Suzaku observation of the LINER NGC 4102

Astronomy & Astrophysics manuscript no. NGC4102˙suzaku˙OGM˙arxiv December 15, 2010

c ESO 2010

Suzaku observation of the LINER NGC 4102 Gonz´ alez-Mart´ın, O.1,2,⋆ , Papadakis, I.1,2 , Braito, V.3 , Masegosa, Márquez, I.4 , Mateos, S.3 , Acosta-Pulido, J.A.5,6 , Mart´ınez, M.A.4,7 , 8 Ebrero, J. , Esquej, P.3 , O’Brien, P.3 , Tueller, J.9 , Warwick, R.S.3 , Watson, M.G.3 J.4 ,

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IESL, Foundation for Research and Technology, 711 10, Heraklion, Crete, Greece Physics Department, University of Crete, P.O. Box 2208, Gr-710 03 Heraklion, Crete, Greece 3 Department of Physics and Astronomy, Leicester University, LE1 7RH, UK 4 Instituto de Astrof´ısica de Andaluc´ıa (CSIC), Granada, Spain 5 Instituto de Astrof´ısica de Canarias (IAC), C/Via Lactea, s/n, E-38205 La Laguna, Tenerife, Spain 6 Departamento de Astrof´ısica, Universidad de La Laguna, E-38205 La Laguna, Tenerife, Spain. 7 Grupo de Mec´ anica Espacial and Instituto Universitario de Matem´ atica y Aplicaciones, Universidad de Zaragoza, Zaragoza 50009 Spain. 8 SRON Netherlands Institute for Space Research, Sorbonnelaan 2, 3584 CA,Utrecht, The Netherlands 9 NASA Goddard Space Flight Center, Astrophysics Science Division, Greenbelt, MD 20771. USA

arXiv:1012.3080v1 [astro-ph.CO] 14 Dec 2010

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Preprint online version: December 15, 2010 ABSTRACT Context. Low ionisation nuclear emission-line region (LINER) nuclei have been claimed to be different than other active galactic nuclei (AGN) due to the presence of complex absorbing structures along the line-of-sight and/or an inefficient mode of accretion onto the supermassive black hole. However, this issue is still open. Aims. To investigate the broad band X-ray spectrum of NGC 4102, one of the most luminous LINERs in the Swift/BAT survey. Methods. We studied a 80 ksec Suzaku spectrum of NGC 4102, together with archival Chandra and Swift/BAT observations. We also studied the optical (3.5m/TWIN at Calar Alto observatory) and near-infrared (WHT/LIRIS at Observatorio Roque los Muchachos) spectra that were taken contemporaneous to the Suzaku data. Results. There is strong evidence that NGC 4102 is a Compton-thick AGN, as suggested by the Swift/BAT detected intrinsic continuum and the presence of a strong narrow, neutral FeKα emission line. We have also detected ionised FeXXV emission lines in the Suzaku spectrum of the source. NGC 4102 shows a variable soft excess found at a significantly higher flux state by the time of Suzaku observations when compared to Chandra observations. Finally, a complex structure of absorbers is seen with at least two absorbers apart from the Compton-thick one, derived from the X-ray spectral analysis and the optical extinction. Conclusions. All the signatures described in this paper strongly suggest that NGC 4102 is a Compton-thick Type-2 AGN from the X-ray point of view. The “soft excess”, the electron scattered continuum component, and the ionised iron emission line might arise from Compton-thin material photoionised by the AGN. From variability and geometrical arguments, this material should be located somewhere between 0.4 and 2 pc distance from the nuclear source, inside the torus and perpendicular to the disc. The bolometric luminosity (Lbol = 1.4 × 1043 erg s−1 ) and accretion rate (m ˙ Edd = 5.4 × 10−3 ) are consistent with other low-luminosity AGN. However, the optical and near infrared spectra correspond to that of a LINER source. We suggest that the LINER classification might be due a different spectral energy distribution according to its steeper spectral index. Key words. galaxies:active - galaxies:nuclei - galaxies:Seyfert - galaxies:individual (NGC 4102) - X-ray:galaxies

