Suzaku Observation of the Brightest Broad-Line Radio Galaxy 4C 50.55 (IGR J 21247+5058)
Fumie Tazaki, Yoshihiro Ueda, Yukiko Ishino, Satoshi Eguchi, Naoki Isobe, Yuichi Terashima, Richard F. Mushotzky
aa r X i v : . [ a s t r o - ph . H E ] A ug Draft version October 31, 2018
Preprint typeset using L A TEX style emulateapj v. 11/10/09
SUZAKU
OBSERVATION OF THE BRIGHTEST BROAD-LINE RADIO GALAXY 4C 50.55 (IGR J 21247+5058)
Fumie Tazaki , Yoshihiro Ueda , Yukiko Ishino , Satoshi Eguchi , Naoki Isobe ,Yuichi Terashima , Richard F. Mushotzky Draft version October 31, 2018
ABSTRACTWe report the results from a deep
Suzaku observation of 4C 50.55 (IGR J 21247+5058), the brightestbroad-line radio galaxy in the hard X-ray ( >
10 keV) sky. The simultaneous broad band spectra over1–60 keV can be represented by a cut-off power law with two layers of absorption and a significantreflection component from cold matter with a solid angle of Ω / π ≈ .
2. A rapid flux rise by ∼ × sec is detected in the 2–10 keV band. The spectral energy distribution suggests thatthere is little contribution to the total X-ray emission from jets. Applying a thermal Comptonizationmodel, we find that corona is optically thick ( τ e ≈
3) and has a relatively low temperature ( kT e ≈ Subject headings: galaxies: active – galaxies: individual (4C 50.55) – X-rays: galaxies INTRODUCTION
Radio galaxies are a class of Active Galactic Nuclei(AGNs) spouting powerful radio jets whose axis is notaligned along the line of sight. While the formationmechanism of relativistic jets is not fully understood, itmust be closely related to mass accretion flow onto thecentral supermassive black hole. In fact, studies of Galac-tic black holes have revealed that the relative power ofthe jets to accretion critically depends on the state of theaccretion disk, which is predominantly determined by themass accretion rate (e.g., see Fender et al. 2004b). ForAGNs, however, detailed studies of accretion disk state inrelation to the jet formation are still limited. Broad bandobservation of nearby, bright radio galaxies hence give usideal opportunities to unveil this problem, because, un-like “blazars”, the innermost disk can be well observablein the X-ray band with much smaller contribution fromthe jet emission.4C 50.55 (IGR J21247+5058) is a Fanaroff-Rileytype II broad line radio galaxy (BLRG), discov-ered by
INTEGRAL in its first survey catalogue(Bird, Barlow & Bassani 2004). The source is also de-tected in the first 9 months data of
Swift
Burst AlertTelescope (BAT) survey of AGNs (Tueller et al. 2008).Though being the brightest BLRG in the hard X-ray sky above 10 keV, 4C 50.55, located at ( l, b ) =(93 . , . INTEGRAL and
Swift era, due to theobscuration by the Galactic plane. This source is also ofgreat interest for understanding the accretion flow ontosupermassive black holes at high fractions of Eddingtonluminosity, which is estimated to be L bol /L Edd ∼ . Department of Astronomy, Kyoto University, Kyoto 606-8502, Japan Department of Physics, Ehime University, Matsuyama 790-8577, Japan Department of Astronomy, University of Maryland, CollegePark, MD, USA shift to be z = 0 . ± .
001 based on the detection of abroad H α emission line in the optical spectrum. From theobserved flux densities at 1.4 GHz with the Very LargeArray (VLA), Molina et al. (2007) estimate the inclina-tion angle to be θ ∼ ◦ , assuming a Lorentz factor of γ = 5 and a typical power ratio between the core andtotal known for radio galaxies (Giovannini et al. 1988).The inferred viewing geometry of the nucleus is well con-sistent with the classification of 4C 50.55 as a BLRG.The initial results on the X-ray spectra are reportedby Molina et al. (2007), who used the XMM-Newton and
Swift /XRT data combined with a time-averaged
INTE-GRAL spectrum. They find that the spectra below 10keV are apparently hard, and complex, multiple layersof absorber are required. Iron-K features are not sig-nificantly detected and the strength of reflection com-ponents, R ≡ Ω / π , where Ω is the solid angle of thereflector, is not tightly constrained ( R < ∼ . Suzaku (Mitsuda et al.2007), obtained in a different epoch of the
XMM-Newton observation.
