Coronal properties of the Seyfert 1 galaxy 3C 120 with NuSTAR
Priyanka Rani C. S. Stalin






 
^1Indian Institute of Astrophysics, Koramangala, Bangalore 560034, India. e-mail: priyanka@iiap.res.in


  


We present measurement of the cut-off energy, a proxy for
the temperature of the corona in the nuclear 
continuum of the Seyfert 1 galaxy 3C 120 using ∼120 ks
of observation from NuSTAR. The quality broad band spectrum
from 3-79 keV has enabled us to measure
the Compton reflection component (R) and to constrain the  temperature 
of the coronal plasma.
Fitting one of the advanced Comptonization models, compPS to the observed broad band spectrum
we derived the kinetic temperature of the electrons in the 
corona to be kT_e = 25 ± 2 keV with Compton y parameter 
of y = 2.2 ± 0.1 for a slab geometry and  
kT_e = 26_-0^+2 keV with a y of 2.99_-0.18^+2.99 
assuming a spherical geometry. 
We noticed excess emission from ∼10-35 keV arising due to Compton 
reflection and a broad Fe Kα line at 6.43 keV with an 
equivalent width of 60 ± 5 eV. The variations in count rates in the soft 
(3-10 keV) band is found to be more compared to the hard 
(10-79 keV) band  with mean fractional
variability amplitudes of 0.065±0.002 and 0.052±0.003 for the soft
and hard bands respectively. 3C 120 is known to have a strong jet, however, 
our results indicate that it is either dormant or its contribution if any 
to the X-ray emission
is negligible during the epoch of NuSTAR observation.








§ INTRODUCTION


Active galactic nuclei (AGN) are energetic extragalactic sources believed
to be powered by a complex physical process, namely, the  accretion of matter
on to supermassive black holes (SMBHs) residing at the centres of galaxies
<cit.>. The  matter accreted by the SMBH  forms an
optically thick, geometrically thin accretion disk 
<cit.>, with a temperature of about 
10^4-10^5 K for a SMBH of mass 10^6 to 10^9 M_⊙, and that emits 
predominantly in the optical/UV region of the electromagnetic spectrum.
The components of an AGN therefore  include an accretion disk surrounding the
central SMBH, an X-ray emitting corona, an obscuring torus
surrounding the accretion disk and a relativistic jet in about
15% of AGN <cit.>.  While the accretion disk emits in the 
optical/UV bands, the observed hard X-ray continuum is believed to 
be caused by thermal Comptonization of
the accretion disk photons in a hot (∼10^9 K) optically thin
corona <cit.> 
in the radio-quiet category (those without radio jets) of 
AGN. This Comptonization process gives rise to the observed 
power-law X-ray  spectrum in AGN and the shape of this observed spectrum 
depends on the seed photon field, the kinetic temperature of the plasma 
kT_e, the optical depth τ  and the geometry of the corona. Arguments 
in literature indicate the corona to lie between 3 - 10 R_g above the 
central SMBH <cit.>. Here, R_g = GM/c^2 is the 
gravitational radius
of a SBMH with mass M_BH.  In addition to
the primary power law continuum, the X-ray spectrum also contains 
a Fe Kα line at 6.4 keV. The origin of this line, unambiguously 
detected for the first time in the source MCG-6-30-15 
<cit.> is attributed
to the reflection of the power law photons from the hot corona by the
relatively cold accretion disk <cit.>. This line
is now observed in many AGN <cit.>.
Therefore, X-ray observations of AGN provide very important clues to the
physical processes that happen close to the SMBH. 



A feature in the hard X-ray spectra of AGN  that arises from the 
thermal Comptonization
process is the presence of a high energy cut off (E_cut) which is related
to the plasma electron temperature (kT_e) of the corona that is found to 
range between 50 to 
100 keV. 
Measurements of E_cut 
for about hundred AGN <cit.> are 
available as of today based on 
observations using older observatories such as the CGRO
<cit.>, BeppoSAX <cit.>
and the currently operating satellite 
INTEGRAL <cit.>. 
 For example, <cit.>  using BeppoSAX data in the energy range 2 - 100 keV 
has provided E_cut measurements for 34 sources and lower limits
for another 54 sources. 
Also, <cit.> using data from XMM, INTEGRAL and
Swift has provided E_cut values for 26 AGN and lower limits for 11 others.
Most of these measurements have large error bars, 
which may be due to  
the quality of the data itself 
as well as the degeneracy that is known to prevail between E_cut 
and other physical properties of the sources such as the slope of
the primary X-ray power law, and the amount of radiation that are
Compton up scattered by the circumnuclear material <cit.>.