1. Introduction Active galactic nuclei (AGN) emit over the entire electromagnetic spectrum and are widely believed to be powered by the accretion of matter onto a supermassive black hole (SMBH, Rees, 1984). Several families within the AGN category have been established from the observational point of view. Although their classification is sometimes misleading, it is widely believed that a unified model can explain them all under a single scenario (Antonucci, 1993). A key ingredient in this scheme is a dusty torus whose inclination with respect to the observer’s line of sight is responsible for ⋆

e-mail: [email protected]

the dichotomy between optical Type-1 (with broad permitted lines, face-on view) and Type-2 (with narrow permitted lines, edge-on view) AGN. However, this scheme needs to be further refined since there are several sub-classes of objects that cannot be easily fitted into this scenario (for example unobscured Type 2 Seyferts, e.g. Mateos et al., 2005; Dewangan & Griffiths, 2005; Panessa & Bassani, 2002). One of the most intriguing cases are low ionisation nuclear emission-line regions (LINERs, Heckman, 1980). As suggested by their low X-ray luminosities (L(2 − 10 keV) ∼ 1039−42 erg s−1 , see González-Mart´ın et al., 2009a) they could be the link between AGN (L(2 − 10 keV) ∼ 1041−45 erg s−1 ) and normal galaxies (∼ 1038−42 erg s−1 , Fabbiano, 1989). Moreover, they are the dominant popu-

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O. Gonz´ alez-Mart´ın et al.: A Suzaku observation of the LINER NGC 4102

lation of active galaxies in the nearby universe (Ho et al., 1997). However, their nature is not yet well understood. Several samples of LINERs have been analysed at Xray frequencies, a large fraction of them showing AGN signatures (González-Mart´ın et al., 2009a, 2006; Dudik et al., 2005). In spite of this, it still remains unclear how to fit LINERs into the AGN Unification scenario. A radiatively inefficient accretion flow onto the SMBH (Ho, 2009) and/or the presence of highly obscuring matter have been proposed to explain the differences between LINERs and more luminous AGN (Goulding & Alexander, 2009; Dudik et al., 2009; González-Mart´ın et al., 2009b). Using the ratio log(Fx(2 − 10 keV) /F([O III])) (RX/[O III] , hereinafter), Gonz´ alez-Mart´ın et al. (2009b) showed that LINERs have a higher fraction of Comptonthick sources than Type-2 Seyfert galaxies. This implies high column densities and significant suppression of the intrinsic continuum emission below 10 keV. Only indirect proof of their Compton-thickness can be obtained with Chandra and XMM-Newton data. Therefore, the nature of these sources is yet to be confirmed. A more direct evidence comes from the determination of the strength of the neutral iron Kα emission line and the direct view of the nuclear continuum above 10 keV. NGC 4102 is a nearby Sb galaxy with a nuclear optical spectrum that was first classified as an HII region by Ho et al. (1997) although its UV emission is not compatible with this classification (Kinney et al., 1993). Gon¸calves et al. (1999) classified its optical spectrum as composite, concluding that the nucleus is dominated by starburst emission although a weak Type-2 Seyfert component is also present. NGC 4102 is included in the Carrillo et al. (1999) sample of LINERs1 and we have reclassified it as LINER by means of the emission lines given in Moustakas & Kennicutt (2006). NGC 4102 has been observed with the Chandra/ACIS snapshot survey (Dudik et al., 2005). They classified it as an AGN-like source. Tzanavaris & Georgantopoulos (2007) pointed out its AGN signatures, and considered it as a good candidate for harbouring a hidden AGN. They claimed the presence of an iron line, although poor statistics did not allow them to accurately constrain its equivalent width. Ghosh et al. (2008) showed that NGC 4102 has an AGN and strong star formation activity. They also pointed out the existence of a reflection component based on a hint of a strong FeKα emission line. According to the FX (2 − 10keV)/F([O III]) ratio, NGC 4102 is a good candidate to be a Compton-thick source (see Sect. 5 in this paper). Therefore, NGC 4102 is an ideal case to study the obscuration in LINERs. Here we present the Suzaku spectra of NGC 4102. We also present optical (TWIN/2.2m in Calar Alto observatory) and near infrared (LIRIS/WHT in El Roque de los Muchachos observatory) spectra which were taken contemporaneously (up to one month apart) with our Suzaku observation. Chandra archival data are also revisited to study the long term variability of this source. This paper is organised as follows. In Section 2 we describe the X-ray data reduction and observations. In Section 3 we present the X-ray spectral fitting. In Section 4 we review the NGC 4102 activity classification as seen 1 This catalogue included all the nuclei classified as LINERs in the literature to data.

