Ezhikode et al.
spectra, averaged between 5400 and 5600 Å, have been scaled
to match the results from the synthetic V-band photometry on
that night. The observed and de-reddened mean spectrum of
these 329 observations, along with the spectra (de-reddened)at
the low- and high-flux states are shown in Figure 1. The strong
emission lines observed, say Hβ(λ
rest
=4861.3 Å),Hγ
(λ
rest
=4340.48 Å),Hδ(λ
rest
=4101.75 Å), and Mg II (λ
rest
=
2795.50 Åand 2802.7 Å), are also identified in the same figure.
3. Optical Spectral Analysis
We derive the optical continuum and emission line flux of the
FSRQ PKS 1222+216. As the optical continuum in blazars is
contaminated by the jet emission, we need a different quantity to
probe the disk emission. The optical emission line components
from BLR and NLR are produced by the photoionization of the
gaseous clouds in these regions by illuminating disk radiation.
Hence, the variable broad optical emission lines can be used to
trackthechangesindiskemissioninFSRQs.Toachievethisgoal,
we obtained the optical spectroscopic monitoring data on PKS 1222
+216 in the observed wavelength range of 4000–7550 Å,as
mentioned in the previous section.
As the available spectra are not corrected for Galactic
extinction and reddening, we applied the correction method
provided by Cardelli et al. (1989)with R
V
=3.1 and
A
V
=0.077 (Schlegel et al. 1998). The errors on the spectra
were calculated from the standard deviation of the flux density
values in the wavelength range where there are no significant
line features. The wavelength range higher than 7000 Åis
affected by the fringing of thinned CCD. Also, there could be
atmospheric absorption features due to water vapor around
7200–7300 Å. Hence, the spectral region above 7000 Åis hard
to analyze. Though Hβ(λ
obs
∼6970.3 Å)appears to be the
strongest among the emission lines in the spectrum, the O
2
B-
band (λ
obs
∼6884 Å)absorption hinders the measurement of
Hβline properties. Therefore, we chose the broad Hγemission
line to explore the variable emission from the BLR region. The
continuum emission was determined from the local spectral
region around the Hγline. Here, we estimated the optical
continuum as well as the emission line properties by modeling
the flux spectra using Sherpa (Version 4.10.2)in Python
(https://doi.org/10.5281/zenodo.593753).
We used Sherpa optical models to analyze the local spectral
region of ∼6020–6450 Å(observer’s frame)where the Hγline
is present. The continuum emission was modeled with
powerlaw component, and Hγemission was modeled with a
broad Gaussian line model. The powerlaw component has
three parameters: the reference wavelength (λ
ref
in Å), the
amplitude (ampl), and the index (α).λ
ref
was fixed at the
average wavelength of the selected region, 6237 Å, while the
other two parameters were left free to vary. The parameters of
Gaussian component are the FWHM in km s
−1
, the central
wavelength (λ
pos
)in Å, the flux, which is the normalization of
the Gaussian, and the skewness parameter skew. All these
parameters were thawed while fitting. Some of the observations
showed atmospheric O
2
C-band absorption features around
6288 Å. Therefore, we removed this part of the spectrum
during the analysis. Examples of the spectral fitting results are
shown in Figure 2.
The emission line flux (F
Hγ
)values were directly obtained
from the parameter Flux of the best-fitGaussian line. The
continuum fluxes (F
C,opt
)for two different regions, on either
sides of the broad Hγcomponent, with λ
obs
: 6024–6092 and
6380–6452 Åare calculated by integrating the power-law
function defined in the model,
fampl . 1
ref
() ( ) ()lll=a
The strength of an emission line can be estimated by
measuring the equivalent width (EW). We estimated the
equivalent width of the Hγline by integrating the quantity
[(F
λ
/F
λ,cont
)−1]over the spectral range local to Hγ. Here, F
λ
Figure 2. Spectral fitting plots of local continuum and Hγline emission with powerlaw and Gaussian (after ignoring the region of atmospheric absorption feature)at
high- (left)and low-optical-flux (right)states. The upper panel in each plot shows the de-reddened spectrum and the best-fit model. The lower panels show the
residuals associated with the spectral fits.
