Nustar Unveils a Heavily Obscured Low-Luminosity Active Galactic Nucleus in the Luminous Infrared Galaxy Ngc 6286 C

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Draft version October 26, 2015 A Preprint typeset using LTEX style emulateapj v. 04/17/13

NUSTAR UNVEILS A HEAVILY OBSCURED LOW-LUMINOSITY ACTIVE GALACTIC NUCLEUS IN THE LUMINOUS INFRARED GALAXY NGC 6286 C. Ricci1,2,*, et al. Draft version October 26, 2015

ABSTRACT We report on the detection of a heavily obscured Active Galactic Nucleus (AGN) in the Lumi- nous Infrared Galaxy (LIRG) NGC 6286, obtained thanks to a 17.5 ks NuSTAR observation of the source, part of our ongoing NuSTAR campaign aimed at observing local U/LIRGs in different merger stages. NGC6286 is clearly detected above 10keV and, by including the quasi-simultaneous Swift/XRT and archival XMM-Newton and Chandra data, we find that the source is heavily obscured 24 −2 [N H ≃ (0.95 − 1.32) × 10 cm ], with a column density consistent with being mildly Compton- −2 thick [CT, log(N H/cm ) ≥ 24]. The AGN in NGC 6286 has a low absorption-corrected luminosity 41 −1 (L2−10 keV ∼ 3 − 20 × 10 ergs ) and contributes .1% to the energetics of the system. Because of its low-luminosity, previous observations carried out in the soft X-ray band (< 10 keV) and in the infrared excluded the presence of a buried AGN. NGC 6286 has multi-wavelength characteristics typical of objects with the same infrared luminosity and in the same merger stage, which might imply that there is a significant population of obscured low-luminosity AGN in LIRGs that can only be detected by sensitive hard X-ray observations.

1. INTRODUCTION 2012; Schawinski et al. 2012), spanning from < 1% at a 41 −1 4 11 12 2–10 keV luminosity of L2−10 ∼ 10 ergs to 70-80% Luminous [L IR(8−1000 µm) = 10 −10 L⊙, LIRGs] 46 −1 12 in the most luminous quasars L2−10 ∼ 10 ergs . and ultra-luminous (L IR ≥ 10 L⊙, ULIRGs) infrared galaxies were first discovered in the early seventies (Low The contribution of AGN to the overall luminosity of & Kleinmann 1968; Kleinmann & Low 1970). With the U/LIRGs has been shown to increase with the luminos- advent of the Infrared Astronomical Satellite (IRAS, see ity of the system (e.g., Veilleux et al. 1995, 1999; Alonso- Sanders & Mirabel 1996 for a review), which discovered a Herrero et al. 2012; Ichikawa et al. 2014). Due to the large number of U/LIRGs, the cosmological importance great opacity of the nuclear region, a clear identifica- of these objects became evident. Although they are rel- tion of AGN in U/LIRGs is often complicated. Mid-IR atively rare at low redshift, their luminosity function is (MIR) properties have been used to estimate the relative very steep, and they are the major contributor to the IR contribution of accretion onto SMBH and star forma- energy density at z ≃ 1 − 2 (Le Floc’h et al. 2005). tion