Long term X-Ray Observations of Seyfert 1 Galaxy Ark 120: On the origin of soft-excess
Prantik Nandi, Arka Chatterjee, Sandip K. Chakrabarti, Broja G. Dutta
aa r X i v : . [ a s t r o - ph . H E ] J a n MNRAS , 1–16 (2020) Preprint 21 January 2021 Compiled using MNRAS L A TEX style file v3.0
Long term X-Ray Observations of Seyfert 1 Galaxy Ark 120: On theorigin of soft-excess
Prantik Nandi ★ , Arka Chatterjee † , Sandip K. Chakrabarti ‡ , Broja G. Dutta , § Department of Astrophysics & Cosmology, S. N. Bose National Centre for Basic Science, Salt lake, Sector III, Kolkata 700091, India Indian Centre for Space Science, Garia Station Road, Kolkata 700084, India Department of Physics, Rishi Bankim Chandra College, Naihati, West Bengal, 743165, India
Accepted XXX. Received YYY; in original form ZZZ
ABSTRACT
We present the long-term X-ray spectral and temporal analysis of a ‘bare-type AGN’ Ark120. We consider the observations from
XMM-Newton , Suzaku , Swift , and
NuSTAR from 2003to 2018. The spectral properties of this source are studied using various phenomenologicaland physical models present in the literature. We report (a) the variations of several physicalparameters, such as the temperature and optical depth of the electron cloud, the size ofthe Compton cloud, and accretion rate for the last fifteen years. The spectral variations areexplained from the change in the accretion dynamics; (b) the X-ray time delay between 0.2-2keV and 3-10 keV light-curves exhibited zero-delay in 2003, positive delay of 4 . ± . negative delay of 4 . ± . nthcomp , of the soft-excess and the primary continuum show a correlationwith a Pearson Correlation Co-efficient of 0 . Key words: galaxies: active – galaxies: Seyfert – X-rays: galaxies – X-rays: individual: Ark120
Active Galactic Nuclei (AGNs) are the most energetic phenomenain the universe. The emitted radiation is observed over the entirerange of the electromagnetic spectrum. The high energy X-rays arebelieved to be emitted from the innermost region of an accretiondisc which surrounds the central black hole (Shakura & Sunyaev1973; Pringle et al. 1973). The X-ray spectra of Seyfert 1 galax-ies, a subclass of AGNs, is mostly fitted by a power-law compo-nent with photon index in the range
Γ = . − . ∼ 𝛼 line, which is observed near6.4 keV, and a Compton hump in the energy range of 20.0 to ★ E-mail: [email protected] † E-mail: [email protected] ‡ E-mail: [email protected] § E-mail: [email protected] © P Nandi et al. sion from a dense and relatively cold accretion disc. Moreover, it isbelieved that the Compton hump could be due to the Compton scat-tering dominated above 10 keV in a relatively cold dense medium.Nevertheless, the complex broad-band spectrum of AGNs requiresa proper physical explanation of the flow dynamics and radiativeproperties around the central engine across the soft and hard energyregime of the X-ray.In this scenario, the Two-Component Advective Flow (TCAF)(Chakrabarti & Titarchuk 1995) model, which combines theessence of all the salient features of a viscous transonic flow(Chakrabarti 1989, 1990, 1995) around black holes is worth ex-ploring. It is a physical solution encompassing hydrodynamics andradiative processes. The transonic flow solution allows two typesof accretion flows depending on how efficiently angular momentumis being transported: a viscous, geometrically thin, optically thickstandard Keplerian component (Shakura & Sunyaev 1973) and aweakly viscous, geometrically thick, optically thin sub-Keplerianhalo component (Chakrabarti & Titarchuk 1995). The latter is ba-sically an inefficiently radiating generalized Bondi flow with highradial velocity till it forms the centrifugal barrier after which it be-comes efficient in radiating at higher energies. The Keplerian discis formally truncated at the centrifugal barrier, the outer boundaryof which is the shock location (Chakrabarti 1989). The post-shockregion (i.e., the region between the shock and the innermost sonicpoint) is known as CENtrifugal barrier supported BOundary Layeror CENBOL and it acts as the Compton cloud. The soft photonsfrom the Keplerian disc are upscattered by Comptonization pro-cess in the post-shock region and produce the high energy X-rayphotons. TCAF, a self-consistent model, is quantified by four flowparameters: two types of accretion rates, namely, the disc rate ( ¤ 𝑚 𝑑 )and halo rate ( ¤ 𝑚 ℎ ), size and density of the Compton cloud, throughthe shock location ( 𝑋 𝑠 ) and the compression ratio ( 𝑅 ), ratio of thepost-shock and the pre-shock flow densities ( 𝜌 + 𝜌 − ). It also requires anintrinsic parameter, namely, the mass of the central black hole (inthe units of 𝑀 ⊙ ), and an extrinsic parameter, namely, the normal-ization which is required to place the observed spectrum over thetheoretical spectrum of TCAF. The broadband spectra of M87 wasexplained with this model by fitting the data from multi-wavelengthobservations(Mandal & Chakrabarti 2008). Later, TCAF has beenimplemented in the xspec as a local table model and has been suc-cessful to fit the data of the Galactic black holes (Debnath et al.2014) and has also been able to estimate the mass of nearby Seyfert1 galaxy NGC 4151 using NuSTAR data (Nandi et al. 2019).Arakelian 120 (Ark 120) is a nearby ( 𝑧 = . ) radio-quietSeyfert 1 AGN with radio-loudness 𝑅 ≈ . The redshift is taken from the NASA/Infrared Process and Analysis center(IPAC) Extragalactic Database. https://ned.ipac.caltech.edu nor by neutral intrinsic absorbers (Reeves et al. 2016) around thecentral engine. Furthermore, Ark 120 is nearly free from intrin-sic reddening in the IR-optical-UV continuum (Ward et al. 1987;Vasudevan et al. 2009). Therefore, it provides one of the cleanestviews ( 𝑁 𝐻 ∼ × cm − ; (Vaughan et al. 2004)) of the cen-tral region. This type of AGNs are called “ bare nucleus” Seyfertsor bare AGNs. The estimated mass of the central black hole ofArk 120 is M 𝐵𝐻 = . ± . × M ⊙ (Peterson et al. 2004)which was measured using the reverberation-mapping technique.From the spectroscopic monitoring data of Ark 120 during 1976 to2013 using a 70 cm telescope, Denissyuk et al. (2015) estimatedthe mass of the central SMBH to be M 𝐵𝐻 = . ± . × M ⊙ . This source has a low Eddington ratio of 𝐿 𝑏𝑜𝑙 / 𝐿 𝐸 ∼ . 𝛼 line(Vaughan et al. 2004; Nardini et al. 2011). Nardini et al. (2011)analyzed Ark 120 spectra, where, in the absence of absorber ofcomplex morphology, soft-excess was explained by reflection fromthe centrally located hot and cold medium located at a distance.Marinucci et al. (2019) used the Monte-Carlo technique to inves-tigate the favourable shape of the Compton cloud considering thefuture polarimetric missions such as IXPE (Weisskopf et al. 2016).Although Ark 120 is a widely studied source, the evolution ofthe X-ray spectra over the last two decades is yet to be understood.However, a steepening of the X-ray spectrum was observed duringsix-month monitoring in 2014 with Swift. The observed spectralvariability was attributed to the possible existence of a large discreprocessing region (Gliozzi et al. 2017). Again during 2017-18,a longer time delay was observed (Lobban et al. 2018) betweenlonger wavelength difference (i.e., optical and X-ray). They pre-dicted that the accretion disc could exist in a longer scale as pre-dicted by standard accretion disc theory. The soft-excess part of Ark120 could be originated due to the Comptonization within the hotelectron cloud of various shape (Marinucci et al. 2019), reflectionfrom a cold medium (Nardini et al. 2011) or the shock heating nearthe inner edge of the disc (Fukumura et al. 2016). We analyzed thelong term X-ray archival data of Ark 120 which provides an idealtestbed to understand the soft-excess as well as its interaction withthe harder (>2 keV) photons. Along with the observations, we per-form Monte-Carlo simulations to find the effect of Comptonizatonwithin the energy range of soft-excess. We also study the X-rayvariability of the source over a longer period and to calculate theapproximate time-delays in X-ray bands. For the first time, we alsofind the flow and system parameters by fitting of the X-ray data.The paper is structured in the following way: in Sec 2, we providethe details of the observational data and their reduction procedure.The results of the spectral and temporal analysis are presented inSec 3 and 4. We discuss our findings in Sec 5 and finally, draw ourconclusions in Sec 6.