One of the important physical prameters of the Comptonizing corona in an AGN is its plasma temperature kT_e. The value of 
kT_e in many instances is indirectly obtained by fitting simple phenomnological models such as the 
cut-off power law to the observed X-ray spectrum to find E_cut and subsequently kT_e is deduced
using certain approximations. However, kT_e values are available for many
AGN based on physical model fits to AGN spectra <cit.>.
 Disentangling the different spectral components that are present in the 
X-ray spectra of AGN and constraining their characteristic parameters require
high S/N data covering a wide energy range.
These limitations are now overcome to a large extent owing to the availability
of the focussing hard X-ray telescope, 
the Nuclear Spectroscopic Telescope Array, NuSTAR 
<cit.>, due to its 
high sensitivity and  wide energy coverage from 3 - 79 keV.
 However, NuSTAR too can provide precise measurements only for sources
with E_cut < 150 keV <cit.>. Measurements beyond this from NuSTAR
will not be reliable due to its own lack of effective area beyond 79 keV.
Using NuSTAR, E_cut as well as the 
more physical coronal plasma temperature kT_e has been measured in about a 
dozen AGN <cit.>
and also summarized by <cit.>. 
Though NuSTAR observations have enabled us to characterise the 
coronal properties of about a dozen AGN, to have a complete knowledge of 
the geometrical and physical properties of the X-ray corona in AGN,
 there is a need to 
extend such studies on the X-ray properties to a large sample of AGN.

In this paper, we present the results  of our analysis of X-ray data 
on 3C 120 observed by NuSTAR for a total duration of about 120 ks.


3C 120 is a X-ray bright Seyfert 1 galaxy at z = 0.033
<cit.> and  
having a black hole mass of 5.6 × 10^7 M_⊙ <cit.>. 
It is also classified as a broad line radio galaxy (BLRG) by 
<cit.>. It has a radio morphology similar to 
the FRI category of AGN <cit.>.
Its one sided jet has an inclination to the line of sight 
of ∼ 14^∘ <cit.> which is  based on 
the  apparent superluminal speed β_app reported by
<cit.>. The jet is known to extend on scales up to
100 kpc <cit.>. It has been found to be variable in
X-rays. A broad Fe Kα line well fitted
by a Gaussian with a σ of 0.8 keV and having an equivalent width of
400 eV has been found from ASCA observations <cit.>.
It has not been detected in γ-rays by the Compton Gamma Ray 
Observatory (CGRO,). However, using data from 
the Oriented Scintillation Spectroscopy Experiment (OSSE,) 
on board CGRO, <cit.> found the
presence of a spectral break in 3C 120 between X-rays and soft γ-rays.
3C 120 was detected in Fermi
using the first 15 months of data <cit.>, but
not detected in the second Fermi-LAT catalog (2FGL, 
) and the third Fermi-LAT
catalog (3FGL, )
indicating that the source is variable in the high energy γ-rays.
Using 180 and 365 days binning on the data obtained between
August 2008 - December 2013, <cit.> found 
γ-ray flux variations. Using 5 days binned light curve
<cit.> noticed that 3C 120 was detected only 
at certain epochs.

In this work, we focus on both the timing and broad band (3-79 keV) spectral 
analysis. Though 3C 120 has
been studied for flux variability as part of the analysis of a large sample
of AGN by <cit.>, analysis of the broad band
X-ray data to constrain its coronal properties using NuSTAR has 
been carried out for the first time. However, from BeppoSAX
observations, <cit.> have estimated a 
E_cut of 100-300 keV. Also, <cit.> using the average OSSE spectrum 
together with ASCA data reported a E_cut of 130_-40^+150 keV. Using data 
from several telescopes <cit.> obtained a value of kT_e = 176_-23^+24 keV. 
This paper is organised as follows.
 In Section 2 we report on the NuSTAR observations and data reduction. 
In Section 3 we present the analysis of the 
NuSTAR data. The results are discussed in Section 4 followed by
the summary in the final section.