3
The Astrophysical Journal, 939:76 (11pp), 2022 November 10 Ezhikode et al.
and F
λ,cont
are, respectively, the total flux density and cont-
inuum flux density at each wavelength (λ)in the region. The
Hγequivalent widths, continuum fluxes, and the other para-
meters obtained from the spectral analysis of all the observa-
tions are shown in Figure 3. We also estimated the flux density
ratio F
5044 Å
/F
6064 Å
for wavelengths 5044 and 6064 Å. The
dependence of the emission line flux, equivalent width, and
F
5044 Å
/F
6064 Å
on the optical continuum flux are shown in
Figure 4.
Please note that the entire optical spectral analysis has been
done in the observed frame, and all the parameters we derived
are in the same frame of reference.
4. Temporal Correlations
The light curves of the optical continuum, Hγline and
γ-ray
>100 MeV
fluxes shown in Figure 5exhibit variability. The
strength of variability of these emission components is discussed
in the next section (Section 6). To check the presence of any
delayed response between the optical and γ-ray emissions from
PKS 1222+216, we performed a cross-correlation analysis. As
our light curves have a lot of gaps, we used the discrete
correlation method developed by Edelson & Krolik (1988). This
method can perform the cross-correlation analysis of unevenly
sampled time-series data without using any interpolation. We
estimated the discrete correlation function (DCF)of the light
Figure 3. Best-fit parameters from the analysis of the optical spectral region (6024–6452 Å)local to Hγline using the models powerlaw and Gaussian. The left panels
show the power-law properties such as the continuum flux (F
C,opt
)in the 6024–6092 Åand 6380–6452 Åwave bands (observer’s frame), spectral index (α)and
amplitude (ampl)of the model. The right panels give the emission line properties of Hγ, say, line flux (F
Hγ
), FWHM, best-fit central wavelength (λ
pos
), and the
equivalent width (EW)of the emission line.
4
The Astrophysical Journal, 939:76 (11pp), 2022 November 10 Ezhikode et al.
curves using PyDCF
6
(Robertson et al. 2015)method. The
cross-correlation plots we obtained for γ-ray/optical continua and
γ-ray/Hγlight-curve pairs, for a lag range of ±1200 days and a
lag bin of 20 days, are shown in Figure 6. No strong correlation is
observed for γ-ray
>100 MeV
/Hγlight curves for the time-lag range
of ±1200 days. However, the plot shows a moderately strong
positive correlation between the γ-ray
>100 MeV
and F
C,opt
(6024–
6092 Å)light curves with a DCF value around 0.45.
In order to find the confidence limits of the observed correlation
between γ-ray and optical continuum fluxes, we followed the
bootstrap method. We simulated 10,000 random subsets of the
original light-curve pairs (γ-ray
>100 MeV
and F
C,opt
(6024–6092 Å)
in our case). Each subset consists of a maximum of 80% of the
original data points. Then, using PyDCF, we estimated the cross-
correlation of each (γ-ray/optical continuum)pair of the simulated
light curves for a lag range of ±1200 days and lag bin size of 20
days. The centroid DCF of the cross-correlation functions were
determined from the average of the DCF values higher than 60%
of the maximum DCF. Then, we estimated the peak and the
confidence intervals (68%, 95%, and 99%)of the DCF from the
distribution of the centroid values obtained with the simulations.
To determine the significance of correlations, we estimated the 1σ,
2σ,and3σconfidence levels for the distributions of simulated
DCF after subtracting the original DCF value for each lag. We
found that the observed correlation is significant at the 3σlevel
overabroadrangeoflags.
We determined the centroid lag from the average of the time
lag values corresponding to the top 60% of DCF values (for
γ-ray
>100 MeV
−F
C,opt
)mentioned above. The peak and con-
fidence limits of the centroid time lags were also obtained in the
same manner. The distribution of centroid lag values, peaking
around 98 days, obtained from the bootstrap analysis is shown in
Figure 7. The determined lag is, however, consistent with zero at
the 2σlevel. In addition, we tried the block bootstrap method to
estimate the time lag of the correlation, considering that the data
points in blazar light curves are mutually dependent. Block
bootstrap is a simulation method used to estimate the distribution
of test statistics (Loh 2008; Goncalves & Politis 2011; Itoh et al.
2016). Here we considered the nonoverlapping block bootstrap
method to re-sample time series data to estimate the correlation
time lag. The original light curves were split into nonoverlapping
blocks of 10 days length. We then randomly resampled the data
sets based on these blocks from the original data over 10,000
times to obtain the bootstrapped samples for each light curve.