We use the publicly available archival data of
XMM-Newton , NuS-TAR , Chandra , and
Suzaku using HEASARC . We reprocessedall data using HEAsoft v6.26.1 (Arnaud 1996), which includes
XSPEC v12.10.1f . http://heasarc.gsfc.nasa.gov/ MNRAS , 1–16 (2020) rigin of soft-excess of Ark 120 Table 1.
Observation Log
ID Date Obs. ID Instrument Exposures(yyyy-mm-dd) (ks)XMM1 2003-08-24 0147190101
XMM-Newton /EPIC-pn 112.15S1 2007-04-01 702014010
Suzaku /XIS-HXD 100.86XRT1 2008-07-24 00037593001
Swift /XRT 10.86-2008-08-03 -00037593003XMM2 2013-02-18 0693781501
XMM-Newton /EPIC-pn 130.46N1 2013-02-18 60001044004
NuSTAR /FPMA 65.46XMM3 2014-03-22 0721600401
XMM-Newton /EPIC-pn 124.0N2 2014-03-22 60001044002
NuSTAR /FPMA 55.33XRT2 2014-09-04 00091909002
Swift /XRT 22.81-2014-10-19 -00091909022XRT3 2014-10-22 00091909023
Swift /XRT 20.18-2014-12-05 -00091909044XRT4 2014-12-09 00091909045
Swift /XRT 23.48-2015-01-26 -00091909068XRT5 2015-01-26 00091909069
Swift /XRT 21.66-2015-03-15 -00091909090XRT6 2017-12-07 00010379001
Swift /XRT 44.14-2018-01-24 -00010379048
Ark 120 has been observed by
XMM-Newton (Jansen et al. 2001)during three epochs from 2003 to 2014. In 2003 and 2013, it hasmade ∼
112 ks (XMM1) and ∼
130 ks (XMM2) observations re-spectively. The XMM1 data is used by (Vaughan et al. 2004) andreported that the source Ark 120 is one of the cleanest Sy1 typeAGN. In 2014,
XMM-Newton observed Ark 120 four times betweenMarch 18 and March 24. Out of these, one (XMM3) was simulta-neous with
NuSTAR observation. The details of the observation logare presented in Table 1. It was observed that the X-ray flux of thissource was about a factor of two higher in 2014 than the XMM2 ob-servation (Matt et al. 2014; Marinucci et al. 2019) made in 2013.A similar trend of flux variation was also reported in optical/UV(Lobban et al. 2018) band.Due to the high brightness of the source, the European PhotonImaging Camera (EPIC-pn (Strüder et al. 2001)) operated in SmallWindow (SW) mode to prevent any pile-up. The details of the
XMM-Newton /EPIC-pn observations of this source are listed in Table-1. We reprocessed the raw data to level 1 data for EPIC-pn byScientific Analysis System (
SAS v16.1.0 ) with calibration filesdated February 2, 2018. We have used only the unflagged ( FLAG== 0 ) events for excluding the edge of CCD and the edge of thebad pixel. Besides this, we also use
PATTERN ≤ GTI files toacquire the maximum signal to noise ratio. After that, we use anannular area of 30 ′′ outer radii and 5 ′′ inner radii centered at thesource to extract the source event. For the background, we use acircle of 60 ′′ in the lower part of the window that contains no (ornegligible) source photons. The response files ( arf and rmf files)for each EPIC-pn spectral data set were produced with SAS tasks
ARFGEN and
RMFGEN , respectively. The
GRPPHA task is used with100 counts per bin for 0.3 - 10.0 keV EPIC-pn spectra.
Suzaku observed Ark 120 on 2007 April 1 (Obs ID: 702014010)in HXD normal position with exposure of ∼
101 ks using X-ray imaging spectrometer (Koyama et al. 2007) and ∼
89 ks for HardX-ray Detector (Takahashi et al. 2007). The photons were collectedin both 3 × × 𝛼 emission line with full-widthat half maximum of 4700 + − km s − was previously reportedby (Nardini et al. 2016) by using Suzaku observation along with
XMM-Newton , Chandra i, and
NuSTAR .We use the standard data reduction technique for
Suzaku dataanalysis illustrated in
Suzaku
Data Reduction Guide and followedthe recommended screening criteria while extracting Suzaku /XISspectrum and light-curves. The latest calibration files available(2014-02-03) using FTOOLS 6.25 is used to reprocess the eventfiles. The source spectra and lightcurves are extracted from a circularregion of radius 200 ′′ centered on the Ark 120 and the backgroundregion is selected on the same slit with a circular region 250 ′′ .Finally, we merge the two front illuminated detectors (XIS0 andXIS3) to produce the final spectra and lightcurves for Ark 120. Wegenerated the response files through XISRESP script.As
Suzaku has a high energy X-ray detector (HXD), we use theHXD/PIN data for our analysis. We reprocessed the unfiltered eventfiles using the standard tools. The output spectrum and lightcurvesare extracted by using the hxdpinxbpi and hxdpinxblc , respec-tively. Further, we correct the spectrum to take into account boththe non-X-ray and the cosmic X-ray backgrounds and the dead timecorrection.
NuSTAR (Harrison et al. 2013) observed Ark 120 simultaneouslywith
XMM-Newton with FPMA and FPMB on 2013 February 18(N1) and 2014 March 22 (N2) for the exposure of ∼
166 ks and ∼
131 ks respectively. The details of the observation log are given inTable 1. We consider both N1 and N2 observations for our analysis.(Porquet et al. 2018, 2019) used this data along with
XMM-Newton and determined the spin 0 . + . − . and comment on the dimensionof the corona and temperature by analyzing these X-ray data.The level 1 data is produced from the raw data by using the NuSTAR data analysis software (
NuSTARDAS v1.8.0 ). The cleanedevent files are produced with standard
NUPIPELINE task and cal-ibrated with the latest calibration files available in the NuSTARcalibration database (CALDB) . We chose 90 ′′ radii for sourceand 180 ′′ radii for the background region on the same detector toavoid contamination and detector edges. For the final background-subtracted lightcurves, we use 100s bin for both FPMA and FPMB.As both detectors are identical, here we present the results of FPMAonly. The response files ( arf and rmf files) are generated by usingthe numkrmf and numkarf modules, respectively. Swift X-ray telescope (XRT; Burrows et al. (2005)), working in theenergy range of 0.2 to 10.0 keV, is an X-ray focusing telescope. XRTobserved this source in both WT (windowed timing) and pc (photoncount) modes depending on the brightness of the source. Ark 120was observed over ∼
130 times from 2008-07-24 to 2018-01-24. http://heasarc.gsfc.nasa.gov/docs/suzaku/analysis/abc/ http://heasarc.gsfc.nasa.gov/FTP/caldb/data/nustar/fpm/ MNRAS , 1–16 (2020)
P Nandi et al.
In 2008,
Swift observed three times, July 24, July 31 and August3. We stack the spectra to produce a combined spectrum (XRT1).Then, it again observed on 2014-03-22, which has a simultane-ous observation with
XMM and
NuSTAR . We consider the XMM3observation over this particular XRT observation.
Swift observedArk 120 from 2014-09-06 to 2015-03-15 on a nearly daily basis.Further, we stack these observations into four observations (XRT2,XRT3, XRT4, XRT5) with each observations spanning around 50days. In the last epoch,
Swift observed Ark 120 from 2017-12-05to 24-01-2018 over ∼
50 days. We stack the observations to pro-duce the spectra of XRT6. The details of the observation log arestated in Table 1. We use the online tool “XRT product builder” Evans et al. (2009) to extract the spectrum and light curves. Thisproduct builder performs all necessary processing and calibrationand produces the final spectra and lightcurves of Ark 120 in WTand PC mode.
We use
XMM-Newton , Suzaku , NuSTAR , and
Swift data forthe spectral analysis and explore the spectral variation over ∼
15 years (2003-2018) period using
XSPEC v12.10.1f (Arnaud1996). We explore the broad spectral properties with nth-comp model (Zdziarski, Johnson & Magdziarz 1996). Later,we apply Two Component Advective Flow (TCAF) model(Chakrabarti & Titarchuk 1995) to extract the physical flow pa-rameters such as the accretion rates and size of the Compton cloud.Along with these models, we use a
Gaussian component forthe Fe fluorescent emission line. While fitting the data, we use twoabsorption components, namely
TBabs and zTBabs (Wilms et al.2000). The component,
TBabs is used for the Galactic absorption,where hydrogen column density ( 𝑁 𝐻 ,𝑔𝑎𝑙 ) is fixed at 9 . × cm − (Kalberla et al. 2005). To calculate the error for each param-eter in spectral fitting with 90% confidence level, we use ‘ error ’command in XSPEC .We use following cosmological parameters in this work: 𝐻 =70 km s − Mpc − , Λ = 0.73, Ω 𝑀 = 0.27 (Bennett et al. 2003).With the assumed cosmological parameters, the luminosity distanceof Ark 120 is 142 Mpc. We have started the spectral fitting with nthcomp model, and themodel in
XSPEC reads as:
TBabs*zTBabs*(nthcomp+zGaussian)nthcomp is a thermally Comptonized continuum model pro-posed by Zdziarski, Johnson & Magdziarz (1996) and later ex-tended by Zycki, Done & Smith (1999). We fit all X-ray spectrumabove 3.0 keV by this baseline model. The model depends on theseed photon energy ( 𝑘𝑇 𝑏𝑏 ), which we consider at 3 eV for all spec-trum. Although, Marinucci et al. (2019) considered 𝑘𝑇 𝑏𝑏 at 15 eV.It is to be noted that, we vary 𝑘𝑇 𝑏𝑏 from 1 eV to 50 eV, and failedto notice any deviation in the residuals of the fitted spectra. Weconsider these seed photons to be disc-blackbody type. For that,we have opted for the inp-type is 1 for all fit. For the spectral fit-ting, first, we consider the energy range 3.0 to 10.0 keV. The fittedasymptotic power-law photon index Γ = .