Suzaku carries (then available) three CCDcameras called the X-ray Imaging Spectrometers, XIS-0,XIS-3 (Front-side Illuminated XIS; XIS-FI), and XIS-1(Back-side Illuminated XIS; XIS-BI), and a collimated-type instrument called the hard X-ray detector (HXD),which consists of Si PIN photo-diodes and GSO scin-tillation counters. The XISs and HXD-PIN covers the0.2–12 keV and 10–70 keV, respectively. Since AGNsare time variable, the simultaneous coverage is criticalfor studying the continuum shape over the broad band,in particular to accurately constrain the Compton reflec-tion component. Moreover, the
Suzaku exposure is quitedeep (a net exposure of ∼
100 ksec), and thus providesus with the best quality dataset so far obtained from thissource.The organization of our paper is as follows. First, wedescribe our
Suzaku observation and data reduction insection 2. Next, we present the analysis and results in Tazaki et al.section 3. The results of the
Swift /BAT data are alsopresented, whose spectrum averaged over 22 months isutilized to constrain the highest energy band up to 200keV. We plot the spectral energy distribution of 4C 50.55from the radio to Gamma-ray bands to discuss the pos-sible contribution of the jet component. Finally, we dis-cuss our results in comparison with the previous studiesof this target and other radio galaxies in section 4. In allspectral analysis, we apply the Galactic absorption fixedat N galH = 1 . × cm − (Kalberla et al. 2005). Thecosmological parameters ( H , Ω m , Ω λ ) = (71 km s − Mpc − , 0.27, 0.73; Komatsu et al. 2009) are adopted incalculating the luminosities. The errors attached to spec-tral parameters correspond to those at 90% confidencelimits. OBSERVATIONS AND DATA REDUCTION
4C 50.55 was observed with
Suzaku from 2007 April16 14:05 (UT) to April 18 11:04 (UT) (observation ID702027010), focused on the nominal center position ofthe Hard X-ray Detector (HXD). The net exposures afterdata screening, described below, are 85.0 ksec (XIS-0, 1),80.0 ksec (XIS-3), 54.7 ksec (PIN). For the HXD, we useonly data collected with the PIN diodes, because we findthat the signal from the source in the GSO data is notsignificant over the systematic error of ≈
2% in the nonX-ray background (NXB) (Fukazawa et al. 2008). Weuse FTOOLS (heasoft version 6.6.2) to extract data, andXSPEC version 11.3.2ag for spectral fitting.
XIS Data Reduction
To apply the latest calibration, we reprocess the unfil-tered event files of the XIS data according to the pro-cedures written in
The Suzaku Data Reduction Guide(ABC Guide) . We select the data where the time sincethe South Atlantic Anomaly (T SAA) passage is longerthan 436 sec, the elevation angle (ELV) is larger than 5 ◦ ,and the dye-elevation angle (DYE-ELV) is larger than20 ◦ . The XIS events are extracted from a circular re-gion centered on the source peak, and the backgroundis taken from a source-free region with the same dis-tance from the optical axis as the target. We generateRMF files with xisrmfgen , and ARF files with xissimar-fgen (Ishisaki et al. 2007). We examine the spectra ofthe Fe calibration source (producing an Mn K α lineat 5.895 keV) located on the corners of the XIS chipsto check the accuracy of the energy response. By fit-ting them with Gaussians, we are able to reproduce theright central energy within the statistical errors and linewidths of 3 . < .
2) eV (XIS-0), 0 . < .
6) eV (XIS-1), and 0 . < .
0) eV (XIS-3). This verifies that bothenergy scale and energy resolution in the responses arewell reliable.
HXD-PIN Data Reduction
We also reprocess the unfiltered event files of the HXDdata as well. This include the time assignment (with hxdtime ), gain correction ( hxdpi ), and grade classifica-tion ( hxdgrade ). We select data where the time afterthe SAA passage is longer than 500 sec, a cutoff rigid-ity (COR) is larger than 6 GV, and the elevation fromthe earth is larger than 5 ◦ . We utilize the “tuned” NXBevent files provided by the HXD team to produce the − × − [ e r g s / c m / s ] Seconds since 2001 Jan 1 00:0014 − 195 keV
Suzaku ObservationApril 16−18 2007 Bin time: 16 days
Fig. 1.—
Long term light curve of 4C 50.55 in the 14–195 keVband obtained with
Swift /BAT with 16 days bin. The fluxes areconverted from the count rate by assuming a power law photonindex of Γ = 1 .
7. The dotted lines indicate the period of the
Suzaku observation (from 2007 April 16 to April 18). background spectra, to which the cosmic X-ray back-ground (CXB) based on the formula by Gruber et al.(1999) is added. We use the HXD/PIN response fileae hxd pinhxnome3 20080129.rsp. ANALYSIS AND RESULTS
Swift/BAT Data
Figure 1 shows the long term light curve of 4C 50.55in the 14–195 keV band obtained with
Swift /BAT from2004 December 15 to 2009 October 10. Each data pointcorresponds to the averaged flux for 16 days. Timevariability is clearly noticed. The epoch of our
Suzaku observation is indicated by the dashed line in the fig-ure. The time-averaged BAT spectrum over the first 22months covering the 14–195 keV band is plotted in Fig-ure 2. We find that it can be fit with a single powerlaw of Γ = 1 . ± .