§ OBSERVATION AND DATA REDUCTION

3C 120 was observed by NuSTAR <cit.> on 
6^th 
February 2013 (ObsID 60001042003) for 127 ks in the 3-79 keV band. 
The data was reduced using the NuSTAR Data Analysis Software 
package NuSTARDAS v.1.6.0 distributed by the High Energy Astrophysics 
Archive Research Center (HEASARC). We generated the cleaned and screened 
event files taking also into account the passage of the 
satellite through the South Atlantic Anomaly using the nupipeline task and using CALDB 20161207. A 
circular region of radius 60” was taken centered
 on the peak of the source image to extract 
the source spectrum and light curve. To extract the background spectrum
and light curve we again selected a circular region of
 radius  60” away from the 
source  on the same detector. Light curves were generated with 300 seconds 
binning in the 3-79 keV band for both the focal
plane modules, FPMA and FPMB. They were further 
divided into soft (3-10 keV) and hard (10-79 keV) bands. To get the final 
light curves, the count rates from the two modules FPMA and FPMB were 
combined using the lcmath task in FTOOLS V6.19. 

We generated the source and background spectra and response files in the 
energy range of 3-79 keV in each focal plane module FPMA and FPMB using the 
nuproducts package available in NuSTARDAS. Instead of combining 
the spectra or responses from FPMA and FPMB, we fitted them simultaneously, 
allowing the cross normalization for both modules to vary freely in all fits. 
We used XSPEC (version 12.9.0; ) for 
all spectral analysis. During the various model fits in XSPEC, we 
used <cit.> set of solar abundances and the <cit.> photoelectric cross 
sections. The χ^2 minimization technique in XSPEC was used 
to get the best model description of the data and all errors were 
calculated using χ^2=2.71 criterion, i.e., 90% confidence level.






 To check for any possible effects
on our choice of the background region used to get the light curves and the
source spectrum, we also
used a background found in an annular region around the source having an
inner radius of 50 pixels and an outer radius of 80 pixels (1 pixel = 2.46 arcsec). The mean
count rates of the source light curve in the 3-79 keV band using the background taken from the circular 
region away from the source and using the annular region surrounding the source
are 3.88 ± 0.20 and 3.84 ± 0.20 respectively. The choice of the background thus has neglible
effect on the light curves presented here. Similarly, the average fluxes
of the source in the 3-79 keV band obtained from a simple power law fit to the spectrum  
are (1.372 ± 0.007) × 10^-10 and  (1.237 ± 0.007) × 10^-10 erg s^-1 cm^-2 respectively
using the background taken from the circular region away from the source and the annular region surrounding the source. 
The difference in the fluxes deduced from the spectra using two choices of the background
selection is less than 10%. 
Thus, the selection of background region has no significant impact on the spectral and timing analysis carried out on 
3C 120.



§ ANALYSIS OF THE DATA



 §.§ Flux variability

In Figure <ref> we show the light curves in two energy bands 3-10 keV and
10-79 keV as well the hardness ratio (HR) curve. The HR is evaluated as
the ratio between the count rates in the  10-79 keV to 3-10 keV bands. The source
is found to show variations in both the soft and hard bands. To 
characterise the flux variations from observed count rates, we used the 
fractional root mean
square variability amplitude (F_var) which gives an estimate of the 
intrinsic variability amplitude relative to the mean count rate 
<cit.>.   We found
F_var values of 0.065 ± 0.002 and 0.052 ± 0.003 respectively for the 
soft and hard bands. To characterise the presence of spectral variations
if any, we calculated the HR and show in Figure <ref> the plot of HR against
flux variations (in the units of counts/sec) in the total (3-79 keV) band. Fit of a linear function 
of the form HR = m × counts/sec (3-79) keV + c, that also
takes into consideration the errors in both HR and the 3-79 keV count rates, 
gave values of -0.019 ± 0.005 and 0.465 
± 0.022 for m and c respectively. The spectrum is thus found not to change
with the brightness of the source. 


  

  



 §.§ Spectral analysis

Analysis of the spectrum of 3C 120 along with  model fittings was
carried out using the XSPEC package.
To find the best fit models, the χ^2 minimization technique 
in XSPEC was used.











































Analysis of X-ray spectra are generally attempted by fitting simple 
phenomenlogical models such as the simple power law model as well as
the power law with exponential cut-off. Using power law fits, <cit.> 
obtained a value of Γ = 1.85 ± 0.01. Similarly, using the 
pexrav model, the same authors obtained value of the high energy
cut-off (E_cut) to be 83^+10_-8 keV. 
However, these models are too simplistic. 
Also, the power law with an exponential cut off 
that is incorporated in 
pexrav has limitations in its approximation of the steep decline of the 
true high energy cut off 
<cit.>. 
Thus, both the models are too simplistic to apply to the good quality of the 
data from NuSTAR that is analysed here. Therefore, the following more physical models
were fit to the data.