90, electron temperature 𝑘𝑇 𝑒 = .
45 keV and an iron K 𝛼 line at 6.40 keV with equivalent http://swift.ac.uk/user_objects/ width (EW) of 116 + − eV with reduced chi-square ( 𝜒 / 𝑑𝑜 𝑓 )=1.04for degrees of freedom (dof) = 300 is obtained. Next, we analysethe data from the 2007 Suzaku observation. We have combined the
Suzaku /XIS observation with
Suzaku /HXD and make a spectrumfrom 0.5 to 40.0 keV. But, we fit 3.0 to 40.0 keV spectrum using thebaseline model. The fitted parameters are
Γ = . 𝑘𝑇 𝑒 = . 𝛼 line at 6.38 keV with equivalent width (EW)of 710 + − eV. We are also in need of an additional powerlaw and Gaussian to take care of high energy (above 10.0 keV) spec-trum and emission lines. We have obtained the reduced chi-square( 𝜒 / 𝑑𝑜 𝑓 )=1.02 for degrees of freedom (dof) = 1093 for this fitting.We have fitted the combined spectrum of XMM2+N1 (MJD-56341)and XMM3+N2 (MJD-56738) spectrum using this model for theenergy range 3.0 to 79.0 keV with the model parameters such as Γ = .
75 & 1 .
87 and corresponding 𝑘𝑇 𝑒 = .
56 & 205 .
95 respec-tively. We have applied a zGaussian for a Fe K 𝛼 line at 6 . + . − . & 6 . + . − . keV with equivalent widths (EW) of 136 + − & 227 + − eV for these combined spectra and the ( 𝜒 / 𝑑𝑜 𝑓 )=644.55/641 &( 𝜒 / 𝑑𝑜 𝑓 )= 508.07/469 respectively. Next, we analyse the data ob-tained from Swift /XRT observation for the energy range of 3.0 to10.0 keV. Fe K 𝛼 line is not detected for all the six XRT spectra.We have fitted the Swift /XRT spectra by removing
Gaussian com-ponent from the baseline model. The power-law index Γ vary from1.60 to 1.88 and the corresponding electron temperature 𝑘𝑇 𝑒 varyfrom 274.40 to 201.58 keV respectively. The nthcomp model fittedspectral analysis result is presented in Table 2. Furthermore, wecalculate the optical depth for each observation using the formula: 𝜏 = s + 𝜃 𝑒 ( Γ + )( Γ − ) − , (1)by inverting the relation A1 is presented inZdziarski, Johnson & Magdziarz (1996). Here, 𝜃 𝑒 = 𝑘𝑇 𝑒 𝑚 𝑒 𝑐 is the electron energy with respect to the rest mass energy. Thevalue of optical depth 𝜏 for each observation is provided inTable 2. The maximum error in optical depth is obtained from Δ 𝜏 ∼ ( Δ 𝜃 𝑒 𝜃 𝑒 + ΔΓΓ ) × 𝜏 , where Δ 𝜃 𝑒 and ΔΓ are considered fromthe fitted errors presented in Table 2.We address the soft-excess (< 3 keV) part by adding another powerlaw component. We freeze the Γ obtained earlier while fittingthe primary continuum alone. The second power-law fits the soft-excess, and the results are presented in Table 3. It should be notedthat the spectral index of soft-excess ( Γ 𝑆𝐸 ) is higher than the spectralindex of the primary continuum ( Γ 𝑃𝐶 ) for every observation. From the nthcomp model fitting, we have extracted several valuableinformation on the spectral hardness and electron temperature ofthe emitting system in a time duration of ∼
15 years. We havealso calculated the optical depths from these parameters, which areshown in Table 2. However, the fundamental properties, such as thecentral black hole mass, accretion rates, the size of the Comptoncloud radius could provide a deeper physical understanding of thesystem. To estimate these quantities, we use the Two-ComponentAdvective Flow (TCAF) model (Chakrabarti & Titarchuk 1995) forour spectral analysis. For the spectral fitting, the model in
XSPEC reads as:
TBabs*zTBabs*(TCAF+zGaussian)TCAF is based on one black hole parameter and four flowparameters: (i) black hole mass in units of the solar mass ( 𝑀 ⊙ ); MNRAS , 1–16 (2020) rigin of soft-excess of Ark 120 Table 2. nthcomp fitting result for the spectrum above 3.0 keV. The optical depth 𝜏 is calculated from equation-1.ID MJD Γ 𝑛𝑡ℎ 𝑘𝑇 𝑒 Fe 𝐾 𝛼 EW 𝜒 / 𝑑𝑜 𝑓 𝜏 ∗ (keV) (keV) (eV)XMM1 52875 1 . + . − . . + . − . . + . − . + − . ± . . + . − . . + . − . . + . − . + − . ± . . + . − . . + . − . - - 75.68/74 0 . ± . . + . − . . + . − . . + . − . + − . ± . . + . − . . + . − . . + . − . + − . ± . . + . − . . + . − . − - 306.65/290 0 . ± . . + . − . . + . − . − - 319.98/320 0 . ± . . + . − . . + . − . − - 269.17/280 0 . ± . . + . − . . + . − . − - 246.53/261 0 . ± . . + . − . . + . − . − - 327.78/318 0 . ± . Table 3.
Soft-excess spectral indices are generated while keeping the spectral slope of nthcomp ( Γ 𝑛𝑡ℎ ) frozen. Intrinsic luminosities are calculated for both ofthe components using clum in the energy energy 0.5 to 10.0 kev.ID Γ 𝑃𝐶 𝑁 𝑜𝑟 𝑚 𝑃𝐶 𝐿 𝑃𝐶 Γ 𝑆𝐸 𝑁 𝑜𝑟 𝑚 𝑆𝐸 𝐿 𝑆𝐸 = Γ 𝑛𝑡ℎ ( − ) ( − ) XMM1 1 .
90 1 . + . − . . + . − . . + . − . . + . − . . + . − . S1 2 .
08 18 + . − . . + . − . . + . − . + . − . . + . − . XRT1 1 .
76 0 . + . − . . + . − . . + . − . . + . − . . + . − . XMM2+N1 1 .
75 0 . + . − . . + . − . . + . − . . + . − . . + . − . XMM3+N2 1 .
86 1 . + . − . . + . − . . + . − . . + . − . . + . − . XRT2 1 .
60 0 . + . − . . + . − . . + . − . . + . − . . + . − . XRT3 1 .
84 0 . + . − . . + . − . . + . − . . + . − . . + . − . XRT4 1 .
72 0 . + . − . . + . − . . + . − . . + . − . . + . − . XRT5 1 .
88 0 . + . − . . + . − . . + . − . . + . − . . + . − . XRT6 1 .
65 0 . + . − . . + . − . . + . − . . + . − . . + . − . (ii) Keplerian disc accretion rate ( ¤ 𝑚 𝑑 ) in the unit of the Eddingtonrate ( ¤ 𝑀 𝐸𝐷𝐷 ); (iii) Sub-Keplerian halo accretion rate ( ¤ 𝑚 ℎ ) in unitsof Eddington rate ( ¤ 𝑀 𝐸𝐷𝐷 ); (iv) shock compression ratio (R) and(v) shock location ( 𝑋 𝑠 ) in units of the Schwarzschild radius ( 𝑟 𝑔 = 𝐺 𝑀 / 𝑐 ). The upper and lower limits of all the parameters are putin a data file called lmodel.dat provided in Table-4 as an input to runthe source code using initpackage and lmod task in XSPEC . Forthe final spectral fitting of a specified observation, we run the source code for a vast number of times and select the best spectrum frommany spectra using minimization of 𝜒 method. First, we have startedfitting by the baseline model described as above. Some spectra,like XMM1, S1, XMM2+N1, XMM3+N2 have high reduced 𝜒 ( 𝜒 𝑟𝑒𝑑 >
2) value. We noticed that the model has deviated fromthe actual data at the high energy end. To compensate for that, wehave added a powerlaw/pexrav with the baseline model. Thus themodel became:
MNRAS , 1–16 (2020)
P Nandi et al.