25. The time averaged 14–195 keVflux is 1 . × − ergs cm − s − (based on the best-fitpower law model), which corresponds to a luminosity of1 . × ergs s − in the rest-frame 14.3–199 keV band.In the subsequent subsections, we will apply more real-istic spectral models to this spectrum. Suzaku Light Curve
We make the
Suzaku light curves of 4C 50.55 witha 5760 sec bin, the orbital period of the satellite, toremove any possible modulation related to the orbitalcondition. Figure 3 shows the X-ray light curves ob-tained with the XIS-FIs (2–10 keV, upper ) and with theHXD/PIN (15–40 keV, middle ), and their hardness ratio(PIN/XIS, lower ). The zero point of time correspondsto the start time of the
Suzaku observation. The XISlight curve in the 2–10 keV band indicates a rapid fluxincrease by ∼
20% in the last 20 ksec exposure. The timescale of this variability is ∼ sec, indicating that theemission region is within ∼ r g ( r g ≡ GMc is the gravita-tional radius) for a black hole mass of M = 10 . M ⊙ (seesection 3.5). By contrast, the light curve in the 15–40keV band does not show evidence for significant variabil-ity above the statistical errors in the same epoch. Thus, uzaku Observation of 4C 50.55 3 − − − no r m a li z ed c oun t s / s e c / k e V − χ channel energy (keV) Fig. 2.—
The time-averaged
Swift /BAT spectrum of 4C 50.55over 22 months. The best-fit model is a pexriv model (see Table 1). . . . . C oun t/ s e c Epoch 1 Epoch 2 . . . C oun t/ s e c . . . H a r dne ss R a t i o time [sec]15−40keV / 2−10keV Fig. 3.—
Light curves of 4C 50.55 obtained by the
Suzaku observation, with a 5760 sec bin. The start time corresponds to2007 April 16. The dotted line shows the border between epochs1 and 2. ( upper ) the light curve of XIS-FIs in the 2–10 keVband. ( middle ) that of PIN in the 15 – 40 keV band. ( lower ) thehardness ratio between them (15–40 keV / 2–10 keV). there is a hint for decrease in the hardness ratio betweenthe PIN and XIS, although its significance is marginal.For the following spectral analysis, we separate the ob-servation period into two, epoch 1 (0–1.4 × sec) andepoch 2 (1.4 × –1.7 × sec). Spectral Analysis with Phenomenological Models
Individual Fit to the Suzaku and Swift Data
We analyze the
Suzaku spectra of epoch 1 and epoch 2separately, by performing simultaneous fit to those of theXIS-FIs in the 1–9 keV band, the XIS-BI in the 1–8 keVband, and the HXD/PIN in the 12–60 keV band. Giventhe fact that the spectrum of 4C 50.55 is subject to aheavy Galactic absorption, we do not utilize the XIS-BIdata below 1 keV to avoid any possible uncertainties inthe response. We find that the effects of pile-up are sig-nificant above 9.0 keV for the XIS data. Further, wediscard the XIS data of the 1.7–1.9 keV band because ofcalibration uncertainties associated with the instrumen- tal Si-K edge. In the simultaneous fit, the flux normal-izations for the XIS-FIs and XIS-BI are set free, whilethat of the PIN is fixed at 1.18 relative to that of theXIS-FIs (Maeda et al. 2008).We firstly apply a single power law model (modified bythe Galactic absorption fixed at N galH = 1 . × cm − )to the XIS and PIN data in epoch 1, covering the 1–60 keV band. The fit is found to be far from accept-able ( χ / dof = 13035 / ≈ .
2. This is because the spectral shape be-low 10 keV is much harder than that above 10 keV.Next we apply a cut-off power law model, in the formof E − Γ × exp( − E/E cut ), over which a narrow Gaus-sian is added to represent an iron-K emission line at6.4 keV. We obtain Γ ≈ .
83 and E cut ≈
14 keV with χ / dof = 4616 / z = 0 . . ± . E cut = 45 ± N H =(0 . ± . × cm − with χ / dof = 997 / N and N , whose covering fraction is f and (1 − f ), respectively. The fit is significantly improved( χ / dof = 940 / . +0 . − / , E cut = 140 +46 − keV, N ≈ . × cm − , N ≈ . × cm − , and f = 0 .