  §.§.§ CompTT model

We used the Comptonization model (CompTT; 
) convolved  with a reflection component so as to 
get the coronal parameters. 
This model has the form TBabs × zTBabs × 
(zgauss+CompTT+refl(CompTT)). 
The first component of this model TBabs <cit.> 
includes galactic absorption, with
the galactic neutral hydrogen column density frozen to the value of 
1.11 × 10^21 cm^-2 obtained from <cit.> 
using the nH tool in HEASARC[https://heasarc.gsfc.nasa.gov/cgi-bin/Tools/w3nh/w3nh.pl] and the second component 
zTBabs represents absorption intrinsic to the host galaxy of the source. 
Redshift was fixed to z=0.033 and the column density zTBabs was kept as a free parameter 
in the fitting. 
The CompTT component in this model assumes 
a geometry for the corona (a slab or spherical) and models the intrinsic 
coronal continuum, and refl convolves it with reflection features 
<cit.>. 


For slab geometry the reduced 
χ^2 was 0.986 (χ^2/ν=3734/3788) and for spherical
geometry too it was 0.986 (χ^2/ν=3734/3788). For the slab 
geometry, 
we found the mean value of kT_e = 9_-3^+2 keV and τ = 2.4_-1.1^+0.6 considering 
primary and reflected emission. 
at the 90% confidence. For the spherical geometry 
the best fit yielded the mean value of kT_e = 16_-7^+6 keV 
and τ = 5.1_-0.4^+0.6 for 
primary and reflected emission at the 90% confidence. The CompTT model 
gave huge error bars in the normalization constant.
 The observed spectrum along with the 
fit and residuals are shown in 
Figure <ref> for the spherical geometry 
and Figure <ref> for the slab 
geometry.
The best fit parameters and their errors at 
90% confidence levels are given in Table <ref>.
The 2-10 keV flux determined from the fit is (5.19 ± 0.01) 
× 10^-11 erg cm^-2 sec^-1. 
This gives an unabsorbed luminosity of 
(1.29 ± 0.01) × 10^44 erg sec^-1. Using the 
bolometric correction of 20.6 ± 0.1 found by 
<cit.> we obtained a bolometric luminosity of
(26.656 ± 0.001) × 10^44 erg sec^-1. For a BH mass 
of 5.6 × 10^7 M_⊙ <cit.>,
 the calculated Eddington accretion rate is 
λ_Edd = L_bol/L_Edd = 0.353, where the Eddington luminosity,L_Edd = 
1.36 × 10^38 (M_BH/M_⊙) erg sec^-1. This is similar to the value of λ_Edd = 0.352 
found by <cit.> using data from many telescopes including INTEGRAL.




  §.§.§ CompPS

Though CompTT model well represents the observed spectrum, this 
has limitations such as its simplistic treatment of the seed photons that participate
in the Comptonization process. We therefore fit the spectrum using one of 
the most
advanced Comptonization models available in XSPEC namely 
CompPS <cit.>. CompPS that produces
the continuum through thermal Comptonization processes
incorporates proper treatment of the Comptonization process through
exact numerical solution of the radiative transfer equations.  It also
offers several choices for the geometries. In the fitting process, when
all the parameters were kept free, the fitting failed to converge. Therefore, to avoid  
non-convergence owing to the presence of many free parameters in CompPS fitting, we fixed 
the centroid energy and deviation of the Fe Kα line to be 
6.43 keV and 0.29 keV respectively obtained from CompTT for a slab
geometry. The parameters obtained from the fit along with their
associated errors including the Compton y parameter (y = 4 τKT/m_ec^2, where 
τ is the Thomson optical depth <cit.> and the normalization (N_CompPS)
are given in Table <ref>. 
To compare our results with CompTT, we used 
CompPS model only for slab and spherical geometries. The seed photons 
were assumed to be 10 eV.
 The observed spectra along with the 
fit and residuals are shown in 
Figure <ref> for the spherical and the slab 
geometries. 
For slab geometry we obtained a reduced 
(χ^2/ν=3773/3790) of 0.996, whereas, for the spherical geometry the reduced
(χ^2/ν=3783/3790) was 0.998. For slab and spherical geometry, we found 
R values of 0.80_-0.10^+0.11 and 0.43±0.06 respectively. 
We found that 
the kT_e values obtained from CompPS model is larger than that
obtained from CompTT for both spherical and slab geometries. 