to the bolometric luminosity. This has been done The discovery that most, if not all, U/LIRGs are trig- by exploiting 5–8 µm spectroscopy (e.g., Nardini et al. gered by galaxy mergers led to the development of an 2010), and the characteristics of several features in the evolutionary scenario (Sanders et al. 1988) in which two L (3–4 µm) and M (4–5 µm) bands (Imanishi & Dud- gas rich disk galaxies collide, triggering an intense phase ley 2000; Risaliti et al. 2006; Sani et al. 2008; Risaliti of star formation in which they are observed as U/LIRGs. et al. 2010): the 3.3 µm Polycyclic Aromatic Hydrocar- This is then followed by a blowout phase, during which bon (PAH) emission feature, the 3.4 µm absorption fea- most of the material enshrouding the supermassive black ture, and the slope of the continuum. The 6.2µm (e.g., hole (SMBH) is blown away and the system is observed Stierwalt et al. 2013, 2014) and 7.7µm PAH features as a luminous red quasar (e.g., Glikman et al. 2015 and (e.g., Veilleux et al. 2009), the presence of high-excitation references therein). When most of the dust is removed, MIR lines (e.g., [Ne V] 14.32µm, Veilleux et al. 2009), or the system is eventually observed as a blue quasar. This the radio properties (e.g., Romero-Cañizales et al. 2012a, model is consistent with the observed increase of the frac- Vardoulaki et al. 2015) have also been used to infer the tion of obscured sources with redshift up to z ≃ 3 (Treis- presence of a buried AGN. ter et al. 2010a). Numerical simulations (e.g., Springel X-ray observations are a very powerful tool to detect et al. 2005) have also shown that tidal interactions can accreting SMBHs and to disentangle the contributions drive an inflow of material that triggers and feeds both of star formation and AGN emission to the total lumi- accretion onto the SMBH and star formation. There- nosity of U/LIRGs. Studies performed so far by using fore mergers might play an important role in fuelling the XMM-Newton (e.g., Franceschini et al. 2003; Imanishi SMBH, as it has been suggested by the discovery that et al. 2003; Pereira-Santaella et al. 2011) and Chandra the fraction of Active Galactic Nuclei (AGN) in merg- (e.g., Ptak et al. 2003; Teng et al. 2005; Iwasawa et al. ers increases with the AGN luminosity (Treister et al. 2011) have been able to characterise the properties of a significant number of these systems. However, a sig- 1 Pontificia Universidad Catolica de Chile, Instituto de Astrof- nificant fraction of U/LIRGs might be heavily obscured sica, Casilla 306, Santiago 22, Chile (e.g., Treister et al. 2010b; Bauer et al. 2010), and X-ray 2 EMBIGGEN Anillo, Concepcion, Chile * observations at energies . 10 keV are strongly limited by [email protected] 24 −2 4 As defined by Sanders & Mirabel (1996). absorption from Compton-thick (CT, N H ≥ 10 cm ) 2 Ricci et al.