19. The photon index becomes a reasonable valuefor AGNs. Note that our “double absorber” model hasthree free parameters for the absorber, while the “dou-ble partial covering” model ( “pcf*pcf*” in the XSPECterminology) adopted by Molina et al. (2007) has four.Since the latter model does not give a significant im-provement for our data (∆ χ ≈ Suzaku spectra are quite useful toconstrain the reflection component, which is indicatedby the presence of the iron-K emission line. Thus, weinclude it by utilizing our modified version of “pexriv”reflection code (Magdziarz & Zdziarski 1995) that as-sumes a cutoff power law continuum and contains a self-consistent fluorescence iron-K line calculated accordingto the same algorithm as described in Zycki et al. (1999).The additional free parameter is the relative reflectionstrength, R ( ≡ Ω / π ), while we fix the ionization param-eter, temperature, and inclination angle at 0, 10 K, and35 degrees (Molina et al. 2007), respectively. The so-lar Fe abundance by Anders & Grevesse (1989) (Fe/H =4 . × − ) is assumed. Considering that the reflec-tion most likely occurs in the accretion disk, we smearboth reflected continuum and iron-K emission line by the “diskline” kernel (Fabian et al. 1989). The innermostradius r in is set as a free parameter by assuming an emis-sivity law of r − for a fixed outer radius r out = 10 r g .When constraining r in , we utilize only the XIS spectraaround the iron-K band (3–9 keV for the XIS-FIs and3–8 keV for the XIS-BI) and fix all the other parametersexcept for the normalization. Thus, the spectral fit isperformed by iteration; after r in is determined from theXIS-only fit, it is then fixed when finally determining thecontinuum parameters in the XIS+PIN fit.The results of the spectral fit to the individual Suzaku spectra in epochs 1 and 2 are summarized in Table 1. Tazaki et al.
TABLE 1Best-Fit Parameters of Cut-off Power Law Model tothe Individual
Suzaku and
Swift
Data
Parameters Epoch 1 Epoch 2
Swift/BAT (a) f −
10 keV . × − . × − – (a) f −
60 keV . × − . × − – (a) f −
195 keV – – 1 . × − N galH § § –Γ 1 . ± .
05 1 . ± .
03 1 . +0 . − . E cut [keV] 80 +36 − +29 − +340 − R (= Ω / π ) 0 . ± . ∗ § (c) f . +0 . − . § – (d) N . +1 . − . § – (d) N . ± .
03 0.73 § – EW [eV] 22 19 – r in [ r g ] 720 § ( > † § § χ / dof 916 . /
805 190 . /
175 8 . / Note . — Errors are 90% confidence level for a singleparameter. (a)
Observed fluxes in the 2–10 keV, 10–60 keV, and 14–195keV bands, in units of ergs cm − s − . (b) Galactic absorption column density in units of 10 cm − . (c) Covering fraction (d)
Intrinsic absorption column density at the source redshiftin units of 10 cm − . § Parameters fixed at these values. ∗ We assume the same reflection component as that deter-mined in epoch 1. † We constrain the inner radius only from the 3—9 keV XISspectra in epoch 1, which is then fixed at the best-fit whendetermining the continuum parameters.
Figure 4 shows the XIS+PIN spectra folded by the en-ergy responses, over which the best-fit models are plot-ted, with residuals in the lower panel. The expandedfigure of the XIS spectra between 3–9 keV in epoch 1 isplotted in Figure 5. To emphasize the iron-K line fea-ture, the residuals when the line is excluded from themodel are shown in the lower panel of this figure. Fromepoch 1, we obtain Γ = 1 . ± . E cut = 80 +36 − keV,and R = 0 . ± .
04. The innermost radius is constrainedto be r in = 720 r g ( > r g ) from the XIS data. For theanalysis of the epoch 2 spectra, we assume the same pa-rameters of the reflection component (including its abso-lute flux) as those found from the epoch 1 data, since it isvery unlikely that it varied on such a short time scale of < sec. The parameters of the absorption are fixed tothe epoch 1 values as well. Thus, only free parameters areΓ, E cut , and the normalization. Finally, we also performspectral fit to the time-averaged Swift /BAT spectrum byadopting the same model. The reflection strength is fixedat R = 0 .
18 referring to the epoch 1 result. The best-fitmodel is over-plotted in Figure 2, whose parameters aresummarized in Table 1.
Simultaneous Fit to the Suzaku and Swift Data
From the above analysis, we find no significant differ-ences in the spectral parameters (except for the normal-ization) within the statistical errors between the
Suzaku epoch 1, epoch 2, and
Swift /BAT data, although thereis a hint that the spectrum became slightly softer inepoch 2. Thus, to best constrain the continuum parame-ters, in particular the cutoff energy, we study the
Suzaku
TABLE 2Best-Fit Parameters of Cut-off Power Law Model to theCombined
Suzaku and
Swift
Data
Parameters Epoch 1 Epoch 2 (1) Epoch 2 (2) (a) L −
10 keV (b) N galH § § § Γ 1 . ± .
04 1 . ± .