  §.§.§ EQPAIR

The models used above to fit the NuSTAR data of 3C 120
assumes that the electrons involved in the Comptonization process are 
thermal electrons with a Maxwellian energy distribution.
 However, hybrid models for the corona that involves the contribution
of both thermal and non-thermal electrons have been applied to
AGN. For example, in NGC 4151, the contribution of non-thermal 
electrons is found to be less than 15% <cit.>. 
The non-thermal fraction can also be as large as 30% <cit.>.



3C 120 is 
known to have a jet emission <cit.> and
it is likely the observed X-ray emission is a combination of various
components.  
We therefore model the spectrum 
with the EQPAIR model <cit.>, the most
advanced Comptonization model in XSPEC. This model
evaluates the emission spectrum resulting from Comptonization, Coulomb
collisions and pair production. This model  can 
treat the  Comptonization for a different nature of plasma (thermal, 
non-thermal and hybrid plasma) and even incorporates Compton hump from cold 
reflection. This is because in EQPAIR, only electrons with optical depth
(τ_p) are accelerated with total power characterised by the
compactness parameter l_h (such that l_h = L_h σ_T/Rm_ec^2, 
where L_h is the luminosity or the power supplied to the  electrons
in the Comptonization region  and R  
the size of the Comptonization region <cit.> that is split
between thermal distribution with power (l_th) and non-thermal 
distribution with power l_nth and l_h = l_nth + l_th. These electrons then cool either through
Compton scattering of soft photons or through Coulomb collisions. The parameter
that plays an important role in characterising the overall spectral shape in 
EQPAIR is the parameter l_h/l_s. The soft compactness parameter (l_s)
refers to the luminosity of the soft photons that is injected into the
corona and the hard compactness parameter l_h refers to the power
supplied to the accelerated electrons in the source. To model the observed spectrum with EQPAIR, we fixed the centroid 
energy of the Fe Kα emission line to the best fit 
value found from CompTT above for a slab geometry. However, in EQPAIR
model the geometry is spherical and the photons are induced 
homogeneously throughout the spherical cloud. We assumed that the 
input source of soft photons  
in EQPAIR is diskpn, a black body spectrum <cit.> with a peak temperature of 
10 eV . The seed photon distribution can be modified in the model
by changing l_s which for this model fitting was fixed to 10. 
The inclination was fixed to 17 degrees and the iron abundance was taken to be
solar. 
For the purpose of this modelling we  
considered the accelerated particles to be  electrons from the thermal pool.

The best fit parameters are given in Table <ref>. The observed
and fitted spectra along with the residuals are given in Figure <ref>. Similar to CompPS and CompTT models, 
the fit of the data with EQPAIR model too provides a good description of the 
data with a reduced χ^2 of 1.047(χ^2/ν=3972/3791). However, the temperature is not among
the default output parameters returned by EQPAIR though it is calculated
in the model. We therefore used the chatter command (chatter level 
= 15) <cit.>  and obtained kT_e = 23. The  
error in kT_e was obtained using χ^2 minimization technique at the 90 % significance level. Thus, using the
EQPAIR model we found kT_e = 23^+1_-7 keV. 
 We found the best fit ratio of the hard to the 
soft compactness parameter, l_h/l_s = 0.90 ± 0.32. This points to
 similar power in the irradiating soft photons that enter the source region and in the heating of the 
electrons. The EQPAIR model yields the value of ionization parameter 
of the reflector to be ξ = 5.14 ± 11.15. The 
electron optical depth obtained by the fit was τ_p = 0.60 ± 0.08. The ratio l_nth/l_h, 
which gives the
fraction of power supplied to energetic particles that goes into accelerating 
non-thermal particles was found to be 0.78 ± 0.10. The value
of l_nth/l_h is zero for a purely thermal model, while it is unity for a purely non-thermal
model. Though l_nth/l_h obtained from the fit deviates much from zero, the detection of 
a high energy cutoff in the NuSTAR spectrum not much beyond the sensitivity of NuSTAR 
<cit.> and the non-detection of the source in γ-rays suggest that Comptonization 
by non-thermal electrons if any is non-significant.