However, even with the block bootstrap method, we could not
constrain the time lag of the correlation between the Fermi-LAT
and optical continuum light curves. The peak of the centroid lag
distribution obtained through this method is also consistent with
zero time lag at the 1σlevel.
As Fermi normally operates in an all-sky scanning mode, the
γ-ray band light curve is obtained with a sampling time of 3
days. The optical data, however, suffer from observational
gaps. There are two types of gaps (1)seasonal observing cycles
and (2)gaps within the observing cycle. Therefore, we also
estimated the correlations between the Fermi-LAT γ-ray band
and optical continuum by considering the optical observational
cycles separately. However, we could not obtain a significant
estimate of the time lag for these seasonal light curves.
5. Spectral Energy Distribution
PKS 1222+216 is known to show significant disk contrib-
ution in the optical/UV band (e.g., Kushwaha et al. 2014a),
apart from the synchrotron emission. To understand the
variability of disk emission in the source, we generated
optical–γ-ray SEDs for flaring (MJD: ∼56974.6–56977.8)and
quiescent (MJD: ∼55651–55685)epochs in the γ-ray band. The
γ-ray spectra were obtained from Fermi-LAT observations,
while the optical/UV and X-ray data were obtained from Swift
X-Ray Telescope (XRT)and UltraViolet and Optical Telescope
(UVOT)observations. The XRT spectra were generated using
theonlinetoolBuildSwift-XRTproducts
7
(Evans et al. 2009).
The corresponding UVOT filter data (de-reddened)were
obtained from the Multi-Mission Interactive service Archive
provided by the Space Science Data Center (SSDC). The high-
energy Fermi-LAT spectra were generated as described in
Section 2.1.
The assembled data for the chosen epochs were used to
investigate the spectral change during the flaring and quiescent
states (see Figure 8). The double-hump SED shape of blazars is
usually interpreted within the leptonic scenario. Standard one zone
leptonic scenario for FSRQ considers a single emitting region
covering the jet cross section responsible for extended emission
from IR to GeV energies. The variability period constraints the
size of the emitting region, usually within the BLR (Dermer et al.
2009; Ghisellini & Tavecchio 2009); however, regions outside the
BLR are equally viable (Marscher et al. 2008;Sikoraetal.2008).
Figure 4. Variation in the Hγflux (left), the equivalent width (middle), and the flux density ratio between 5044 and 6064 Å(right)with the narrowband optical
continuum flux.
6
https://github.com/astronomerdamo/pydcf
7
https://www.swift.ac.uk/user_objects/index.php
5
The Astrophysical Journal, 939:76 (11pp), 2022 November 10 Ezhikode et al.
We use the open source package Jetset to model the broadband
SED (Massaro et al. 2006; Tramacere et al. 2009,2011;
Tramacere 2020). Three components are considered to be
contributing to the high-energy emission in the second hump:
disk,BLR,andtorus.
The disk emission is probably imprinted in the UVOT data
points. We use the highest-frequency filter (UVW2)in the UVOT
energy range to estimate the disk luminosity (L
disk
)as in
Tavecchio et al. (2011). This produces a lower limit of disk
luminosity at L
disk
=3.75 ×10
45
erg s
−1
, which is of the same
order of magnitude as in Tanaka et al. (2011). The radiation from
the disk photoionizes the BLR, modeled as a spherical shell of
radius R
BLR
with inner and outer radii of
R
R0.9
BLR BLR
in ~and
R
R1.1
BLR BLR
out ~, respectively. We set
R
L10 d
BLR 17 ,4
5
12
=as in
Ghisellini & Tavecchio (2009). A fraction of disk emission is
reemitted by the dusty torus. We consider torus to be emitting as a
blackbody with temperature T
DT
=1000 K approximated as a
spherical volume of radius R
DT
=7×10
18
cm. We consider the
emitting region to be a sphere covering the entire cross section of
the jet and comoving with the bulk Lorentz factor. We considered
an emitting region outside BLR at 5.2 ×10
17
cm corresponding to
a variability time of 1 day for the flaring epoch and 2 ×10
18
cm
for the quiescent period corresponding to a variability time of
Figure 5. Light curves of Hγemission line flux (upper), optical continuum fluxes for two wavelength regions (red: 6024–6092 Åand blue: 6380–6452 Å; middle),
and γ-ray flux (Fermi-LAT: 3σdetection; lower). Different markers for the optical light curves (upper and middle panels)represent various cycles used in the analysis.