1) A Chandra (0.2-2 keV) 2) Chandra (3-8 keV) 1.4 GHz contour

Core B

Core

C 4'' 4''

3.91636"

3.9551"

0 0.6 1.2 1.8 2.4 3 3.6 4.2 4.8 5.4 6 0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2

3) NuSTAR (3-24 keV) 4) Spitzer/IRAC (3.6 micron)

NGC 6285 A NGC 6286

B Core C

12''

1' 11.9054"

1.00002'

0 0.3 0.6 0.9 1.2 1.5 1.8 2.1 2.4 2.7 3 0.2 0.3 0.5 1.0 1.8 3.5 7.0 13.8 27.6 54.8 109.0

Figure 1. Chandra ACIS-S (panel one, 0.2–2 keV; panel two, 3–8 keV), NuSTAR FPMA (panel three, 3–24 keV) and Spitzer IRAC (panel four, 3.6 µm) images of NGC 6286. The Chandra 0.2–2 keV image shows a clear extended structure of ∼ 12 arcsec size (∼ 4.4 kpc). The four regions shown in panel one and four represent the 3–8 keV core, the north, central, and south regions discussed in § 3.1. The 1.4 GHz VLA FIRST radio contour is illustrated in panel two, and shows that the radio emission coincides with the hard X-ray core, suggesting the presence of a buried AGN. The black circle and the blue dashed annulus in panel three correspond to the NuSTAR source and background extraction regions, respectively. The blue dashed circle in panel four represents the source region used for XMM-Newton EPIC/PN. AGN. Observations carried out in the hard X-ray band This paper is based on the detection of an heav- (≥ 10keV) are less affected by absorption, and could ily obscured AGN in NGC 6286 (also referred to as be used to probe nuclear X-ray emission even in highly NGC 6286S), a LIRG (log L IR/L⊙=11.36, Howell et al. obscured systems. Previous hard X-ray observations of 2010) located at z=0.018349 (i.e., a luminosity distance U/LIRGs have been carried out with BeppoSAX (e.g., of d L = 76.1 Mpc), which was not previously detected Vignati et al. 1999), Suzaku (e.g., Teng et al. 2009) and above 10kev (Koss et al. 2013). The source has a star- Swift/BAT (Koss et al. 2013). formation rate (SFR) of 41.3M⊙/yr (Howell et al. 2010), The recent launch of the NASA X-ray satellite NuS- is in an early merging stage (i.e., stage B or 2, following TAR, the first focussing telescope in orbit operating at the classification of Stierwalt et al. 2013), and is inter- E ≥ 10 keV, has opened a new window in the study acting with the galaxy NGC 6285 (NGC 6286N), located of U/LIRGs thanks to its unprecedented characteris- at ∼ 1.5 arcmin (∼33kpc). The paper is structured as tics. The first study of the hard X-ray emission of lo- follows. In § 2 we present the X-ray data used and de- cal ULIRGs carried out with NuSTAR has been recently scribe the data reduction, in § 3 we report on the X-ray reported by Teng et al. (2015), who have shown the im- spectral analysis of NGC 6286, in § 4 we discuss our re- portance of sensitive hard X-ray spectra to well constrain sults by taking into account the multi-wavelength prop- the value of the line-of-sight column density. erties of NGC6286, and in § 5 we summarise the main re- We report here on the first results of a series of NuS- sults of our work. Throughout the paper we adopt stan- −1 −1 TAR observations awarded to our group during AO-1 as dard cosmological parameters (H0 = 70kms Mpc , a part of a campaign aimed at observing ten local LIRGs Ωm =0.3, ΩΛ =0.7). in different merger stage (PI: F. E. Bauer). The sources were selected from the Great Observatories All-sky LIRG 2. X-RAY OBSERVATIONS AND DATA REDUCTION 5 Survey (GOALS , Armus et al. 2009) sample. GOALS 2.1. NuSTAR is a local (z < 0.088) sample which contains 181 LIRGs and 21 ULIRGs selected from the IRAS Revised Bright The Nuclear Spectroscopic Telescope Array (NuSTAR, Galaxy Sample (Sanders et al. 2003). Harrison et al. 2013) observed NGC6286 on May 29th 2015 for 17.5ks. We processed the data using the NuS- TAR Data Analysis Software nustardas v1.4.1 within 5 http://goals.ipac.caltech.edu Heasoft v6.16, adopting the latest calibration files (Mad- NuSTAR unveils a low-luminosity heavily obscured AGN in the LIRG NGC 6286 3 sen et al. 2015). The source is clearly detected in the 3– 24keV image (panel three of Fig.1). For both focal plane modules (FPMA and FPMB) we extracted source and background spectra and light-curves with the nuprod- ucts task. A circular region of 45arcsec was used for the source6, while the background was extracted from an annulus centred on the X-ray source, with an inner and outer radius of 90 and 150 arcsec, respectively. The 3–10 and 10–50keV light-curves of the sources do not show any evidence of flux variability.