 The χ^2 from
EQPAIR model fit is poorer compared to CompPS and CompTT, though, the kT_e 
value from EQPAIR model fit agrees to that obtained from CompPS. 
Thus, it is likely that in 3C 120, for the observations analysed here, the 
electrons involved
in the Comptonization process are predominantly thermal.





















































































   



§ DISCUSSION










 §.§ Coronal properties

The availability of high quality NuSTAR data from observations of 
about 120 ks has enabled the determination of the coronal properties of 
3C 120.  The time averaged spectrum covering the 
3-79 keV band, when fitted with the  phenomenological power law model gave 
the continuum power law index of Γ = 1.85 ± 0.01
 <cit.>. However, values of 1.70 and 2.08 were found from 
XMM <cit.> and INTEGRAL <cit.>
observations. From BeppoSAX observations,  
<cit.> found the continuum to be well described
by a power law with Γ∼ 1.85 ± 0.05, which is in 
close agreement with what is found from NuSTAR data
by <cit.>. 
The difference in the  photon index values obtained
from different sets of observations acquired from
different telescopes could point to spectral variations in the source.



Using the pexrav model
with the inclusion of a Gaussian component to account for the presence
of the Fe Kα line in the spectrum, <cit.> found values of 
Γ= 1.87 ± 0.02 and E_cut = 83_-8^+10 keV.



 3C 120 has been observed before by BeppoSAX and OSSE.
 By modelling the BeppoSAX data  with an e-folded power law or a thermal
Comptonization model, <cit.> found a value of  E_cut = 150_-30^+230 keV. 
Using ASCA observation that was contemporaneous with an OSSE observation, 
and modelling the spectra with a broken power law multiplied
by an exponential factor, <cit.> found E_cut = 110_-50^+130
keV. Within error bars, the value of E_cut obtained from 
NuSTAR data using simple model fits matches with that known from BeppoSAX and OSSE data, however,
has improved precision, with  a manifold  reduction in 
the errors. As these observations were taken at different epochs, it 
is also likely the E_cut is variable, but, this cannot be ascertained 
because of the large error bars in its values from earlier observations.
The Fe Kα line 
is well fit by a Gaussian incorporated in CompTT with 
σ of 0.29_-0.10^+0.09 keV  and 0.29_-0.10^+0.09 keV respectively 
for the slab and 
spherical geometry. This is much narrower than the value of σ
= 0.8 keV obtained from ASCA observations <cit.>. 


In this work, we applied physical models to the data in 
contrast to the simple phenomenological models
used earlier to understand the spectral characteristics of 3C 120 <cit.>. 
We fitted CompTT, to the observed 
spectrum and
used it to characterise the temperature and optical depth of the electrons
in the corona for two geometries, namely a sphere
and a slab.  The goodness of the fit (with a nearly identical chi-square per degree of 
freedom of 
χ^2/ν≈ 3734/3788) is found to be insensitive to the assumption of the coronal geometry as
 assumption of both the slab and spherical geometry fit the data 
equally well and we obtained kT_e = 9_-3^+2 keV for slab geometry and kT_e = 16_-7^+6 keV for the spherical geometry. 
 These two model assumptions about the geometry of the
corona gave different values of the optical depth  
with τ = 2.4_-1.1^+0.6 and τ = 5.1_-0.4^+0.6 for the slab 
and 
spherical geometry.  This
is expected  because the optical depth for a slab geometry is measured 
vertically 
while for a sphere it is measured radially.
Using CompPS an advanced Comptonization model available 
in XSPEC, we found kT_e values of 25± 2 and 26^+2_-0 for the
slab and spherical geometry. Within errors, these values of kT_e 
matches with that obtained from the fit of the EQPAIR model
to the NuSTAR data that returned a value of kT_e = 23^+1_-7 keV.







This value of kT_e is much lower than the value 
of kT_e of 176 keV  obtained by
<cit.>. This discrepancy might be attributed to the presence of a significant
jet contribution during the epoch of observations done from INTEGRAL. 










Considering CompPS model, the derived value of kT_e is 
nearly identical for both the slab and sphere geometry of the corona.
This could mean that the shape of the X-ray spectra emerging out of these
two geometries is quite similar and the available spectral data from 
NuSTAR 
is not sufficient to distinguish between these two geometries.

