The error bars for the optical continuum flux values are smaller than the size of the markers, hence are not visible in the plot.
6
The Astrophysical Journal, 939:76 (11pp), 2022 November 10 Ezhikode et al.
4 days. The parameters used for the SED fitting are listed in
Table 1. The value of N during SED fitting of flaring state
increases by 2 orders of magnitude, hinting at a particle
acceleration process (shock or magnetic reconnection).The
dominating source of external seed photons for the observed
high-energy emission seems to be the BLR during the flaring
period. However, the dusty torus contributed most of the observed
high-energy emission during the quiescent state.
6. Results
In this work, we studied the connection between the optical
and γ-ray emissions from PKS 1222+216 monitored with
Steward Observatory and Fermi-LAT. We have analyzed the
long-term optical spectroscopic observations of the source. We
estimated the broad Hγemission line and the continuum fluxes
for all the observations by fitting the spectra with the powerlaw
and Gaussian models in Sherpa. The optical continuum flux
varies by a factor of ∼7 from about 7.64 ±0.04 ×
10
−14
erg cm
−2
s
−1
to 52.39 ±×10
−13
erg cm
−2
s
−1
.The
power-law index αranges from −0.32 ±0.07 to −1.56 ±
0.13. The parameters of the Gaussian component range as
follows: F2.10 0.13 0 3.69 10 erg cm s
H0.18
0.14 14 2 1
()= - ´
g-
+---
,
F
WHM 2739.46 5078.85 km s
161.48
890.16 406.15
412.38 1
()=-
-
+
-
+-,pos
l
=
6216.01 6245.54
0.84
3.68 4.25
4.40
(
-
-
+
-
+)Å,andEW=(2.96 ±0.08 −
31.42 ±1.56)Å. We observed significant variability in the
parameters that were confirmed by fitting the light curve of each
parameter by a constant, which resulted in reduced χ
2
>2.
However, we note that the variability in the Gaussian parameters
could also be contributed by the variable instrumental resolution
of the spectra used in the analysis.
We measured the strength of variability of the parameters over
the total monitoring period in terms of the fractional rms
variability amplitude (F
rms
; Vaughan et al. 2003). The optical
continuum emissions show significant variability with F
rms
∼0.4,
whereas the Hγflux is less variable with F
rms
∼0.08. The F
rms
values of the parameters for the overall monitoring period and
separately for different cycles are quoted in Table 2.We
calculated the equivalent width of the Hγline in each observation
from the continuum and Hγemission line flux density for the
spectral range. There is an inverse relationship between the
equivalent width and the underlying continuum, as observed in
other blazars (e.g., Patel et al. 2018). The observed variation of
EW with the continuum flux is shown in Figure 4. The inverse
trend between these quantities suggests that the Hγline is less
variable than the continuum emission. Also, the flux density ratio
between 5044 and 6064 Ådecreases as the flux increases showing
the “redder-when-brighter”behavior (see Figure 4). A similar
trend has been previously reported for this object for the flux
density ratio between 4700 and 6600 Åwith V-band magnitude
obtained from SO observations (Smith et al. 2011). The other
parameters obtained from the analysis did not show any direct
correlations using the nonparametric Spearman’s rank-order
method.
The Fermi-LAT light curve of PKS 1222+216, obtained
roughly for the same observation period (∼2011–2018), shows
strong variability (F
rms
∼0.9)with flaring events at various
epochs. The optical continuum is also found to have a few
flaring episodes. It is known that the optical and γ-ray
emissions from blazars can be correlated with or without time
lag (e.g., Cohen et al. 2014; Majumder et al. 2019).To
investigate the possible temporal correlation in optical and γ-
ray bands, we performed the cross-correlation analysis of the
F
γ‐ray>100 MeV
/F
C,opt
(6024–6092 Å)light curves. The analysis
provided no significant correlation for Hγemission with γ-ray
continuum. The long-term optical continuum and LAT light
curves show a moderately strong positive correlation, sig-
nificant at 99% level (see Figure 7). However, we caution that
the time lag of the correlation is not significant at the 2σlevel.