02 1.65 § E cut [keV] 105 +28 − +25 − § R (= Ω / π ) 0 . ± . ∗ ∗ EW [eV] (Fe K α line) 21 18 18 (c) f . ± .
03 0.21 § . +0 . − . N . +1 . − . § . +1 . − . N . ± .
03 0.75 § . +0 . − . χ / dof 926 . /
812 201 . /
182 192 . / Note . — Errors are 90% confidence level for a single parameter. (a)
Intrinsic luminosity in the 2–10 keV band corrected for bothGalactic and intrinsic absorptions in units of 10 ergs s − . (b) Galactic absorption column density in units of 10 cm − . (c) Covering fraction. (d)
Intrinsic absorption column density in units of 10 cm − . § Parameters fixed at these values. ∗ We assume the same reflection component as that determined inepoch 1. spectra (either of the two epochs) in the 1–60 keV bandand
Swift spectrum in the 14–195 keV band simultane-ously, in all following analysis. The flux normalizationbetween the
Suzaku (XIS-FIs) and BAT spectra are setfree, to take into account the time variability. In analyz-ing the spectra of epoch 2, we always fix the reflectioncomponent to that determined from the epoch 1 data.Table 2 summarizes the results using the same phe-nomenological model (cutoff power law) as adopted insection 3.3. For epoch 2, we consider two extreme casesas the cause of the spectral variability from epoch 1 that(1) only the continuum changed without change of theabsorber and that (2) only the absorber changed with thesame continuum except for its normalization. We obtainsimilarly good fits for the two cases, and thus both pos-sibilities are plausible from the spectral analysis. In re-ality, however, it may be difficult to explain such a shorttime ( < sec) variability by the absorber alone. If theabsorber makes Kepler motion at ∼ r g , a typical lo-cation of the broad line region in AGNs, it moves only ∼ r g in 10 sec, by assuming the black hole mass of 10 . M ⊙ . Thus, unless the emitting region is extremely small(like < several r g ), it is unlikely that crossing blobs in theline of sight can cause the large variability as observed. Spectral Analysis with Comptonization Model
In this subsection, we analyze the spectra of 4C 50.55with a physically motivated model instead of the phe-nomenological “cutoff power law” model, which is amathematical approximation of the X-ray spectra ofAGNs. Such analysis of AGN spectra has been verylimited so far, since it requires high quality broad banddata. As we will discuss in section 3.5, the contributionfrom the jet components is very small in the X-ray band.Hence, we consider that the origin of the continuum emis-sion is predominantly thermal Comptonization of soft(ultra-violet) photons off hot electrons in the corona lo-cated above the accretion disk. Accordingly, we adopt a uzaku
Observation of 4C 50.55 5 − − . . r m a li z ed c oun t s / s e c / k e V
102 5 20 50 − − χ channel energy (keV) − − . . r m a li z ed c oun t s / s e c / k e V
102 5 20 50 − − χ channel energy (keV) Fig. 4.— (a) ( left ) The folded spectra of 4C 50.55 in epoch 1 obtained with the XIS-FIs in the 1–9 keV band (black), the XIS-BI in the1–8 keV band (red, open circle), and the PIN in the 12–60 keV band (blue). The solid curve represents the best-fit model (see Table 1).The dotted curve represents the reflection component. (b) ( right ) those in epoch 2. . . . r m a li z ed c oun t s / s e c / k e V − − χ channel energy (keV) Fig. 5.— ( upper ) The spectra of 4C 50.55 obtained with theXIS-FIs (black) and XIS-BI (red, open circle) in epoch 1. Thebest-fit model (see Table 1) is plotted by the solid curve. Thedashed curve indicates the model from which the iron-K emissionline is excluded. ( lower ) The residuals of the fit in units of χ forthe model without the iron-K line. thermal Comptonization model, thComp (Zycki et al.1999), for the primary continuum. It has two free pa-rameters, the slope Γ and electron temperature kT e . Theelectron scattering optical depth τ e is related to T e and Γby the following formula (Sunyaev & Titarchuk 1980): τ e = s .
25 + 3( T e /
511 keV)[(Γ + 0 . − . − . . (1)For seed photons, we assume a multicolor disk compo-nent with the innermost temperature of 0.01 keV. Thischoice is not important to constrain the Comptoniza-tion parameters. As described in the previous subsec-tion, the reflection component and fluorescence iron-Kline are self-consistently included, which are blurred bythe “diskline” profile with the same parameters as ob-tained above ( r in = 720 r g ).We perform simultaneous fit to the Suzaku (epoch 1 or2) and
Swift /BAT spectra using this model. The best-fit
TABLE 3Best-Fit Parameters of Comptonization Model to theCombined
Suzaku and
Swift
Data
Parameters Epoch 1 & BAT Epoch 2 & BAT (a) L −
10 keV (b) N galH § § Γ 1 . ± .