by optical and near-IR observations. Finally, we discuss the nature of the emission seen in NGC 4102 in Section 5 and report the main conclusions in Section 6. A distance of 17 Mpc is assumed for NGC 4102 throughout the analysis (Tully, 1988). A ΛCDM cosmology with (ΩM , ΩΛ ) = (0.3, 0.7) and H0 = 75 km s−1 Mpc−1 (i.e. z=0.0042) is also assumed.

2. X-ray observations and data reduction 2.1. Suzaku data Suzaku (Mitsuda et al., 2007) observed NGC 4102 for a total exposure time of 80 ksec on 2009 May 30th. The data were taken using the X-ray Imaging Spectrometer (XIS) and the Hard X-ray Detector (HXD) at the HXD nominal point2 . For the data reduction and analysis we followed the latest Suzaku data reduction guide3 . We reprocessed all the data files using standard screening within XSELECT (“SAA==0” and “ELV>5”). The net exposure time of XIS detectors is 79 ksec. We reprocessed the Spaced-row Charge Injection (CTI) data of the XIS instrument using xispi task in order to use the latest calibration files at the time of writing. We also excluded data with Earth day-time elevation angles less than 20o using XSELECT (“DYE ELV>20”). XIS data were selected in 3 × 3 and 5 × 5 edit-modes using grades 0, 2, 3, 4, 6. Hot and flickering pixels were removed using the sisclean script. Spectra were extracted by using circular regions of 2 arcmin radius4 centred in the NED nuclear position of NGC 4102 (R.A. (J2000)= 12:06:23.0 and Dec (J2000) = +52:42:40). CXO J120627.3+524303 is reported by the Chandra Source Catalogue (CSC) as a source within this extraction region with F(0.5 − 10 keV) = 4.8 × 10−14 erg s−1 cm−2 , which is much smaller than that of NGC 4102 (see Table 2). In addition to NGC 4102, four sources were detected in the XIS field of view: CXO J120543.3+523806, CXO J120548.4+524306, CXO J120600.6+523831, and CXO J120633.2+524022. According to the CSC these sources have a 0.5–10 keV flux of 7.5, 4.8, 11.0, and 6.7 in units of 10−14 erg s−1 cm−2 . Background spectra were extracted using two circles of 2.5 and 1.7 arcmin radii on the field, excluding the four sources mentioned above and chip corners to avoid the calibration lamps. The response matrix RMF and ancillary response ARF files were created using the tasks xisrmfgen and xissimarfgen, respectively. Spectra from the two front illuminated XIS 0 and XIS 3 chips were combined to create a single source spectrum (addascaspec task), while data from the back illuminated XIS 1 chip were kept separate. Both resulting spectra were then binned with a minimum of 20 counts in each energy bin in to allow the use of χ2 statistics using grppha task. Suzaku HXD/PIN is a non-imaging instrument with a 34’ square (FWHM) field of view. We reprocessed the HXD/PIN files using standard screening within XSELECT 2

Suzaku data has been obtained centring the source in the nominal position of HXD in order to maximise the S/N. 3 http://heasarc.gsfc.nasa.gov/docs/suzaku/analysis/abc/ 4 This includes ∼90% of the emission of the source.