Hence, we do not confirm a strong time-delayed correlation as
Figure 6. Cross-correlation plots obtained with PyDCF for a time range of ±1200 days and lag bin size of 20 days for γ-ray (F
γ‐ray>100 MeV
)and optical continuum
(F
C,opt
(6024–6092 Å)) light curves (left)and γ-ray F
γ‐ray>100 MeV
and Hγ(F
Hγ
)light curves (right).
7
The Astrophysical Journal, 939:76 (11pp), 2022 November 10 Ezhikode et al.
Figure 7. Results from the bootstrapping method for F
γ‐ray>100 MeV
and F
C,opt
(6024–6092 Å)light curves. (a)PyDCF output with the centroid values of DCF
(DCF
cent
; dotted horizontal line)and lag τ
cent
(dashed vertical line)for the original data. (b)DCF
cent
distribution obtained from cross-correlation results of the
bootstrap output light curves. The peak of the distribution is represented by the solid magenta horizontal line. The 68%, 95%, and 99% confidence limits of DCF
peak
are shown by red, green, and blue dashed lines, respectively. (c)Distribution of the centroid lag values obtained from cross-correlation analysis of the simulated light
curves. The solid brown vertical line indicates the peak of the distribution (τ
peak
). The confidence limits of τ
peak
are plotted in red, green, and blue dashed lines. (d)
Plot showing the significance of the correlations at the 1σ(red dashed line),2σ(green dashed line), and 3σ(blue dashed line)levels for different lags.
Figure 8. SEDs of the source fitted with the Jetset code for the γ-ray flaring and quiescent epochs. The optical/UV photometric data were obtained from Swift UVOT
filters (V,B,U,UVW1, UVM2, and UVW2), and the X-ray (0.3–10 keV)spectra were taken from XRT observations. LAT spectra were used for the high-energy γ-ray
emission. The upper limits in the LAT spectra are plotted as downward arrows.
8
The Astrophysical Journal, 939:76 (11pp), 2022 November 10 Ezhikode et al.
the time lag is too broad in this case. It is probable that the gaps
between the observational cycles of optical monitoring dilute
the possible strong correlations. We also carried out a similar
analysis for the seasonal light curves, but no significant
correlation was found among the light curves.
Previous studies (Ackermann et al. 2014; Cohen et al. 2014)
have analyzed the temporal correlation between the γ-ray and
optical emissions from PKS 1222+216 at the flaring episode in
2010. Ackermann et al. (2014)found that γ-ray is leading the
optical by ∼35 days with cross-correlation value of ∼0.4
whereas Cohen et al. (2014)detected the γ-ray lead with a time
lag of ∼8.6 days. In a γ-ray (0.1–300 GeV), X-ray (0.2–10
keV), and optical (Rband)cross-correlation study of a sample
of low-synchrotron-peaked (LSP)and high-synchrotron-
peaked (HSP)blazars, Meyer et al. (2019)found multiband
variability with no time delay. Another work by Sbarrato et al.
(2012)studied the relation between the accretion rate and the
jet power of a sample of blazars using the Sloan Digital Sky
Survey and Fermi observations. They found a clear positive
correlation between the broad emission lines and γ-ray
luminosities in Eddington units, supported by the Kendall test.
7. Summary and Discussion
The high-energy emission in FSRQs is expected to be produced
by the external Comptonization scenario. The source of seed
photons for EC process could be the accretion disk, BLR, or torus
(e.g., Dermer et al. 1997; Bottcher 1999; Arbeiter et al. 2002).As
the observed optical continuum luminosity is a combination of
disk and jet emissions, the broad emission line luminosity in
FSRQs provides an excellent probe of disk emission. In an
attempt to study the relationship between various emission
components in the FSRQ PKS 1222+216, we carried out a
comprehensive investigation of the ∼8 yr long optical/γ-ray
monitoring observations, as described in the previous sections.