2.2. XMM-Newton Two XMM-Newton (Jansen et al. 2001) observations of NGC6286 (ID: 0203390701 and 0203391201; PI: Maiolino) were carried out on May 7 2005, with expo- Figure 2. XMM EPIC/PN, Chandra ACIS-S and NuSTAR sures of 20.8 and 8.9ks. Both PN (Strüder et al. 2001) FPMA/FPMB spectra of NGC 6286. The continuous lines and MOS (Turner et al. 2001) data were analysed by represent the model used in Brightman & Nandra (2011) reducing first the observation data files (ODFs) using (apec+zpowerlaw) to reproduce the 0.3–10 keV spectrum of the source. The figure clearly shows the importance of hard X-ray cov- XMM-Newton Standard Analysis Software (SAS) ver- erage to fully understand the characteristics of the X-ray emission. sion 12.0.1 (Gabriel et al. 2004), and then the raw PN and MOS data files using the epchain and emchain tasks, respectively. In order to filter the observations which does not appear in the 0.2–2 keV image. This for periods of high background activity we analyzed the source is located at the center of the galaxy (see panel EPIC/PN and MOS background light curves in the 10–12 three) and could be associated with AGN emission. The keV band and above 10 keV, respectively, and found that spectra of these different regions are discussed in § 3.1. both observations show a significant background contam- In order to be consistent with the spectral extraction of ination. Observation 0203391201 was not used because XMM-Newton and Swift/XRT, which have a much lower the background flux dominates the whole observation spatial resolution than Chandra, the ACIS-S source spec- (with an average count-rate of 6cts−1 and a minimum of trum used for the broad-band X-ray fitting was extracted 2cts−1). Observation 0203390701 showed less contam- from a circular region of 10.5 arcsec radius. The back- ination, and we filtered the periods of high background ground spectrum was extracted from a circular region of activity using a threshold of 2 ct s−1 for both PN and the same size on the same CCD, where no other source MOS, which resulted in net exposure times of 2.3 and was detected. 4.7ks, respectively. For both cameras we extracted the 3. spectrum of the source using a circular region of 20arcsec X-RAY SPECTRAL ANALYSIS radius, while the background was extracted from a circu- The X-ray spectral analysis was performed within lar region of 40arcsec radius, located on the same CCD xspec v.12.8.2 (Arnaud 1996). Galactic obscuration in G 20 −2 of the source and in a zone devoid of other sources. No the direction of the source (N H = 1.8 × 10 cm , significant flux variability is found in the 0.3–10 keV band Kalberla et al. 2005) was taken into account by adding during the XMM-Newton observation. photoelectric absorption (TBabs in xspec, Wilms et al. 2000). Abundances were set to the solar value. Spectra 2.3. Swift/XRT were rebinned to have at least 20counts per bin in order 2 The X-ray telescope (XRT, Burrows et al. 2005) on to use χ statistics, unless reported otherwise. board Swift (Gehrels et al. 2004) observed NGC6286 In the following we first present the X-ray spectral quasi-simultaneously with NuSTAR on May 29 2015 for analysis of the extended and nuclear emission revealed 2 ks. XRT data were reduced using the xrtpipeline by Chandra (§ 3.1) and then discuss the spatially inte- v0.13.0 within Heasoft v6.16. grated broad-band X-ray emission (§ 3.2) considering all observations available. 2.4. Chandra 3.1. Extended and nuclear emission A Chandra (Weisskopf et al. 2000) ACIS-S (Garmire et al. 2003) observation of NGC6286 was carried out The diffuse soft X-ray emission detected by Chandra on September 18 2009 (PI: Swartz) with an exposure has an angular size of ∼ 12arcsec, which at the distance of 14.2 ks. The data reduction was performed following of the source corresponds to ∼ 4.4 kpc. This diffuse emis- the standard procedure, using CIAO v.4.6. The data sion might either be related to thermal plasma in a star- were reprocessed using chandra repro, and then the forming region, to photo-ionisation from an AGN, or to spectra were extracted using the specextract tool. shocks created by the interaction between outflows from The 0.2–2keV Chandra image shows clear evidence of the AGN and the galactic medium. To analyse the dif- extended emission (panel one of Fig.1). The 3–8keV im- fuse and nuclear emission we extracted the spectra of age (panel two) shows instead only a point-like source, the four regions shown in panel one of Fig. 1. Due to the low number of counts we rebinned the spectra to have 6 In the 3–24 keV band for for photon indices Γ = 0.6 − 1.8 this at least 1 count per bin, and used Cash statistics (Cash aperture encloses ∼ 65% of the full PSF energy (Lansbury et al. 1979) to fit the data. In the following we discuss the 2014). spectral properties of the Core, and of the regions A, B 4 Ricci et al.

Table 1 Results obtained by applying the phenomenological photoionisation model.