Table 4.
The TCAF parameter space is defined in the file lmod.dat.Model parameters Parameter units Default value Min. Min. Max. Max. IncrementM BH M Sun . × × × . × . × . ¤ 𝑚 𝑑 Edd 0 .
001 0 . . . . . ¤ 𝑚 ℎ Edd 0 .
01 0 . . . . . s r g . . . . . .
0R 1 . . . . . . Table 5.
TBabs*zTBabs*(TCAF+zGaussian) model fitted Parameters in 0.2-79.0 keV energy band for Ark 120. The
TBabs is fixed at 𝑁 𝐻 𝑔𝑎𝑙 = 9 . × cm − . The second column shows the variation of zTBabs for z = 0.033. ID MJD 𝑁 𝐻 𝑀 𝐵𝐻 ¤ 𝑚 𝑑 ¤ 𝑚 ℎ 𝑋 𝑠 𝑅 𝑁
𝑇 𝐶𝐴𝐹 Γ 𝑝𝑒𝑥𝑟𝑎𝑣 𝑅 𝑟𝑒 𝑓 𝑁 𝑝𝑒𝑥𝑟𝑎𝑣 𝜒 / 𝑑𝑜 𝑓 ( 𝑐𝑚 − ) ( M ⊙ ) ( ¤ 𝑚 𝐸𝑑𝑑 ) ( ¤ 𝑚 𝐸𝑑𝑑 ) ( 𝑟 𝑔 ) ( − ) ( − )XMM1 . + . − . . + . − . . + . − . . + . − . . + . − . . + . − . . + . − . . + . − . . + . − . . + . − . . / S1 . + . − . . + . − . . + . − . . + . − . . + . − . . + . − . . + . − . . + . − . . + . − . . + . − . . / XRT1 . + . − . . + . − . . + . − . . + . − . . + . − . . + . − . . + . − . − − − . / XMM2+N1 . + . − . . + . − . . + . − . . + . − . . + . − . . + . − . . + . − . . + . − . . + . − . . + . − . . / XMM3+N2 . + . − . . + . − . . + . − . . + . − . . + . − . . + . − . . + . − . . + . − . . + . − . . + . − . . / XRT2 . + . − . . + . − . . + . − . . + . − . . + . − . . + . − . . + . − . − − − . / XRT3 . + . − . . + . − . . + . − . . + . − . . + . − . . + . − . . + . − . − − − . / XRT4 . + . − . . + . − . . + . − . . + . − . . + . − . . + . − . . + . − . − − − . / XRT5 . + . − . . + . − . . + . − . . + . − . . + . − . . + . − . . + . − . − − − . / XRT6 . + . − . . + . − . . + . − . . + . − . . + . − . . + . − . . + . − . − − − . / χ r e d χ r e d E (keV) -4-2024 χ r e d TCAFTCAF+zGaussianTCAF+zGaussian+Pexrav
Figure 1.
Variation of 𝜒 𝑟𝑒𝑑 is shown for each model components on broad-band spectra of Ark 120 during 2014 epoch. Primarily, we have started with TCAF , and then added zGaussian and
Pexrav upon necessity.
TBabs*zTBabs*(TCAF+powerlaw/pexrav+zGaussian) .We have fitted the spectra with this model and found 𝜒 𝑟𝑒𝑑 ≈ powerlaw componentby pexrav (Magdziarz & Zdziarski 1995). The pexrav model hasa power-law continuum with a reflected component from an infiniteneutral slab. We have estimated the relative reflection coefficient( 𝑅 𝑟𝑒 𝑓 ) with photon index ( Γ 𝑝𝑒𝑥𝑟𝑎𝑣 ) and cosine of inclinationangle cos 𝜃 from the model fitting. We find 𝜃 to vary from 40 ° to72 ° . We fix abundances for heavy elements, such as iron at the Solarvalue (i.e., 1). For the photon index ( Γ 𝑝𝑒𝑥𝑟𝑎𝑣 ), first, we freeze itsvalue to the value of Γ obtained from nthcomp . For this, we havefound 𝜒 𝑟𝑒𝑑 >
2. Thereafter, we thaw this parameter and fit it againwhich have resulted 𝜒 𝑟𝑒𝑑 ≈ Γ 𝑝𝑒𝑥𝑟𝑎𝑣 .We first fit the XMM-Newton observation (XMM1) during2003 (MJD-52875) in the energy range of 0.2 to 10.0 keV with
TBabs*zTBabs*(TCAF+zGaussian) model. However, we found ahigh 𝜒 𝑟𝑒𝑑 . The model has deviated after 9.2 keV from the actualdata. As mentioned above, we then add a powerlaw with the base-line model, and then the powerlaw is replaced by pexrav . The fittedparameters are, 𝑀 𝐵𝐻 = . × 𝑀 ⊙ , ¤ 𝑚 𝑑 = . ¤ 𝑚 ℎ = . 𝑋 𝑠 = . 𝑅 = .
95 with Γ 𝑝𝑒𝑥𝑟𝑎𝑣 = . 𝑅 𝑟𝑒 𝑓 = .
96, and 𝐸 𝑓 𝑜𝑙𝑑 = .
08 keV and the corresponding 𝜒 = .
20 with de-grees of freedom (dof)= 842. The Fe line is found at 6 . Suzaku observation (S1) of 2007 (MJD-54191). We combine the
Suzaku /XIS and
Suzaku /HXD spectra and
MNRAS , 1–16 (2020) rigin of soft-excess of Ark 120 XMM1 F l ux -3 0 3 0.2 1 5 χ r e d Energy in keV
1 5 25 S1 F l ux -3 0 3 1 5 25 χ r e d Energy in keV
1 5
XRT1 F l ux -3 0 3 1 5 χ r e d Energy in keV -4 -3 -2 -1 XMM2+N1 F l ux -3 0 3 0.5 1 10 20 40 80 χ r e d Energy in keV -4 -3 -2 -1 XMM3+N2 F l ux -3 0 3 0.5 1 10 20 40 80 χ r e d Energy in keV
1 5
XRT2 F l ux -3 0 3 1 5 χ r e d Energy in keV
1 5
XRT4 F l ux -3 0 3 1 5 χ r e d Energy in keV
1 5
XRT5 F l ux -3 0 3 1 5 χ r e d Energy in keV
1 5
XRT6 F l ux -3 0 3 1 5 χ r e d Energy in keV
Figure 2.
TCAF model fitted spectra of Ark 120 from the
XMM-Newton , Suzaku , NuSTAR and
Swift observations along with the residuals obtained from thespectral fitting. make a broadband spectrum in the energy range of 0.5 to 40 keV. Wefollow the similar steps as described in XMM1 fitting and the fittedparameters are 𝑀 𝐵𝐻 = . × 𝑀 ⊙ , ¤ 𝑚 𝑑 = . ¤ 𝑚 ℎ = . 𝑋 𝑠 = . 𝑅 = .
66 with Γ 𝑝𝑒𝑥𝑟𝑎𝑣 = . 𝑅 𝑟𝑒 𝑓 = . 𝜒 𝑟𝑒𝑑 / 𝑑𝑜 𝑓 = . / .
38 keV with an equivalent width of 710 eV. It is to benoted that, within 6 − 𝑀 𝐵𝐻 = .
50 & 1 . × 𝑀 ⊙ , ¤ 𝑚 𝑑 = .
068 & 0 . ¤ 𝑚 ℎ = .
111 & 0 . 𝑋 𝑠 = .
83 & 28 . 𝑅 = .
83 & 2 .
43 with Γ 𝑝𝑒𝑥𝑟𝑎𝑣 = .
96 & 1 .
66 respectively. The details of data fitting aregiven in Table-5.We fit all the six
Swift /XRT spectra using the baseline model.Here, we do not find any Fe line in all these spectra. From thefitting, it is noticed that the mass of the central black hole 𝑀 𝐵𝐻 =1 . × 𝑀 ⊙ , the disc ¤ 𝑚 𝑑 ∼ .
065 and halo accretion rates ¤ 𝑚 ℎ ∼ .
110 are more or less constant except XRT6 observation. Here,we find ¤ 𝑚 𝑑 = .
081 & ¤ 𝑚 ℎ = .