We implemented a detailed spectral analysis to retrieve the
optical emission properties that reveal strong variability in the
optical continuum flux, whereas the Hγline is less variable. The
equivalent width of the Hγline shows an inverse correlation with
the optical continuum emission. This trend, along with the nearly
constant line flux, show that the line emission is varying slowly
compared to the underlying continuum as suggested by the well-
known Baldwin effect (Baldwin 1977). The source also exhibits a
redder-when-brighter trend of decreasing flux density ratio
Table 1
Comparison of Parameters from SED Fitting on the Quiescent and Flaring States
Parameter Parameter Type Quiescent Flaring
[MJD 55651–55685][MJD 56974–56978]
R
em
Size of emitting region 1 ×10
17
cm 2.6 ×10
16
cm
Γ
j
Bulk Lorentz factor 10 10
θJet viewing angle 3 3
zRedshift 0.432 0.432
L
disk
Disk luminosity 3.75 ×10
45
erg s
−1
4.5 ×10
45
erg s
−1
T
disk
Peak disk temperature 2.5 ×10
4
K 2.75 ×10
4
K
R
BLRin Inner radius of BLR 1.9 ×10
17
cm 1.9 ×10
17
cm
R
BLRout Outer radius of BLR 2.3 ×10
17
cm 2.3 ×10
17
cm
τ
BLR
Fraction of disk luminosity reflected by the BLR 0.15 0.15
R
DT
Radius of the dusty torus 7 ×10
18
cm 7 ×10
18
cm
T
DT
Dust temperature 1000 K 1000 K
τ
DT
Fraction of disk luminosity reflected by the torus 0.2 0.2
BMagnetic field within the emitting region 0.26 G 0.6 G
NParticle number density 20 cm
−3
220 cm
−3
min
gLow-energy cutoff 100 100
max
gHigh-energy cutoff 4 ×10
3
4.0 ×10
3
γ
break
Turnover energy 800 800
p Low-energy spectral slope 2.3 2.1
p
1
High-energy spectral slope 3.3 3.4
Table 2
F
rms
for the Parameters for Different Cycles and Total Periods (Overall)of Observations
Cycle F
rms
F
Hγ
F
C,opt
(6024–6092 Å)F
C,opt
(6380–6452 Å)αF
γ‐ray>100 MeV
C-3 0.08 ±0.01 0.3438 ±0.0003 0.348 ±0.001 0.21 ±0.01 0.51 ±0.03
C-4 0.06 ±0.01 0.1429 ±0.0003 0.147 ±0.001 0.14 ±0.01 0.38 ±0.04
C-5 0.05 ±0.01 0.0742 ±0.0003 0.080 ±0.001 0.11 ±0.01 0.49 ±0.04
C-6 0.06 ±0.01 0.3235 ±0.0002 0.316 ±0.001 0.29 ±0.01 0.59 ±0.02
C-7 0.07 ±0.01 0.1767 ±0.0002 0.175 ±0.001 0.16 ±0.01 0.63 ±0.04
C-8 0.06 ±0.01 0.2185 ±0.0004 0.223 ±0.001 0.12 ±0.02 0.34 ±0.10
C-9 0.02 ±0.01 0.0673 ±0.0003 0.066 ±0.001 0.15 ±0.02 L
C-10 0.06 ±0.01 0.0232 ±0.0007 0.021 ±0.002 0.07 ±0.03 L
Overall 0.084 ±0.004 0.4291 ±0.0001 0.4270 ±0.0003 0.235 ±0.004 0.92 ±0.01
(C-3—C-10)
9
The Astrophysical Journal, 939:76 (11pp), 2022 November 10 Ezhikode et al.
(F
5044 Å
/F
6064 Å
)with an increase in the optical continuum flux.
As the synchrotron emission peaks at the infrared wavelengths in
FSRQs, and the accretion disk contributes more to the bluer part
of the SED, the redder-when-brighter behavior clearly points
toward an increasing contribution from the jet at high-flux states
in the optical band. Similar trend was observed by Smith et al.
(2011)inthesamesourceandotherblazars(e.g., Zhang et al.
2015). All these trends indicate that the contribution of the
accretion disk to the optical continuum variability is less
significant in the source.
The variability studies of the various emission components
clearly show that the γ-ray (>100 MeV)flux is highly variable in
the whole period of observation, with an F
rms
of ∼0.9. The γ-ray
emission appears to be more variable than the optical line and
continuum components in the individual cycles of the optical
monitoring program as well. The SED analysis and previous
studies rule out the contribution of thermal disk emission to the
variable γ-ray (>100 MeV)emission. The higher variability in
F
γ‐ray>100 MeV
and the moderate positive correlation we observed
between the F
γ‐ray>100 MeV
and F
C,opt
(6024–6092 Å)emissions
can be attributed to the enhanced particle acceleration or cooling
process at the flaring/quiescent epochs.