Energy Flux Association −6 −2 −1 [ keV] [10 ph cm s ]

Region A

+0.03 0.75−0.03 3.3 O VIII; Fe-L of Fe XVII–Fe XIX +0.03 0.87−0.02 3.3 Fe-L of Fe XVIII–Fe XIX or Fe XXI +0.03 1.04−0.03 2.4 NeIX;NeX;Fe-LofFeXXI–FeXXIII +0.13 1.27−0.06 0.5 MgXI;NaX;NaXI,NeX

Region B

+0.03 0.79−0.03 2.7 O VIII; Fe-L of Fe XVII–Fe XIX +0.02 0.98−0.03 2.0 Fe-L of Fe XX–Fe XXII Region C

+0.06 0.90−0.04 1.5 Ne IX; Fe-L of Fe XVIII–Fe XXI. and C. can be improved by adding photoelectric absorp- Core. The spectrum of the core was extracted from tion (tbabs Gal×ztbabs×apec, C-stat/DOF=43.7/33). a circular region of radius 1.5 arcsec centred on the The shock plasma model fails to reproduce well the peak of the 3–8keV emission. Ignoring the data be- spectrum both without (C-stat/DOF=48.7/33) and with low 1.2 keV to avoid contamination from the diffuse (C-stat/DOF=43.6/32) an absorption component. The soft X-ray emission, and fitting with a power-law model photo-ionisation model on the other hand reproduces ex- (tbabs ×zpowerlaw in xspec) we obtain a photon tremely well the spectrum (C-stat/DOF=29.9/31) with Gal the scattered continuum plus two emission lines. index of Γ = −0.17+1.01. This low value is indicative −1.03 Region C. A thermal plasma model with kT = of heavy absorption in the nuclear region. Fitting the +0.37 X-ray spectrum using the whole energy range with a 1.23−0.35 keV yields a good fit (C-stat/DOF=34/34), model that includes also a collissionally ionized plasma while a shock plasma model cannot reproduces well model (tbabs Gal×zpowerlaw+apec) we obtain C- the data (C-stat/DOF=45.1/33), and results in kT s = +0.73 8 −3 stat/DOF=20.9/21, Γ ≤ −0.03 and a temperature of 1.09−0.38 keV and τ u ≤ 1.4 × 10 s cm . The photoion- +0.28 ized model requires only an emission line, besides the the plasma of kT =0.99−0.35 keV. The 3–8keV core coincides with the 1.4 GHz radio emission measured by the scattered X-ray continuum, to well fit the X-ray spec- VLA FIRST survey (Becker et al. 1995). At 1.4 GHz trum (C-stat/DOF=29.4/32). ′′ ′′ the angular size of the radio emission is ≤ 0.25 × 0.25 3.2. Spatially integrated X-ray emission (Vega et al. 2008), which confirms that this component is likely associated to an obscured AGN. The XMM-Newton EPIC spectrum of NGC6286 was analysed by Brightman & Nandra (2011), who found that Region A. Fitting the spectrum with a collissionally it could be well represented by an unabsorbed power- ionized thermal plasma model (tbabs Gal×apec) re- law continuum plus a thermal plasma, with a photon sults in a good fit (C-stat/DOF=31.0/37), with kT = 7 +0.10 index fixed to Γ = 1.9 . This is in disagreement with 0.91−0.18 keV. Applying a spectral model which repro- the hard (Γ = −0.17+1.01, see § 3.1) 1–8keV spectrum of duces a non-equilibrium plasma created in a shock −1.03 the 3–8 keV core. Fitting the NuSTAR FPMA/FPMB (pshock in xspec) yields C-stat/DOF=31.4/36, a tem- +0.11 data with a simple power-law model we also find a very perature of the plasma of kT S = 0.88−0.13 keV and an +0.46 flat continuum (Γ = 0.49−0.41). These low values of the upper limit on the ionization timescale of τ u ≥ 1.6 × 12 −3 photon index could indicate that the X-ray emission is 10 s cm . We also tested a simple photo-ionisation highly absorbed. model, which consists of a power-law continuum with While the model used by Brightman & Nandra (2011) photon index fixed to Γ = 1.9, which takes into account can well reproduce the XMM-Newton and the spatially- the scattered X-ray radiation, and four Gaussian emis- integrated Chandra spectra, it severely under predicts sion lines with widths fixed to σ = 10 eV [tbabs Gal×(po the hard X-ray flux inferred by NuSTAR, as illustrated + 4×zgauss)]. This model is also successful in repro- in Fig. 2. This might be related either to heavy obscu- ducing the X-ray emission (C-stat/DOF=23.1/29), and ration of the X-ray source or to flux variability between requires lines that could be associated to He or H-like XMM-Newton and NuSTAR observations, although vari- ions of O, Mg, Ne, Na or to Fe-L transitions of ionized ability would not be able to explain the very flat hard X- iron (Table 1). ray spectrum. The Swift/XRT observation allows us to Region B. Using the thermal plasma model yields a rather poor fit (C-stat/DOF=48.0/34). This 7 zpowerlaw+apec in xspec NuSTAR unveils a low-luminosity heavily obscured AGN in the LIRG NGC 6286 5