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(“T SAA HXD>500” and “COR>8”). We extracted the spectra and corrected them for dead-time intervals. We used the variable non-X-ray background (NXB) model D (or tuned background, see Fukazawa et al., 2009) that the HXD instrument team provides to correct for particle or detector background. This NXB was added to the cosmic X-ray background (CXB) to produce the final background spectrum. The source was not detected by HXD (see Section 3.1.2).

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2.2. Chandra data Level 2 event data from the ACIS instrument were extracted from Chandra’s archive5 (ObsID 4014). The source was observed on 2003 April 3rd as part of a snapshot survey of LINERs by Dudik et al. (2005). The data were reduced with the ciao 3.46 data analysis system and the Chandra Calibration Database (caldb 3.4.0). The observation was processed to exclude background flares, using the task lc clean.sl in source-free sky regions of the same observation. The net exposure time after flare removal is 4.9 ksec and the net count rate in the 0.5–10 keV band is (6.8±0.3) × 10−2 counts s−1 . The source spectrum was extracted from a circular region with 5 arcsec radius centred at the same position than the extraction used for Suzaku/XIS data. The background spectrum was extracted using also a circular region of 18 arcsec centred at R.A. (J2000) = 12:06:20.7 and Dec (J2000) = +52:42:13. The net number of counts of the spectrum is 340 counts in the 0.5 to 10 keV band. Response and ancillary response files were created using the CIAO mkacisrmf and mkwarf tools. The spectrum was binned to give a minimum of 20 counts per bin. 2.3. Swift/BAT data The Swift /BAT reduced spectrum was kindly provided to us by the Swift team. It has been derived from an allsky mosaic in high energy bins, averaged over 22 months of data, from 2004 December 15th to 2006 October 27th, and extracted from a 17 arcmin circular extraction region (see Winter et al., 2008; Tueller et al., 2010, for a detailed explanation of the data processing). NGC 4102 is reported in the 22 months catalog as one of the 461 sources detected above a 4.8σ level in the 14-196 keV band with BAT (Tueller et al., 2010). It has been detected at the 6.96σ confidence level, and has an average flux of F(14 − 196 keV) ∼ 2.2 × 10−11 erg cm−2 s−1 .

3. X-ray spectral fitting of NGC 4102 In this section we present the results from various model fits to the Suzaku, Chandra, and Swift data. All the spectral analysis was done using version 12.5.0 of XSPEC. All spectral fits include neutral Galactic absorption (NH (Gal) = 1.68 × 1020 cm−2 ; Dickey & Lockman, 1990). Spectral parameter errors are computed at the 90% confidence level. 5 6

http://cda.harvard.edu/chaser/ http://asc.harvard.edu/ciao

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Fig. 1. Ratio between XIS/Suzaku data and a power-law model fitted to the 2–6 keV band. Black (dark) points indicate the XIS-0+3 spectrum and red (light) points the XIS-1 spectrum.

3.1. Suzaku spectra 3.1.1. XIS spectra NGC 4102 is detected with the XIS instrument. The spectral range of 0.6–9 keV for back-illuminated detectors and 0.7–9 keV for front-illuminated detectors were used for the Table 1. Results from spectra fitting of the XIS/Suzaku data.