 §.§ Nature of the corona in 3C 120

3C 120 is classified as a Seyfert 1 galaxy <cit.> 
and is also identified as a BLRG by <cit.>. It has an 
one sided jet and is also 
known to be a γ-ray emitter in  Fermi 
data <cit.>, which provides
additional evidence for the presence of a powerful relativistic jet, already seen
in radio observations <cit.>. It is known 
that BLRGs have harder X-ray spectra in comparison
to radio-quiet Seyfert galaxies <cit.>. However, spectral fits to the NuSTAR
data analysed here by <cit.> using pexrav model gave a photon index Γ of 1.87 ± 0.02. This 
value is 
similar to that known for non-jetted Seyfert 1 galaxies and different from
the X-ray spectrum of AGN with relativistic jets (blazars) that have
Γ < 1.5 
<cit.>. Though the 
derived X-ray spectral index points to negligible contribution of the jet 
emission we checked
for the signature of jets in our data by looking at the multi-wavelength
properties during the epoch of NuSTAR observations. Using the
light curves taken in the optical from the Catalina Realtime Transient
Survey (CRTS; ) and in the 15 GHz band in the radio from
the Owens Valley Radio Observatory (OVRO, 
),
we found that 3C 120 was in a moderately low flux state during the time of NuSTAR
observation analysed here. The optical and radio 
light curves are given in Figure <ref> with the 
epoch of NuSTAR observations indicated as a blue dashed line.  Also, 
during the epoch of the NuSTAR observations used here, the source was
not detected in γ-rays by Fermi <cit.>. 
The F_var for 3C 120 in the soft and hard bands are 0.065 ± 0.002 and 0.052 ± 0.003 
respectively. This is much lower than the average F_var in X-rays shown by
the blazar class of AGN <cit.>
Also, the variations seen in the NuSTAR data
is similar to that of Seyfert galaxies and not blazars 
<cit.>. Model fits to the observed spectrum
using CompPS that considers thermal Comptonization gave a
value of kT_e = 26^+2_-0 keV for a spherical geometry. On the other hand, fits
to the observed spectrum using EQPAIR that treats Comptonization from
hybrid plasma gave kT_e = 23_-7^+1 keV. 



 Comparing CompTT and CompPS models for a spherical geometry using F-test we
find a F-value of 1.013. The test does not rule out the null hypothesis that
the two chi-square distributions are the same at the 90% confidence level.
Between CompPS and EQPAIR model fits for a spherical geometry we find a
F-value of 1.0637, larger than the F_critical value for a 90% confidence,
rejecting the null hypothesis that the two chi-square distributions are the
same. As the chi-square value of CompPS matches close to unity compared to
EQPAIR, we consider CompPS model better represents the spectrum of 3C 120. 
Therefore, based on both spectral
 (presence of X-ray high energy cut-off and the X-ray photon index
being close to that known for Seyfert galaxies) and timing analysis
 (non-detection of the source in γ-rays during the epoch of
NuSTAR observations), it is clear that  the X-rays observed by
NuSTAR from 3C 120 
 is similar to that found in non-jetted Seyfert 1 galaxies
considering a model of a thermal Comptonizing corona producing the 
X-ray in 3C 120. We note that the strength of the reflection component
 obtained here showed significant differences between various model fits, 
which might the due to the low S/N of the data beyond 30 keV.



 §.§ Comparison with the  coronal properties of other AGN

Because of the degeneracies involved in the evaluation of the 
properties of the corona from the observed X-ray spectrum, it is needed 
to simultaneously measure the 
power law slope and the cut off energy. Measurements of this demands
high quality X-ray spectra. Measurements of E_cut 
were known for several AGN from observatories such as
BeppoSAX and INTEGRAL. However, most of these measurements
have large error bars. Recently, observations from NuSTAR
have started to provide reliable estimates of E_cut in 
few AGN, even though it might not be sensitive to sources with E_cut much larger
that its spectral coverage.

To compare the coronal measurements reported here for 3C 120 with that of
other AGN, we searched the literature for the availability of coronal
properties of AGN based on observations either from NuSTAR 
alone or NuSTAR observations
coupled with other telescopes. Focussing only on those sources that have E_cut 
 measurements (with out lower limits) we arrived at a sample 
of twelve sources including 3C 120. They are given in Table <ref>. 