02 1 . ± . kT e [keV] 31 +10 − +37 − ( τ e . +0 . − . . +0 . − . ) R (= Ω / π ) 0 . ± . ∗ EW [eV] (Fe K α line) 19 17 (d) f . ± .
02 0.27 § (e) N . +1 . − . § (e) N . ± .
03 0.80 § χ / dof 942 . /
812 200 . / Note . — Errors are 90% confidence level for a singleparameter. (a)
Intrinsic luminosity in the 2–10 keV band correctedfor both Galactic and intrinsic absorptions in units of10 ergs s − . (b) Galactic absorption column density in units of 10 cm − . (c) Electron-scattering optical depth calculated from theequation (1). (d)
Covering fraction. (e)
Intrinsic absorption column density in units of 10 cm − . § Parameters fixed at these values. ∗ We assume the same reflection component as that deter-mined in epoch 1. parameters are summarized in Table 3. We obtain theelectron temperature of ≈
30 keV with an optical depthof ≈
3. Again, the reflection component in epoch 2 arefixed at the same one determined from epoch 1. We findthat the continuum parameters are consistent each otherbetween epochs 1 and 2 within the statistical errors atthe 90% confidence level. Figure 6 shows the results inthe two epochs, in the form of unfolded spectra (i.e.,,those corrected for the effective area of the instruments)in units of EI ( E ), where E is photon energy and I ( E )the energy flux. Multi-Wavelengths Spectral Energy Distribution
Tazaki et al. − − − . . k e V ( k e V / c m s k e V ) Energy (keV) 1 10 100 − − − . . k e V ( k e V / c m s k e V ) Energy (keV)
Fig. 6.— (a) ( left ) The unfolded spectra of 4C 50.55 determined from the
Suzaku data in epoch 1 and from the
Swift /BAT data, based ona thermal Comptonization model. The crosses (black) represent the data points, dashed curve (blue) the transmitted component, dashedcurve (magenta) the reflection component, and solid curve (red) the total. The best-fit parameters are given in Table 3. (b) ( right ) thosein epoch 2. − − − − − − [ e r g / c m s ] [Hz] Fig. 7.—
The SED of 4C 50.55. The solid (black) and dotted(red) curves represent the
Suzaku spectra in epochs 1 and 2, re-spectively, while the thick (purple) one represents their differencespectrum between the two epochs. The spectrum of
Swift /BAT isshown by the blue crosses. The pink arrow represents the upperlimit on the γ -ray flux from the Fermi data (Abdo et al. 2009).The radio data are taken after Molina et al. (2007); the orangefilled-triangle and reddish violet filled-circles represent the core fluxobtained from the GMRT (Pandey et al. 2006) and the VLA, re-spectively. The blue filled-square represent the total flux includingboth core and lobes.
We summarize in Figure 7 the available multi-wavelengths fluxes of 4C 50.55 to discuss its spectral en-ergy distribution (SED) from the radio to γ -ray bands.In the radio band, we utilize those obtained by the VLAand the Giant Metrewave Radio Telescope (GMRT) com-piled by Molina et al. (2007). In the GeV γ -ray band,the upper limit derived from the one-year Fermi data(Abdo et al. 2009) is indicated by the arrow. We plotthe results of cutoff power law fit to the
Suzaku spec-tra in epochs 1 (black) and 2 (red) as well as the time-averaged
Swift /BAT data, correcting for both Galacticand intrinsic absorptions. To examine the origin of thetime variability, we analyze the difference spectrum ofXIS-FIs between epochs 2 and 1. Fitting with a singlepower law with the same double absorption as given inTable 1, we find Γ ≈ .
8. This result is also plotted inthe figure (green curve).Assuming that the global intrinsic SED of the jet emis- sion in 4C 50.55 is similar to that of jet dominated-sources (Kubo et al. 1998) with correction for an esti-mated beaming factor ( δ ∼ δ ∼ ∼ − less than the observed X-ray flux. We thus con-clude that X-ray emission due to a jet via synchrotronemission or inverse Compton should be very small inthe total X-ray emission of this source. Based on the Suzaku results, the ratio of the core luminosity at 5 GHzand that in the 2–10 keV band is estimated to be log R X = − .
6. Thus, while 4C 50.55 should be classified asa radio loud object (Terashima & Wilson 2003), its radioto X-ray power ratio is much lower compared with typi-cal blazars and more powerful radio galaxies, like 3C 120(log R X = − .