Model Γ EW(FeKα)(1) EW(FeXXV )(1) χ2 /dof

A

B C (Models to the 2-9 keV band) 2.3+0.3 1.7+0.1 1.6+0.1 −0.3 −0.1 −0.1 +0.2 +0.2 1.3−0.2 0.76−0.2 0.68+0.11 −0.13 ... 0.18+0.07 0.16+0.06 −0.07 −0.06 214.5/164 188/163 181.7/162

Model

D E (Models to the 0.6–9 keV band) ... 0.7+0.5 NH (2) −0.5 ∗ Γ 2.3 2.3∗ kT(3) 0.78+0.03 0.73+0.03 −0.03 −0.03 EW(FeKα)(1) 0.68∗ 0.68∗ EW(FeXXV )(1) 0.16∗ 0.16∗ 2 χ /dof 456.8/379 445/378 Models A: phabs(powerlaw+zgauss) B : phabs(powerlaw+zgauss+zgauss) C : phabs(pexrav+powerlaw+zgauss+zgauss) D: phabs(apec+pexrav+powerlaw+zgauss+zgauss) E : phabs*(zwabs(apec+powerlaw)+pexrav+ zgauss+zgauss) (1)

EW of emission lines in keV. Intrinsic cold absorber column density NH in units of 1021 cm−2 . (3) Temperature of the thermal component kT and EWs of the emission lines expressed in keV. ∗ Fixed parameters to the values obtained in Model C. See text for details. (2)

O. Gonz´ alez-Mart´ın et al.: A Suzaku observation of the LINER NGC 4102 normalized counts s−1 keV−1

normalized counts s−1 keV−1

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Fig. 2. Best-fit (top panels) and residuals (bottom panels) plots of Suzaku/XIS data for Model A (left) and Model B (right) around the Fe emission region (black and red points as in Fig. 1). The dotted lines indicate the various model components. spectral fit of the XIS data. We excluded energies below 0.6 or 0.7 keV (for back- or front-illuminated respectively), between 1.7-1.9 keV, and above 9 keV because of unsolved calibration uncertainties at these energies and low statistics. There are 4600 and 2800 net counts in the front- and back-illuminated detectors, respectively. To constrain the shape of the intrinsic continuum we used the rest-frame 2– 6 keV band where we do not expect contamination from other components usually found in the X-ray spectra of AGN (e.g. soft excess emission below ∼2 keV and the FeKα emission line at 6.4 keV) and fitted the data to a power-law model. The fit is good (χ2 /dof = 129.4/114), and the best fit spectral index is Γ = 1.9 ± 0.2. Fig. 1 shows the ratio between the 0.6–9 keV data and this model (fit statistics of χ2 /dof = 1332/382). The residuals plot indicate excess emission below 2 keV and around the 6–7 keV band. The FeKα line: the excess emission around 6–7 keV is consistent with the most prominent feature typically observed in the 2–10 keV rest-frame spectra of AGN, i.e. the FeKα emission line at 6.4 keV. We therefore refitted the 2– 9 keV band data with a powerlaw plus Gaussian line model (Model A hereafter). The width of the Gaussian was fixed to 0.01 keV (i.e. it was assumed to be intrinsically narrow) and the line centroid energy was fixed to 6.4 keV. Best fit parameters are listed in Tab. 1, and the best fit model together with the residuals (in the 5.5–7.5 keV range for plotting purposes), are shown in Fig. 2 (left). We also tested the possibility that the line is broad, by letting the width of the Gaussian profile vary. The best-fit width was σ(FeKα) = 130+70 −40 eV but the improvement of the fit was marginal (∆χ2 = 3.7 for one extra parameter, F-statistics probability of 0.09). However, Model A is not good enough (χ2r = 1.31). This is mainly due to the presence of an emission-line feature at ∼6.7 keV (see residuals plot in the left panel of Fig. 2). We therefore included a second Gaussian (which corresponds to the Kα line from FeXXV ) to the models with the line energy and width fixed at 6.7 keV and 0.01 keV, respectively (Model B hereafter). The fit is significantly improved (∆χ2 = 26, for one extra parameter when compared to Model A). The best fit model is shown in Fig. 2 (right