Figure 3. Left panel: unfolded broad-band X-ray spectrum of NGC 6286. The black continuous line represents the best fit to the data, while the dot-dashed line is the thermal plasma, the dashed line is the scattered emission and the dot-dot-dashed line is the sphere model. Right panel: ratio between the data and the model shown in the right panel. constrain the flux level below 10 keV band at the time of 3.2.1. Pexrav the NuSTAR observations. We find that the Swift/XRT To infer the value of the line-of-sight column density 0.3–2 and 2–5keV flux8 is consistent with that inferred 9 10 (N H) we fitted the joint Swift/XRT, Chandra ACIS- by Chandra and XMM-Newton EPIC/PN observa- S, XMM-Newton EPIC/PN and MOS, and NuSTAR tions, which implies the lack of significant variability FPMA and FPMB with a model that consists of: a) between the different observations. To further test the an absorbed power-law with a photon index fixed to variability scenario we fitted NuSTAR and the spatially- Γ=1.9, b) unabsorbed reprocessed X-ray emission from integrated Chandra spectra with a model that consists a slab, c) a Gaussian to reproduce the fluorescent Fe Kα of a power-law plus a thermal plasma [tbabs ×(apec Gal emission line (with the energy fixed to E Kα = 6.4 keV), + power law)], allowing for different normalisations of d) a second power law to reproduce the scattered com- the power-law continuum to vary (fixing Γ = 1.9). We ponent, and e) emission from a collissionally ionized found that the model cannot well reproduce the spectra 2 11 plasma. To reproduce the effect of obscuration we in- (χ /DOF=28.9/20), with the fit showing clear resid- cluded both Compton scattering and photoelectric ab- uals between 10 and 30 keV. This rules out variability sorption. Reprocessed X-ray emission (excluding fluo- as a possible explanation for the large ratio between the rescent lines) was taken into account using the pexrav 10–50keV and 2–10keV flux. model (Magdziarz & Zdziarski 1995). The fraction of In the following we report the results obtained by scattered flux (f scatt) is calculated as the ratio between adopting several different X-ray spectral model to in- the normalization of the primary power law (n po) and fer the properties of the AGN in NGC6286. In order n scatt. The width of the Gaussian line was fixed to to reduce the possible degeneracies in the models, we po constrained the average properties of the diffuse soft X- σ = 40eV. A Fe Kα line at 6.4 keV is usually found ray emission. To do this we first extracted the Chan- in the X-ray spectrum of AGN (e.g., Nandra & Pounds dra X-ray spectrum of the diffuse emission by exclud- 1994, Shu et al. 2010, Ricci et al. 2014b), and is believed ing from the circular region of 10.5 arcsec a circle of to originate in the material surrounding the SMBH (e.g., 1.5arcsec centred on the 3–8keV core. We then fitted Ricci et al. 2014a and references therein). In xspec our the spectrum with a model that includes i) a thermal model is: plasma and ii) a power-law component (Γ = 1.9) to take tbabs Gal(ztbabs×cabs×zpowerlaw + zgauss + into account the scattered emission. We obtained a nor- apec + zpowerlaw). scatt malization of the power law n po = (1.03 ± 0.37) × The model yields a good value of the chi-squared 2 10−5 ph keV−1 cm−2 s−1, and a temperature and normal- (χ /DOF = 47.2/44) and results in a column density +0.07 consistent with border-line Compton-thick obscuration ization of the thermal plasma of kT =0.77−0.08 keV and +82 22 −2 −5 −1 −2 −1 (N H = 132−54 × 10 cm ). Due to the low signal- n apec = (2.02 ± 0.33) × 10 ph keV cm s , respectively. In all the spectral models reported below we set to-noise ratio the Fe Kα is not spectrally resolved, and n scatt, kT and n to the values obtained for the dif- only an upper limit of its equivalent width was obtained po apec (EW ≤ 2318 eV), which is consistent with heavy obscu- fuse emission, and allow them to vary only within their ration. 90% uncertainties. 3.2.2. Torus To further study the absorbing material we used the 8 +4.1 −14 −1 −2 +0.8 F0.3−2 = 8.4−2.9 × 10 erg s cm and F2−5 = 2.0−1.0 × torus model developed by Brightman & Nandra (2011), 10−14 erg s−1 cm−2 in the 0.3–2 and 2–5 keV band, respectively. which considers reprocessed and absorbed X-ray emis- 9 +0.6 −14 −1 −2 +0.3 F0.3−2 = 8.8−0.6 × 10 erg s cm ; F2−5 = 2.0−0.4 × sion from a spherical-toroidal structure. In this model 10−14 erg s−1 cm−2 the line-of-sight column density is independent of the in- 10 +0.8 −14 −1 −2 +0.2 F0.3−2 = 9.5−1.1 × 10 erg s cm ; F2−5 = 2.2−0.4 × clination angle, which we we fixed to the maximum value −14 −1 −2 ◦ 10 erg s cm permitted (θ i = 87.1 ). Similarly to what was done for 11 the ratio of the power law normalisations is ≃ 4. pexrav, we added to the model a power law, to take 6 Ricci et al. into account the scattered emission, and a collissionally- ionized plasma model. In xspec tbabs Gal(atable{torus1006.fits} + apec + zpowerlaw). We fixed Γ = 1.9 and tested several values of the half- ◦ ◦ ◦ opening angle of the torus (θ OA = 40 , 60 , 80 ). The three models are statistically indistinguishable, and in all cases we obtained good fits. For the three values of θ OA the column densities are consistent within the uncertainties with CT absorption.