14 and the corresponding shocklocation has moved inward from 57.87 to 42.95 𝑟 𝑔 . Therefore, theshock location ( 𝑋 𝑠 ) has varied in between 30 . . 𝑟 𝑔 , and thecorresponding variation of the compression ratio (R) is in between2 . . powerlaw to fit the high energy spectra.The details of the parameter variations are presented in Table-5. InFigure 2, we plot the model fitted spectrum with the variation of 𝜒 .Detailed discussions on spectral properties are demonstrated in Sec5.1. X-ray variability of an AGN provides a powerful probe of the nearbyregions of the central black hole. Since Ark 120 has a ‘bare-typenucleus’, the X-ray comes from the Compton cloud and is not in-tercepted by any clouds such as BLR, NLR or molecular torus.Thus, the X-ray variability is originated from the varying Comptoncloud and the central accretion disc. To analyze the temporal vari-ability in X-ray of Ark 120 in different energy bands, we have esti-mated different parameters for the duration of 2003 (MJD-52875) to2018 (MJD-58118). The fractional variability 𝐹 𝑣𝑎𝑟 ((Edelson et al.1996); (Nandra et al. 1997); (Edelson et al. 2001); (Edelson et al.2012); (Vaughan et al. 2003); (Rodríguez-Pascual et al. 1997)) of MNRAS , 1–16 (2020)
P Nandi et al. Γ k T e τ m . d & m . h X s Time (years) R Figure 3.
Variation of different model parameters with time are presented. lightcurves of 𝑥 𝑖 count/s with finite measurement error 𝜎 𝑖 of length 𝑁 with a mean 𝜇 and standard deviation 𝜎 is given by: 𝐹 𝑣𝑎𝑟 = s 𝜎 𝑋𝑆 𝜇 (2)where, 𝜎 𝑋𝑆 is excess variance (Nandra et al. (1997); Edelson et al.(2002)), an estimator of the intrinsic source variance and is givenby: 𝜎 𝑋𝑆 = 𝜎 − 𝑁 𝑁 Õ 𝑖 = 𝜎 𝑖 . (3)The normalized excess variance is given by 𝜎 𝑁 𝑋𝑆 = 𝜎 𝑋𝑆 / 𝜇 .The uncertainties in 𝜎 𝑁 𝑋𝑆 and 𝐹 𝑣𝑎𝑟 are taken from Vaughan et al.(2003) and Edelson et al. (2012).The X-ray variability of Ark 120 in different energy bands(0 . − . . − . . − . . − . 𝑋 𝑚𝑎𝑥 = .
95) in 2003 observation.Then, in 2013 (XMM2), it became half ( 𝑋 𝑚𝑎𝑥 = .
24) from itsinitial value. In 2014 (XMM3), the count increased ( 𝑋 𝑚𝑎𝑥 = . . .
064 from 2003 to 2014 observations. A similar trend is shown by 𝜎 𝑁 𝑋𝑆 (0 .
006 to 0 . . − . 𝜎 𝑁 𝑋𝑆 is 0 . . . . − . Suzaku data. We find higher variability 𝐹 𝑣𝑎𝑟 = . ± .
31 in the2007
Suzaku data as compared to the previous XMM observations.The variability for XRT observations in 0 . − . 𝐹 𝑣𝑎𝑟 , and is not shown in Table 6. Fromthe other observations of Swift /XRT, we observe high fractionalvariability ( 𝐹 𝑣𝑎𝑟 ) from 0 .
14 to 0 .
23 with < 𝐹 𝑣𝑎𝑟 > = .
22. Theaverage value of 𝑥 𝑚𝑎𝑥 / 𝑥 𝑚𝑖𝑛 and 𝜎 𝑁 𝑋𝑆 for these observations are2 .
65 and 0 .
045 with a range from 2 .
16 to 3 .
09 and 0 .
027 to 0 . For temporal analysis of the long term archival data of Ark 120,we stress three epochs of
XMM-Newton , 2003, 2013, and 2014out of which the latter two have high energy (3-80 keV) counter-parts observed by
NuSTAR . We have performed cross-correlationanalysis using
DCF (Edelson & Krolik 1988) and 𝜁 -discrete cross- MNRAS , 1–16 (2020) rigin of soft-excess of Ark 120 Table 6.
Variability statistics in various energy ranges are shown in this Table. We have opted for 100s time bins for variability analysis. In some cases, theaverage error of observational data exceeds the limit of 1 𝜎 , resulting negative excess variance. In such cases, we have imaginary 𝐹 𝑣𝑎𝑟 , which are not shownin the table. ID Energy band 𝑁 𝑥 𝑚𝑎𝑥 𝑥 𝑚𝑖𝑛 𝑥 𝑚𝑎𝑥 𝑥 𝑚𝑖𝑛 𝜎 𝑁 𝑋𝑆 𝐹 𝑣𝑎𝑟 keV Count/s Count/s ( − ) ( − ) XMM1 0.2-2.0 1117 21.95 19.58 1.12 0 . ± .
003 1 . ± . . ± .
015 5 . ± . . ± .
011 6 . ± . . ± .
023 3 . ± . . ± .
035 3 . ± . . ± . . ± . . ± .
007 7 . ± . . ± .
024 3 . ± . . ± .
06 2 . ± . . ± .
31 8 . ± . . ± . − XMM2 0.5-10.0 1294 10.36 6.73 1.54 2 . ± .
02 5 . ± . . ± .
01 5 . ± . . ± .
44 20 . ± . . ± .
42 15 . ± . . ± .
46 18 . ± . . ± .
25 14 . ± . . ± .
49 23 . ± . correlation function ( ZDCF , Alexzander (1997)) for comparison.The likelihood is calculated using 12000 simulation points in the ZDCF code for the lightcurves obtained by
XMM-Newton . The peakerror is calculated using the formula provided by Gaskell & Peterson(1987). We have followed a similar procedure as in Chatterjee et al.(2020). The time resolution of each light curve is 1000s. The0 . − 𝜒 𝑟𝑒𝑑 < . 𝜒 𝑟𝑒𝑑 < . XMM-Newton /Epic-pndata to ensure the simultaneity in their procurements.The
DCF (Edelson & Krolik 1988), performed using thelightcurves, have generated three distinct patterns. The 2003 datahas produced 2 . ± .
67 minutes or ∼ .
16 ks delay. We havefitted the peak using a
Gaussian model (dotted line in Fig. 4).Considering the error, no delay can be seen between two bands ofX-ray. Similar delay pattern is also observed from
ZDCF , and thelikelihood density also maximizes around zero. Likewise, we haveperformed
Gaussian fitting for 2013 data where a positive delayof 78 . ± .
17 minutes or ∼ . DCF . But, the
ZDCF peak maximizesaround 112 . ± .
22 minutes or 6.7 ks and likelihood peak coin-cides with that (see Fig. 4). In 2014, the delay sign have switched,and we find a negative delay of − . ± .
67 minutes or ∼ − . DCF analysis. However,
ZDCF peaks maximize around two positions, − . ± .
67 ( − . ± . − . ± .
46 ( − . ± .
58 ks) minutes having peakvalues of 0.664 and 0.722 respectively. Between these two, the for-mer coincides with the
DCF pattern (see, Table 7 for details). For all three cases, we find the peak values of ZDCF patterns are lesser thanthe corresponding peak values obtained from
DCF patterns.
We have studied the central region of Ark 120 through X-ray (above0.2 keV) using the data of
XMM, Suzaku, NuSTAR and
Swift/XRT in the period 2003 (MJD-52875) to 2018 (MJD-58118). As it is abare type AGN, the X-ray spectra mainly generated from the nearbyregion of the central engine.
The ‘bare-type AGN’ Ark 120 was observed for a period of fifteenyears, 2003 to 2018 using various X-ray satellites. During these ob-servations, the source has exhibited variabilities in both spectral andtemporal domain. The luminosity of the source in the energy rangeof 2.0 to 10.0 keV varied within ∼ . − . erg/s through-out these observations. From the nthcomp model, we report thevariation of the spectral index (1.6< Γ <2.08) where the harder spec-tra were observed after 2014. Following Vaughan et al. (2004), wehave fitted the 2003 spectrum of Ark 120 with ( nthcomp + Gaus-sian ) model. The fitted Γ = . + . − . agrees with the spectral in-dex previously observed (Table 4 of Vaughan et al. (2004)). Corre-sponding temperature of the Compton cloud is 𝑘𝑇 𝑒 = . + . − . keV. The ( TCAF + Gaussian ) model provided a few previously un-known parameters like accretion rates, disc rate ¤ 𝑚 𝑑 = . ± . ¤ 𝑚 ℎ = . ± . 𝑋 𝑠 ), estimated from the fits,is 20 . ± . 𝑟 𝑔 . The shock is found to be moderately strong witha compression ratio of 𝑅 = . ± . Γ = . + . − . is seen during the MNRAS , 1–16 (2020) P Nandi et al.