1) (Kataoka et al. 2007). This confirmsthe conclusion by Molina et al. (2007), who did not findany evidence for an additional power law component rep-resenting the jet emission in the
XMM-Newton spectrum.We note that the optical fluxes of 4C 50.55 are highlyuncertain at present due to the extinction (both Galac-tic and intrinsic ones), which could largely affect theempirical estimate of the black hole mass. In fact,the extinction-corrected AGN continuum flux at 5100 ˚Aadopted by Winter et al. (2010) falls far below an extrap-olation from the X-ray spectra toward lower energies.As a more simple approach, assuming typical SEDs ofAGNs (Elvis et al. 1994; Grupe et al. 2010), we roughlyestimate the 5100 ˚A flux to be (0 . − × − ergcm − s − from the BAT flux averaged for 22 months.Then, from the luminosity at 5100 ˚A, (0 . − × erg s − , and the width of the H β line, 2320 km s − FWHM (Winter et al. 2010), we derive the black holemass of 4C 50.55 to be ∼ . ± . M ⊙ , using the for-mula given by Vestergaard & Peterson (2006). This isabout 10 times higher than the estimate (10 . ± . M ⊙ )by Winter et al. 2010. Nevertheless, the suggestion thatthe black hole in 4C 50.55 accretes with a high Edding-ton ratio still holds. With a typical bolometric correctionfactor ( L bol /L −
10 keV =30), we estimate the fraction ofEddington luminosity of 4C 50.55 to be L bol /L Edd ∼ . SUMMARY AND DISCUSSION uzaku
Observation of 4C 50.55 7We have obtained the first simultaneous, broad bandX-ray spectra of 4C 50.55 over the 1–60 keV band with
Suzaku . These provide one of the best quality high en-ergy data so far obtained from BLRGs. From the SED,we conclude that there is little contribution from jets inthe X-ray emission, which is thus dominated by Comp-tonization by hot corona. We find that the overall contin-uum is represented by a cutoff power law, or a thermalComptonization model, with complex intrinsic absorp-tions. We show that at least two absorber with differentcovering fractions and column densities are necessary tomodel the spectrum. The photon index and cutoff en-ergy of 4C 50.55 are within the distribution of previousobservations of BLRGs (Grandi et al. 2006), althoughboth are somewhat smaller (Γ ≈ . E cut ≈ ≈ E cut ∼
200 keV, Dadina 2008).These findings on 4C 50.55 are fully consistent with theresults by Molina et al. (2007) from the
XMM-Newton and
INTEGRAL data observed on earlier epochs. Weestimate that the corona responsible for Comptoniza-tion is optically thick for scattering τ e ≈ T e ≈
30 keV. The optical depth is largerand the temperature is lower than those obtained fromSeyfert galaxies with thermal Comptonization models(e.g., Lubinski et al. 2010), as expected from the com-parison in Γ and E cut .From our observations, time variability of the X-rayflux on both long ( > ∼ sec) and short ( ∼ sec)time scales is detected. The averaged 2–10 keV flux wasthe highest in our Suzaku observation among those in theprevious observations reported in Molina et al. (2007) bya factor of 1.3–2.5. The hard X-ray flux in the 17–100keV band (1 . × − erg cm − s − ) was also higher by1.4 than the averaged flux over 22-months obtained with Swift /BAT (1 . × − erg cm − s − ). The large long-term variability in the hard X-ray above 10 keV suggeststhat it is mainly produced by the intrinsic emission, notpurely by the change of the absorber as discussed byMolina et al. (2007) in the frame work of a “patch torus”model (Elitzur & Shlosman 2006). While there remainsa possibility that Compton thick absorber ( N H > cm − ) may completely cover a part of the emission regionto cause the flux variability, it would produce signals ofheavy obscuration such as a deep iron-K edge feature inthe X-ray spectra, which are not seen in the data. Thus,at least variability of the continuum flux is required toexplain these results, while that of the absorber can alsocontribute to the variability, in particular below 10 keV(Risaliti et al. see e.g., 2005 for NGC 1365).We significantly detect an iron-K emission line andobtain a tight constraint on the reflection component,even though it is quite weak as reported by Molina et al.(2007). The reflection strength, R ≃ . R ∼ ≈ R ≃ .