panel) and the best-fit parameter values are listed in Tab. 1. The power-law index is consistent with the previous value. We also tested the possibility of this line to be broad by letting its width to be a free parameter. We get a considerable improvement on the fit (∆χ2 = 13.5 for one extra parameter, F-statistics probability of 6.5 × 10−4 ) with σ(FeXXV ) = 390+450 −105 eV. The neutral FeKα line in this case is still consistent with being narrow [σ(FeKα) < 70 eV]. The continuum shape in the 2–9 keV band: even after the inclusion of the 6.7 keV lines the EW of the 6.4 keV line is ∼680 eV, which strongly suggests a Compton-thick source (see Maiolino et al., 1998). In this case, we expect the presence of a strong reflection component as well. For that reason, we refitted the 2–9 keV spectrum with a model consisting on the two narrow lines at 6.4 keV and 6.7 keV, a power-law, and pexrav in XSPEC, which describes reflection from neutral material (Magdziarz & Zdziarski, 1995). The inclination angle of the reflector was set to 60o and the iron abundance was fixed to 1. pexrav was used in such a way that it produces the reflected photons only to test if the spectrum is consistent with a ‘pure’ reflection model. The fit is now acceptable (Model C hereafter, see Tab. 1) and the best fit spectral index is Γ = 2.3 ± 0.3, similar to what is observed in other LINERs (González-Mart´ın et al., 2009a). Finally, we allowed to vary the centroids of the Gaussians. The results do not change, showing centroids at E(FeKα)= 6.40 ± 0.02 keV and E(FeXXV )= 6.73 ± 0.04 keV, consistent with the theoretical energies of these transitions. We also tested the width of the lines. The width of the 6.4 keV line is still consistent with zero [σ(FeKα) < 60 eV] while the 6.7 keV line is now also consistent with zero σ(FeXXV ) < 860 eV. The full energy band spectrum: our best-fit Model C fails to describe well the full band Suzaku data (see Fig. 3, left). The spectra show an excess below 2 keV. Many LINERs show a soft excess that can be fitted with a thermal model (González-Mart´ın et al., 2009a, 2006). Thus, we added a thermal component (apec on XSPEC) to the model (Model D hereafter). Abundances were fixed to the solar value. The spectral index, the parameter values of the reflection component and of the Gaussian lines were kept fixed to those in Model C. Best-fit results are

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Fig. 3. (Left): Extrapolation of the 2–9 keV XIS/Suzaku best fit model (Model C ) to lower energies. (Right): Final fit (top panel) and residuals (bottom panel) to the XIS/Suzaku data using Model E. See Tab. 1 and text for an explanation on the components used in each model. listed in Tab. 1. However, the fit overestimates the spectra below 0.7 keV. In order to account for this, an additional absorber (zwabs in XSPEC) was included, absorbing power-law and thermal components. This model properly describes the XIS/Suzaku dataset (Model E hereafter, see Tab. 1 and Fig. 3, right). The best-fit results and the quality of the fit do not differ if we let free the normalization of the Gaussians and the spectral index: EW(FeKα) = 700 ± 130 eV, EW(FeXXV ) = 170 ± 60 eV, and Γ = 2.4 ± 0.4. The resulting temperature and hydrogen column density NH are consistent with those in other LINERs (González-Mart´ın et al., 2009a). Note that some residuals still appear in the 1–2 keV band (Fig. 3, right bottom panel). See Section 5.4 for a detailed discussion of the soft-excess. The observed fluxes and absorption corrected luminosities using Model E are reported in Tab. 2. Sixty per cent of the 0.5–2.0 keV flux is contributed by the power-law, 38% by the thermal component, and 2% by the reflection component. In the hard band (2–10 keV), 54% of the flux is contributed by the power-law component, while the reflection component, the emission lines and the thermal component contribute 27%, 17%, and 2%, respectively. The flux of the FeKα line using the Model E best-fit results −13 is F(FeKα)=1.5+0.3 erg s−1 cm−2 . −0.2 × 10 3.1.2. HXD/PIN spectrum NGC 4102 is not detected by the HXD/PIN above the total background. The count rate of the source is (4.6±2.5) 10−3 counts/sec, only 1% above the background. We have estimated an upper limit for the source flux of F(1470 keV)