XMM1 C oun t/ s ks (a) 0.2-2.0 keV(b) 3.0-10.0 keV
0 40 80 120
XMM2 ks (a) 0.2-2.0 keV(b) 3.0-10.0 keV
0 40 80 120
XMM3 ks (a) 0.2-2.0 keV(b) 3.0-10.0 keV -1-0.5 0 0.5 1-80 -40 0 40XMM1 D C F ks -1-0.5 0 0.5 1 -80 -40 0 40 80XMM2 D C F ks -1-0.5 0 0.5 1 -80 -40 0 40 80XMM3 ICF D C F ks -1-0.5 0 0.5 1-80 -40 0 40 80 XMM1 Z D C F D C F ks -1-0.5 0 0.5 1 -80 -40 0 40 XMM2 Z D C F p li k e ks -1-0.5 0 0.5 1 -100 -50 0 50 100 0 0.01 0.02XMM3 Z D C F D e n s it y ( ρ ) ks Figure 4.
Top panel:
The light-curves of the energy ranges of 0 . . . . XMM-Newton are plotted for three epochs. Thehigh energy count always remained a fraction of low energy counterpart. In 2013, the low-energy count dropped to nearly 50% as compared to 2003. Again in2014, the 0 . − Middle panel:
Corresponding discrete cross-correlations between light-curves of 0 . − −
10 keV are plotted. All three epochs exhibited different patterns where zero , positive , and negative delays are observed in 2003, 2013, and 2014respectively. We have also presented the ICF (solid-blue line) for XMM3 observation.
Lower panel: 𝜁 -discrete cross-correlations (light-green) are plotted forlight-curves of 0 . − −
10 keV. While 2003 and 2013 patterns remain similar to what have been observed from
DCF , the pattern obtained from 2014data develops twin peak. The likelihoods (dark-green), simulated using 12000 points, are plotted along with the
ZDCF . Table 7.
Parameters used in delay estimations are presented. The error in measurement of delay is considered as the larger between binsize and 𝜖 𝜏 . 𝜖 𝑑𝜏 and 𝜖 𝑧𝜏 represents errors for DCF and
ZDCF patterns.
Id Epochs Bin size 𝜖 𝑑𝜏 Δ 𝜏 𝑑𝑐 𝑓𝑑 𝜖 𝑧𝜏 Δ 𝜏 𝑧𝑑𝑐 𝑓𝑑 Year (ks) (ks) (ks) (ks) (ks)XMM1 2003 1 0.388 0 . ± − . ± . ± . ± . − . ± − . ± . − 𝑑𝑜 − − 𝑑𝑜 − − . ± . Suzaku observation in 2007. It is to be noted that, Nardini et al.(2011) found the spectral index to be
Γ = . + . − . for the Suzaku data using blurred reflection model. We have estimated the temper-ature of the Compton cloud to be 𝑘𝑇 𝑒 = . + . − . keV. Thisis the least of all temperatures obtained from all the observations.Using a single Gaussian , we find the presence of a broad iron line(6 . + . − . ) keV having an equivalent width of 𝐸𝑊 = + − eV.The derived optical depth is 𝜏 = . + . − . . This suggests an opti-cally thin Compton cloud. From the TCAF fits, we find that the size of the Compton cloud has slightly increased to 𝑋 𝑠 = . ± . 𝑟 𝑔 from the earlier observation. Corresponding disc rate, which en-hances the soft seed photons, has increased to ¤ 𝑚 𝑑 = . ¤ 𝑚 ℎ = . 𝑘𝑇 𝑒 could be under-stood easily from TCAF, where the increase in disc rate leads toan enhanced cooling fraction. Thus, within the epochs of 2003 and2007, the temperature of the Compton cloud was varied from 159.45to 124.65 and as a result the spectrum softened. MNRAS , 1–16 (2020) rigin of soft-excess of Ark 120 Γ kT e (in keV)
20 40 60 80 1.4 1.6 1.8 2 2.2(d) X s ( i n r g ) Γ
20 40 60 80 0.6 0.65 0.7 0.75(c) X s ( i n r g ) τ R τ Figure 5.
Correlation of fitted parameters are plotted. Fig. (a) represents the correlation between Γ vs 𝑘𝑇 𝑒 and the corresponding PCC is -0.95. It is also notedthat the 𝑘𝑇 𝑒 is losely bound with Γ . Fig. (b) is the correlation between 𝜏 and R. The PCC for these parameters is -0.72. In Fig. (c) represents the correlationbetween 𝜏 vs 𝑋 𝑠 and the corresponding PCC is -0.45. Fig. (d) provides the correlation of Γ vs 𝑋 𝑠 with PCC -0.53. Later, in 2008,
Swift observed the source where the spectrumhardened from the previous observation having
Γ = . + . − . , 𝑘𝑇 𝑒 = . + . − . keV, and optical depth 𝜏 = . + . − . . Theiron line could not be detected from the XRT spectrum. Correspond-ing TCAF fitted parameters, such as the shock location 30 . 𝑟 𝑔 and 𝑅 = .
80 while ¤ 𝑚 𝑑 and ¤ 𝑚 ℎ have changed to 0 .
064 and 0 .
11 respec-tively.Significant variation of spectral properties is also noted during2013 and 2014. The broad-band spectra (3-78) keV are fitted with( nthcomp + Gaussian ) having the spectral indices 1 . + . − . and1 . + . − . and are in good agreement with parameters obtained byPorquet et al. (2018); Marinucci et al. (2019). The optical depth isreduced from 𝜏 = . ± .
074 to 𝜏 = . ± . ¤ 𝑚 𝑑 changed from0 .
068 to 0 . ¤ 𝑚 ℎ changed from 0 .
111 to 0 . 𝑋 𝑠 changedfrom 52 .
83 to 28 .
24 within 2013 and 2014 observations respectively.As the disc accretion rate increases, Compton cooling increases, andthis lead to the decrease in the 𝑋 𝑠 which finally softens the spectrum.Considering TCAF, the lower optical depth for softer spectrumcould be explained by the weakening of the shock ( 𝑅 = .
43 as compared to 𝑅 = .
83 in February 2013) for this observation.The stronger shock creates a distinct boundary between the haloand CENBOL region where the majority of the hard photons areproduced. However, for the weaker shock, the CENBOL boundaryis less sharp and a fraction of inverse Comptonization could occurwithin the halo component. Thus, the effective optical depth ofthe medium could become lower even though the spectrum hassoftened.Ark 120 has shown significant variabilities after February 2014and is monitored by
Swift . We have tabulated the spectral and tem-poral variabilities in Table 2 and 5. During September-October of2014, we find that the spectral slope was
Γ = . + . − . and the cor-responding temperature was 274 . ± . ¤ 𝑚 𝑑 and ¤ 𝑚 ℎ has changed to 0 .
068 and 0 .
11 respectively andthe corresponding shock location has changed to 53 . ± . 𝑟 𝑔 andthe shock strength has increased from 2 .
43 to 2 . ± . Γ = . ± .
02 with the temperature of Comptoncloud 215 . ± . . 𝑟 𝑔 and 𝑅 = .
74. Like previousobservations, we see the halo rate and disc rates are fixed at 0 . . .
74 and 1 .
88 respectively. The temperature
MNRAS , 1–16 (2020) P Nandi et al. and optical depths have also varied during this time. From TCAFfitting, we find the halo rate has decreased to 0 .
061 in the XRT4observation. However, the disc rate was constant. Again in XRT5observation, halo rate has increased to 0 .
069 while the disc rateremained the same. The shock location and the compression ratioremained constant (considering the errors) within this period. Thus,we can see that Ark 120 exhibited spectral variability (see Fig. 3)within ∼
200 days (since September 2014-March 2015).In XRT6, which was observed from December 2017 to January2018, the spectrum of Ark 120 has hardened with respect to theearlier observations during January 2015. The spectral index andtemperature of Compton cloud are 1 . ± .
02 and 246 . ± ¤ 𝑚 𝑑 = .
081 & ¤ 𝑚 𝑑 = .