3. The weak reflec-tion is also consistent with the narrow iron-K emissionline, which indicates that the reflection is mainly pro-duced by relatively outer parts of the disk (hence with asmall solid angle), unlike the results from typical Seyfert 1 galaxies (Dadina 2008). Our 4C 50.55 result confirmsthe trend reported by Sambruna et al. (2002) for radioloud AGNs.The analysis of the iron-K line profile yields an (appar-ently) large innermost radius, r in ∼ r g , by assumingan emissivity law of r − . Our diskline result suggests itunlikely that a “standard disk” extends down to closeto the innermost stable circular orbit (ISCO) around theblack hole, < r g . Instead, it is possible that the disk isthere but its inner part is covered by an optically thickcorona, as estimated by our Comptonization model fit,which would smear out relativistic iron-K broad lines.Note that the obtained r in value does not directly meanthat the disk is truncated at that radius, because (1) theestimated r in critically depends on the emissivity profileand (2) there may be another line component from dis-tant parts, such as the torus. If the scale height of theX-ray irradiating corona is sufficiently small, then onewould expect a flatter slope for the emissivity law, evenclose to r − . In this extreme case, we were not able toobtain good constraints on r in from our data. To exam-ine the second possibility, we apply a two-componentsline model to the XIS 3–9 keV spectra, consisting of anarrow Gaussian at 6.4 keV and a broad diskline, whichrepresents that from the torus and disk, respectively. Weobtain a worse fit than the single diskline fit by ∆ χ = 5even with a larger degrees of freedom, suggesting that thetwo components model is not a good description of thedata. Nevertheless, when r in of the disk line componentis fixed at 10 r g as found from 3C 120 (Kataoka et al.2007), both lines are found to be significant with equiv-alent widths of 19 ± ±
15 eV, respectively.Thus, we do not completely exclude the possibility forthe presence of a moderately broadened iron-K line inthe observed spectra of 4C 50.55.The inferred geometry of the accretion disk in 4C 50.55(i.e., truncated and/or inner parts covered by corona)may be common features of AGNs with powerful jets.Recent
Suzaku studies indicate that radio galaxies alsohave relatively narrow iron-K emission lines e.g., r in > r g for 3C 390.3 (Sambruna et al. 2009) and r in > r g for 4C +74.26 (Larsson et al. 2008) from the singlediskline fit, and r in = (9 ± r g for 3C 120 from the mul-tiple components fit (Kataoka et al. 2007). This resultis in accordance with an expectation from theories thatjets are more easily produced by radiatively inefficientaccretion flow than by a standard disk.Another key parameter to understand the accretionflow is the Eddington ratio, which is estimated to be L bol /L Edd ∼ . L bol /L Edd ∼ . . M ⊙ (Peterson et al. 2004). Thus,these two sources may belong to a very similar class ofAGNs, except for the radio loudness to the X-ray flux (log R X = − . R X = − . i < ◦ ; see Kataoka et al. 2007)compared with 4C 50.55 ( i ∼ ◦ ). The physical reasonfor the difference in their X-ray spectra that the reflec-tion component and iron-K lines are stronger in 3C 120( R ∼ .
7) is not clear at present.4C 50.55 and 3C 120 are rare objects having Tazaki et al.distinctively high fractions of Eddington luminositycompared with other typical BLRGs, for instance, L bol /L Edd = 0.01–0.07 for 3C 390.3 (Sambruna et al.2009; Lewis & Eracleous 2006), ∼ .
04 for 4C +74.26(Larsson et al. 2008), and 0.001–0.002 for Arp 102B(Lewis & Eracleous 2006). By analogy to the Galacticblack holes, these low Eddington ratio sources likely cor-respond to the low/hard state, where the accretion disk isaccompanied by steady jets, while normal Seyfert galax-ies may do to the high/soft state, where the disk extendsclose to the ISCO with quenched jet activity. The ac-cretion flows in 4C 50.55 and 3C 120 could be explainedas a high luminosity end of the low/hard state. Alter-natively, they may be another state achieved with evenhigher mass accretion rates than in the high/soft state,where the disk structure is also similar to that foundin the low/hard state (i.e., truncated disk). For thispossibility, it is interesting to note the similarly to thehigh-Eddington ratio Galactic black hole GRS 1915+105,which exhibits a similarly narrow iron-K emission lineover a Comptonization dominated continuum, imply- ing that the inner disk is fully covered by a corona(Ueda et al. 2010); in GRS 1915+105, a compact jet isalso detected in a steady state with a hard spectrum,so-called in Class χ (see e.g., Fender & Belloni 2004).In summary, the unified picture of accretion flows overa wide range of black hole mass is far from established.Further systematic studies of the accretion disk structureof radio loud AGNs at various accretion rates based ondetailed X-ray spectroscopy and multi-wavelengths dataare very important to reveal these fundamental problems.We thank Gerry Skinner for providing the Swift /BATlight curve of 4C 50.55, and the
Suzaku team for the cal-ibration of the instruments. Part of this work was finan-cially supported by Grants-in-Aid for Scientific Research20540230 (YU) and 20740109 (YT), and by the grant-in-aid for the Global COE Program “The Next Generationof Physics, Spun from Universality and Emergence” fromthe Ministry of Education, Culture, Sports, Science andTechnology (MEXT) of Japan.team for the cal-ibration of the instruments. Part of this work was finan-cially supported by Grants-in-Aid for Scientific Research20540230 (YU) and 20740109 (YT), and by the grant-in-aid for the Global COE Program “The Next Generationof Physics, Spun from Universality and Emergence” fromthe Ministry of Education, Culture, Sports, Science andTechnology (MEXT) of Japan.