In this work, we have quantified the γ-ray/optical correlation
in PKS 1222+216 using SPOL and Fermi-LAT monitoring
observations and studied the relationship between the emis-
sions from the jet, disk, and BLR regions. The DCF analysis
did not provide any strong correlation between Hγline and γ-
ray (>100 MeV)fluxes. As the broad emission lines arise due
to the photoionization of the gaseous material by the disk
photons in the BLR region (e.g., Netzer & Peterson 1997;
Kaspi et al. 2000; Peterson et al. 2004; Haas et al. 2011; Zhang
et al. 2019), the aforementioned noncorrelation most likely
rules out the possibility of disk photons acting as the source of
variability observed in γ-rays. This, in turn, implies that the
variability of the γ-ray emission is most likely intrinsic to the
high-energy particles. The lack of strong variability of the Hγ
line also suggests weak variability of disk component, and the
observed correlation between F
C,opt
(6024–6092 Å)and
F
γ‐ray>100 MeV
can be attributed to the jet contribution in both
optical and γ-ray emissions. The observed correlation between
γ-ray and optical continuum emissions is consistent with a zero
time lag at the 2σlevel. We note that the absence of a strong
correlation and difficulty constraining the time lags could be
due to the gaps between the optical observing cycles.
Obviously, studies with long-term high-cadence monitoring
can precisely determine the presence of time-delayed correla-
tion in the source.
The results from the SED analysis indicate that the location
of the emitting region of size 2.5 ×10
16
cm was at 0.2 pc for
the flaring epoch, whereas for the quiescent state an emitting
region of size 1 ×10
17
cm was apparently located at 0.6 pc.
This suggests that the BLR photons significantly contribute to
the high-energy emission in the flaring epochs. An increased N
during flaring epochs also hints at an increased number of high-
energy electrons through particle acceleration via shocks or
magnetic reconnection. On the other hand, the dusty torus
component apparently contributes to the seed photons for the
EC process in the quiescent state. This can be explained by the
variation in the energy density of the radiation fields (magnetic
and external)as the distance from the center increases. The
observed results points to a scenario where the blazar output is
dominated by the inverse Compton scattering of the external
radiation, and the jet dissipation occurs at a distance of a few
hundreds of Schwarzschild radius from the central black hole,
as supported by the canonical jet model (Ghisellini &
Tavecchio 2009; Sikora et al. 2009). Our result suggests that
the γ-ray emitting region lies in the BLR/torus regions in the
FSRQ PKS 1222+216, in agreement with former studies on the
source during its active and quiescent states (e.g., Tavecchio
et al. 2011; Ackermann et al. 2014; Chatterjee et al. 2021). This
is also consistent with some recent studies of other FSRQs
(e.g., Paliya 2015; Shukla & Mannheim 2020). In future work,
we will explore the optical and γ-ray correlations of a large
number of sources to establish the disk–jet connection in
blazars.
We thank the anonymous referee for the insightful comments.
Data from the Steward Observatory spectropolarimetric monitor-
ing project were used. This program is supported by Fermi
Guest Investigator grants NNX08AW56G, NNX09AU10G,
NNX12AO93G, and NNX15AU81G. We acknowledge the use
of Fermi-LAT data and analysis tools from Fermi Science
Support Center. This work made use of data supplied by the UK
Swift Science Data Centre at the University of Leicester. Part of
this work is based on archival data, software or online services
provided by the Space Science Data Center—ASI. S.H.E. would
like to thank Kavita Kumari for discussions on time-series
analysis. This research has made use of the NASA/IPAC
Extragalactic Database (NED), which is operated by the Jet
Propulsion Laboratory, California Institute of Technology, under
contract with the National Aeronautics and Space Administration.
ORCID iDs
Savithri H. Ezhikode https://orcid.org/0000-0003-1795-3281
Amit Shukla https://orcid.org/0000-0002-5656-2657
Gulab C. Dewangan https://orcid.org/0000-0003-1589-2075
Pramod K. Pawar https://orcid.org/0000-0003-3188-1501
Sushmita Agarwal https://orcid.org/0000-0001-5507-7660
Blesson Mathew https://orcid.org/0000-0002-7254-191X
Akhil Krishna R. https://orcid.org/0000-0002-6096-3330
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