14 respectively and thecorresponding shock location settled at 42 . ± . 𝑟 𝑔 .In Figure 5, we have plotted the correlations of a few spectralparameters. We find the spectral index and the temperature of theCompton cloud is anti-correlated (Fig. 5a with Pearson Correla-tion Co-efficient (PCC) = -0.9542) for the long term observation.However, the values of 𝑘𝑇 𝑒 are poorly constrained with respect tospectral indices. This is a well-established relation and is gener-ally found in case of AGNs and Galactic black holes. In Fig. 5b,we have presented the correlation between shock compression ratioand optical depth. We find 𝑅 − 𝜏 produces anti-correlation hav-ing PCC=-0.721. In general, stronger shocks are associated withthe harder spectra where the optical depth is expected to be less(Chatterjee et al. 2016) and the corresponding shock location isalso expected to be bigger. Keeping that argument, we also showthe 𝑋 𝑠 − 𝜏 correlation where an anti-correlation (PCC=-0.457) hasbeen observed from the long term data and presented in Fig. 5c. Asa consequence, the spectral softens due to the reduction of the shocklocation 𝑋 𝑠 i.e., the size of the Compton cloud, we find a globaltrend of anti-correlation (PCC=-0.562) between 𝑋 𝑠 − Γ (see Fig. 5)for Ark 120.From the nthcomp fitting, it can be found that the Comptoncloud of the source was optically thin for the entire period of obser-vation. Overall, we also noticed that the disc and halo rate is nearlyconstant and they are ∼ .
07 and ∼ .
11 respectively for the major-ity of observations. But, we find a higher disc and halo rate in 2007and 2014 observation. The shock location and the compression ratiohave varied with time. The variation of these parameters is shownin Figure 3. First, the shock location increases with time from 20to 52 𝑟 𝑔 in the first ∼
10 years. Then the shock location falls to26.7 𝑟 𝑔 within the next ∼
13 months. Later, we find that the shocklocation again moves outward from 26.7 to 57.8 𝑟 𝑔 before movinginward again, and finally settling at 42.95 𝑟 𝑔 in January 2018. TheCompression ratio (R) also varies as the shock location ( 𝑋 𝑠 ). First,the compression ratio increased from 1.95 to 2.83 in ∼
10 years.Then, the value of 𝑅 decreased to 1 .
67 within next 1 year. After that,it increased to 2.73 within less than six months and finally reached2.69 at the end of January 2018.
The Compton delay (Payne 1980; Sunyaev & Titarchuk 1980) foran electron cloud of size R having an optical depth 𝜏 and temperature 𝜃 𝑒 = 𝑘𝑇 𝑒 / 𝑚 𝑒 𝑐 can be described by, 𝑡 𝑐 = R 𝑐 ( + 𝜏 ) 𝑙𝑛 ( 𝐸 ℎ / 𝐸 𝑠𝑠 ) 𝑙𝑛 [ + 𝜃 𝑒 ( + 𝜃 𝑒 )] , where, 𝑐 is the velocity of light, 𝐸 ℎ and 𝐸 𝑠𝑠 are the energy ofhard photons and soft seed photons respectively. For AGNs having a central black hole mass of 1 . × (Peterson et al. 2004), theseed temperature of the photons remains in the 1-10 eV range. Themaximum of the hard and soft energy band is considered to be 10keV and 1 keV and the seed photon temperature is 𝐸 𝑠𝑠 = 𝑟 𝑔 is 𝑟 𝑔 / 𝑐 = nthcomp and TCAF model.We have calculated the Compton delay for XMM1 observationwhere the size of the Compton cloud is ∼ 𝑟 𝑔 , optical depth0 . 𝜃 𝑒 = . 𝑡 ℎ𝑐 = . 𝑡 𝑠𝑐 = . Δ 𝜏 = 𝑡 ℎ𝑐 − 𝑡 𝑠𝑐 =
30 ks. However, from the observed
DCF pattern,we fail to notice any such delay for this case. Here, we find lightcrossing delay ( 𝜏 𝑙𝑐 ) of 30 ks for a ∼ 𝑟 𝑔 Compton cloud. Theobserved zero-delay could be a combined result of 𝜏 𝑐 and 𝜏 𝑙𝑐 . Inthat case, it is to be noted that 𝜏 𝑙𝑐 becomes crucial in presence of asignificant contribution of reflection component ( 𝑅 𝑟𝑒 𝑓 = .
96, seeTable 5).For the broadband observation (XMM2+N1), the size of theCompton cloud is
R ∼ 𝑟 𝑔 , having an optical depth of 0 . 𝜃 𝑒 = . 𝑡 ℎ𝑐 =
208 ks and 𝑡 𝑠𝑐 =
148 ks respectively.Thus, the maximum delay between hard and soft bands of X-raycan be Δ 𝜏 = 𝑡 ℎ𝑐 − 𝑡 𝑠𝑐 =
60 ks. The light crossing delay is around 𝜏 𝑙𝑐 =
75 ks. The combined effects of Δ 𝜏 and 𝜏 𝑙𝑐 should yield a neg-ative delay of 15 ks. However, as discussed previously, 𝜏 𝑙𝑐 coulddominate if reflection becomes dominating (here 𝑅 𝑟𝑒 𝑓 = . transition radius ’ (see, Dutta & Chakrabarti(2016); Dutta, Pal & Chakrabarti (2018) for details) of an AGNhaving mass 1 . × 𝑀 ⊙ . Being an intermediate inclination anglesource (Nardini et al. 2011; Marinucci et al. 2019), Comptoniza-tion dominates the time delay when the size of the Compton cloudis bigger. The theoretical structure of Compton cloud is somewhatdeviated from the sphere (see, Chakrabarti & Titarchuk (1995))and the thermodynamical fluctuations within the inhomogeneousCompton cloud (see, Chatterjee et al. (2017b)) contributes to thedelay patterns. Considering this, the effect of light crossing delaywould be much less and Comptonization could be considered asthe core process, which generates 0 . − R = 𝑟 𝑔 , the optical depth is 𝜏 = . 𝜃 𝑒 = . 𝑡 ℎ𝑐 = . 𝑡 𝑠𝑐 = . Δ 𝜏 = 𝑡 ℎ𝑐 − 𝑡 𝑠𝑐 =
35 ks. Contrary to that, the ob-served delay is − . ± .
67. Clearly, the Comptonization maynot be the dominating radiative process for this observation. FromTable 5, we see that the reflection co-efficient 𝑅 𝑟𝑒 𝑓 = .
96, whichrefers to a stronger reflection. It is also to be noted that Lobban et al.(2018) found the X-ray to be leading the U-band by 2 . ± . 𝜏 𝑙𝑐 becomes 42 ks, which is comparableto compensate for the positive lag obtained from Comptonization.In this particular case, the maximum possible negative delay wouldbe Δ 𝜏 − 𝜏 𝑙𝑐 ∼ − 𝑅 𝑟𝑒 𝑓 is much less thanthe XMM1 observation. Thus, the contribution from 𝜏 𝑙𝑐 could beless effective and we observe a negative delay much less than themaximum allowed delay. MNRAS , 1–16 (2020) rigin of soft-excess of Ark 120 l og ( L i n t S E ) ( . - ) e r g / s log(L intPC ) (0.5-10) erg/s ObservedSimulated l og ( L i n t S E ) ( . - ) e r g / s l og ( L i n t P C ) ( . - ) e r g / s N H (10 cm -2 ) Soft-ExcessPrimary Continuum
Figure 6.
Correlation of intrinsic luminosities of 0.5-10.0 keV obtained using nthcomp . Left: shows a correlation (PCC=0.92) between the observed intrinsicluminosities of primary continuum and soft-excess (blue-circle). Monte-Carlo simulated luminosities for both energy ranges are presented with red-diamondpoints.
Right:
No correlation between intrinsic luminosities and 𝑁 𝐻 from long term observations. Thus, along with the spectral variations, we find the delaypatterns have varied over the three epochs (2003, 2013, and 2014)in which
XMM-Newton observed Ark 120. A significant change inthe delay pattern is observed within a year (2013-2014) where thepositive delay changed sign and becomes negative with a similarmagnitude.