Also, the sources listed in Table <ref>
belong to different types of AGN that includes both radio-quiet Seyferts and BLRGs (3C 390.3 and 3C 120, 3C 282 and 3C 390.3). 
Analysis of a larger sample of AGN do indicate differences between
BLRGs and radio-quiet Seyfert 1 galaxies, with BLRGs having, on average
lesser Compton reflection, weaker Fe Kα line and
harder hard X-ray spectra compared to radio-quiet Seyfert 1 
galaxies <cit.>. These differences between
BLRGs and radio-quiet Seyfert 1 galaxies are further
confirmed by <cit.>,however, the authors
state that the distribution of these parameters in 
these two populations of sources is not distinct. <cit.> 
obtained mean values of Γ = 1.74 ± 0.04 and 1.95 ± 0.05 
for BLRGs and radio-quiet Seyferts respectively. The value of Γ
obtained for 3C 120 by <cit.>
 is closer to what is known for radio-quiet Seyfert 1 galaxies and is steeper
than the other two BLRGs 3C 282 and 3C 390.3. This also supports the 
dormant state of the jet of 3C 120 during the epoch of NuSTAR observations
reported here. Though the kT_e values of 3C 120 and 3C390.3 agree within a factor of two,
the value of kT_e obtained for 3C 382 another BLRG is much larger.
Therefore, based on existing data from NuSTAR, it
is very difficult to say if the coronal properties of 
radio-loud AGN (BLRGs) and radio-quiet AGN (radio-quiet Seyfert 1 galaxies)
are similar or different. Understanding the connection between
radio-emission and coronal properties if any needs observations
on a large number of sources of both types analysed in a
homogeneous manner. For this modest sample of sources with NuSTAR observations
culled from literature, we looked for correlation
of kT_e with other physical parameters of the sources such as
Γ and the black hole mass. No correlation
could be established (Figure <ref>). Therefore, more and more measurement of kT_e
on a large
sample that comprises both radio-loud and radio-quiet AGN  
are needed to know for the existence or absence of such 
correlations and largely to better 
understand the nature of the corona in AGN.








§ SUMMARY

We have carried out timing and spectral analysis of the Seyfert 1 galaxy
3C 120 using ∼120 ks observations from NuSTAR. The results of our 
analysis are summarized below:


  * The source showed variations in the count rates during the duration of the X-ray 
observations. The amplitude of count rate variations characterized by F_var are
found to be 0.065 ± 0.002 and 0.052 ± 0.003 for the soft 
(3 - 10) keV  and the hard (10-79) keV  bands respectively, thus 
showing more variations in the soft band relative to the hard band


  * The X-ray spectrum characterised by HR is found not to show a  
correlation with the flux variations in the total band, indicating 
the spectrum was non-variable during the epoch of NuSTAR
observation.



  * From fit of CompTT model to the time averaged spectrum we found
evidence for the presence of weak Fe Kα line in the data at 6.4 keV with an 
equivalent width of 60 ± 5 eV.
The line is best fit by a Gaussian with a σ of 0.29 keV













  * Using the Comptonization model CompPS to fit the observed 
spectrum, we derived the kinetic temperature of the coronal electrons to be 
kT_e = 25 ± 2 keV  with a Compton y parameter of 
y = 2.2 ± 0.2 for a slab geometry. 
This is similar to the
value of the kinetic temperature of kT_e = 26^+2_-0 keV
obtained for a spherical 
geometry with a y of 2.99^+2.99_-0.18. Also, fitting the observed
spectrum with EQPAIR gave
a best fit value of kT_e = 23^+1_-7 keV.


Thus, fits to the data 
with the two most advanced 
Comptonization models available in XSPEC namely CompPS and
EQPAIR  gave similar values of coronal 
temperature. 




 It is likely that the electrons participating in the comptonization
process is predominantly thermal. Comptonization by non-thermal electrons
if any is in-significant as (i) the source is not detected in γ-rays
during the epoch of NuSTAR observations and (ii) the X-ray photon
index is similar to that known for Seyfert galaxies


  * 3C 120 is known to have a large scale radio jet and is  
also a γ-ray emitter. 
However, NuSTAR data analysed here has made possible the detection of
coronal spectral signatures, constrain kT_e and the reflection
features, which are found  
similar to that known for radio-quiet 
Seyfert galaxies. This indicates
that the contribution of jet emission to the X-ray is negligible in the 
NuSTAR data and is likely to be weak during the epoch of NuSTAR
observations. Additional support to this is provided by similar value of kT_e obtained
by both CompPS and EQPAIR model fits to NuSTAR observations.
This is also supported by the low/moderate radio and optical flux states as well
as non-detection by Fermi during the epoch of NuSTAR observations.
To constrain the contribution of jet emission if any to the X-ray emission 
from 3C 120 requires observations at energies higher than that covered
by NuSTAR.


 

 















We thank the referee for his/her critical and valuable comments that
improved the paper significantly.
This research made use of data from the NuSTAR mission, a project 
led by the 
California Institute of Technology, managed by the Jet Propulsion Laboratory, 
and funded by NASA, XMM-Newton, an ESA science mission with instruments and 
contributions directly funded by ESA Member States and NASA. This research has 
made use of the NuSTAR Data Analysis Software (NuSTARDAS) jointly developed by 
the ASI Science Data Center (ASDC, Italy) and the California Institute of 
Technology (USA).


apj