The origin of ubiquitous soft-excess (Arnaud et al. 1985;Singh et al. 1985; Brandt et al. 1993; Fabian et al. 2002;Gierliński & Done 2004) remains debated. A plausible cause ofsoft-excess was given using reflection Sobolewska & Done (2007).The multi-wavelength campaign of Mrk 509 (Mehdipour et al.2011) revealed the correlation of soft-excess with the optical-UVpart both in the spectral and temporal domains where they con-cluded that the soft-excess was generated due to Comptonizationby a warm optically thick region surrounding the accretion disc.Done et al. (2012) proposed that the high mass accretion rate of thedisc could generate the soft-excess. For lower 𝐿 / 𝐿 𝐸𝐷𝐷 , the energydependent variability in the soft-excess part was found to be less incase of Narrow line Seyfert 1 galaxies. Lohfink et al. (2012) stud-ied Seyfert 1 galaxy Fairfall 9 where the origin of the soft-excesscomponent was found to be connected with source which gener-ates the broad iron line. However, they implied that another sourceof Comptonization might be responsible for the formation of thesoft-excess.A strong soft-excess present in the X-ray spectrum of Ark120 was reported by Brandt et al. (1993); Matt et al. (2014);Porquet et al. (2004). This soft-excess is also free from the ab-sorbers and was reported by Nardini et al. (2011). As a firststep, we investigate the spectral slopes and the relative contri-bution of the soft-excess from 2003 to 2018 using the nth-comp+zGaussian+powerlaw model and the results are presentedin Table 3. Subsequently, we freeze the Γ 𝑛𝑡ℎ obtained from nth-comp while fitting the soft excess below 3 keV. The Γ 𝑝𝑙 fits thesoft-excess < 3 keV. For every observation, we find a soft-excesssteeper than the primary continuum (see, Table 3) which is a char-acteristic associated with the Narrow line Seyfert 1 galaxies. Apartfrom the steeper power law, the variation of soft-excess luminosity and spectral index can be observed from long term observationspresented in Table 3. We have calculated the intrinsic luminositiesof nthcomp and powerlaw within the energy range 0.5 to 10.0 keV.In Fig. 6a, we see a strong correlation (PCC=0.9227) between theintrinsic luminosities of soft-excess ( 𝐿 𝑆𝐸𝑖𝑛𝑡 ) and primary continuum( 𝐿 𝑃𝐶𝑖𝑛𝑡 ). However, as a “bare” type AGN, Ark 120 has not shown anycorrelation (Fig. 6b) among the intrinsic luminosities and the lineof sight hydrogen column density ( 𝑁 𝐻 ).While nthcomp provides a good fit in the high energy range,we have used TCAF+zGaussian+pexrav model (presented in Table5) in the entire range. We find that the
TCAF fits well in the rangeof 0 . −
10 and requires no other additional model for the soft-excess part with the range of 0 . − TCAF , one recognizes that the soft-excess could be originated fromthe photons which are rarely scattered in the Compton cloud. Thesurrounding halo will contribute to this energy band (0.2 - 2 keV).Also, some high energy photons from the Compton cloud whichcould be reflected from the disc will appear in this energy range afterlosing their energy through reflection from the cold disc. We haveperformed Monte-Carlo simulations to show the spectral variationswith 𝑁 𝑠 . This is briefly discussed in Sec. 5.3.1. Radiative and hydrodynamic origin of soft-excess has been inves-tigated in Fukumura et al. (2016) where they proposed that theshock heating near the ISCO could produce the soft-excess. Themodel reproduced the spectra of “bare” Seyfert 1 galaxy, Ark120. We have inspected the possibility of scattering dependentspectral contribution from the pre-shock and the post-shock re-gions (Chakrabarti & Titarchuk 1995). We extend the work ofGhosh et al. (2011); Chatterjee et al. (2018) in case of AGNs con-sidering Ark 120. Using the Total Variation Diminishing (
TVD )scheme (Ryu et al. 1997), we inject matter having a halo rate of 0 . 𝑟 𝑔 . TCAF fitted parameters are usedfor the simulation setup and are mentioned in the Fig. 7. Consideringthe Keplerian disc in the equatorial plane ( 𝑧 = < 𝑟 < 𝑟 𝑔 ) has followed the MNRAS , 1–16 (2020) P Nandi et al. -2 -1 -4 -3 -2 -1 L ν Energy in keV 2
Monte-Carlo simulated spectra for Ark 120 are presented. We have considered 𝑀 𝐵𝐻 = . × 𝑀 ⊙ for Ark 120. Simulation boundary extends upto 100 𝑟 𝑔 . For left panel ¤ 𝑚 𝑑 = 0.06; ¤ 𝑚 ℎ = 0.1, 𝑋 𝑠 = 𝑟 𝑔 , and maximum 𝑘𝑇 𝑒 =
270 keV. For right panel ¤ 𝑚 𝑑 = 0.1; ¤ 𝑚 ℎ = 0.1, 𝑋 𝑠 = 𝑟 𝑔 , and maximum 𝑘𝑇 𝑒 =
100 keV. Notice the spectral contributions due to increasing number of scatterings. 𝐿 𝜈 has been normalized with respect to the observed spectrum. process provided by Pozdnyakov et al. (1983) and later extended byGhosh et al. (2009); Chatterjee et al. (2017a). The simulations areperformed using 10 injected photons for each case. The emergentComptonized spectra are plotted in Fig. 7. We show the variation ofspectral components with respect to the number of scatterings (seealso Ghosh et al. (2011)) within the region. From Fig. 7, we find thatthe spectra harden as the number of scatterings increase. The spectraof the primary component within the energy range 2.0 to 10.0 keV isdominated by the photons where the number of scatterings are ≥ ≤
10 scatterings. A steeper spectral slope ( Γ 𝑆𝐸 ) for soft-excess isachieved with respect to the primary component ( Γ 𝑃𝐶 ) for both ofthe spectrum. This is similar to what has been observed for Ark 120(Table 3). It is to be noted that, Boissay et al. (2016) studied theAGN 102 Sy1 and found that there is no link between the reflectionand the soft excess. Instead, they indicated that the soft-excess couldbe related to the thermodynamical properties of Compton cloud andassociated medium. We have studied ∼
15 years of X-ray data of Ark 120. We find thesource varied considerably within that time span. This source waspreviously reported to be a ‘bare-type AGN’ and we also find asimilar nature of this source from the long term analysis. The X-raycount rate has increased by a factor of two in a few years, and it isnot found to be related to the Hydrogen column density ( 𝑁 𝐻 ) sinceit is a ‘bare-type AGN’. Following are the major findings from ourwork.1. The spectral slopes of the primary continuum ( Γ 𝑃𝐶 ) and thesoft-excess ( Γ 𝑆𝐸 ) are not constant throughout our observationaltime span. Γ 𝑃𝐶 has varied between 1.60 and 2.08 whereas Γ 𝑆𝐸 between 2.52 and 4.23 from 2003 to 2018.2. The variation is reflected in fitted parameters of TCAF , namely,the accretion rates and properties of the Compton cloud. From thespectral fitting using
TCAF , we find that the disc rate ( ¤ 𝑚 𝑑 ) and thehalo rate ( ¤ 𝑚 ℎ ) have varied between 0 .
061 and 0 .
126 and between0 .
108 and 0 .
191 respectively. The shock location ( 𝑋 𝑠 ) or the size ofthe Compton cloud and compression ratio ( 𝑅 ) vary correspondingly. 𝑋 𝑠 varies between 20 .
36 and 57 .
87, whereas 𝑅 varies between 1 . . . − . . − . XMM-Newton to calculate the time delay between them. We find that inXMM1 observation, there is no delay between the low and highenergy band, while a positive delay of 4 . ± . ± 𝑁 𝐻 ). Thisis expected as the source has a negligible line-of-sight hydrogencolumn density ( 𝑁 𝐻 < × ). The luminosity of the primarycontinuum is highly correlated (PCC ∼ .
92) with the soft excessemission. From
TCAF fitting and Monte-Carlo simulations usingTCAF flow configurations, we show that the soft-excess spectralslope ( Γ 𝑆𝐸 ) is the result of a fewer Compton scatterings in theCompton cloud and the primary continuum ( Γ 𝑃𝐶 ) is the resultof the higher number of Compton scatterings. Corresponding in-trinsic luminosities obtained from simulations corroborate with theobserved pattern. ACKNOWLEDGEMENTS
PN acknowledges CSIR fellowship for this work. AC acknowl-edges Post-doctoral fellowship of S. N. Bose National Centre forBasic Sciences, Kolkata India, funded by Department of Scienceand Technology (DST), India. BGD acknowledges Inter-UniversityCentre for Astronomy and Astrophysics (IUCAA) for the Visit-
MNRAS , 1–16 (2020) rigin of soft-excess of Ark 120 ing Associateship Programme. This research has made use of dataand/or software provided by the High Energy Astrophysics ScienceArchive Research Center (HEASARC), which is a service of the As-trophysics Science Division at NASA/GSFC and the High EnergyAstrophysics Division of the Smithsonian Astrophysical Observa-tory. This work has made use of data obtained from the Suzaku , acollaborative mission between the space agencies of Japan (JAXA)and the USA (NASA). This work made use of data supplied bythe UK Swift Science Data Centre at the University of Leicester.This work has made use of data obtained from the
NuSTAR mis-sion, a project led by Caltech, funded by NASA and managed byNASA/JPL, and has utilized the NuSTARDAS software package,jointly developed by the ASDC, Italy and Caltech, USA. This re-search has made use of the NASA/IPAC Extragalactic Database(NED) which is operated by the Jet Propulsion Laboratory, Cal-ifornia Institute of Technology, under contract with the NationalAeronautics and Space Administration. This research has made useof the SIMBAD database, operated at CDS, Strasbourg, France.
DATA AVAILABILITY
We have used archival data for our analysis in this manuscript.All the softwares used in this manuscript are publicly available.Appropriate links are given in the manuscript.
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