3.2.3. Sphere To test the scenario in which the X-ray source is fully covered by the obscuring material we applied the sphere 2 2 2 2 model (Brightman & Nandra 2011), using the same set- Figure 4. Value of ∆χ = χ −χbest (where χbest is the minimum ting as for the torus model: value of the χ2) versus the column density for the different X- ray spectral models discussed in § 3.2. The horizontal dashed line tbabs Gal(atable{sphere0708.fits} + apec + represents ∆χ2 = 2.7. The plot shows that NGC 6286 is heavily zpowerlaw). obscured, with N H consistent with the source being CT for the This model provides a good fit (Fig. 3), and confirms five models considered. +46 4. DISCUSSION the presence of heavy obscuration (N H = 98−35 × 1022 cm−2). The X-ray spectral analysis of NGC6286 reported above clearly shows that the accreting SMBH is heav- 3.2.4. MYTorus ily obscured, possibly by CT material (see Fig. 4). The Next we applied the MYTorus model (Murphy & very flat continuum found by both Chandra (for the hard Yaqoob 2009), which considers absorbed and reprocessed X-ray core) and NuSTAR, together with the fact that ◦ the 1.4 GHz emission coincides with the 3–8 keV Chan- X-ray emission from a smooth torus with θOA = 60 , and can be used for spectral fitting as a combination of dra point-source (Panel2 of Fig.1) confirms the presence three additive and exponential table models: the zeroth- of an heavily obscured AGN. While the buried AGN in order continuum (mytorusZ), the scattered continuum NGC 6286 could be easily identified at hard X-rays, sev- (mytorusS) and a component containing the fluorescent eral other diagnostics failed to detect it because of its emission lines (mytorusL). We used the decoupled ver- low-luminosity. In § 4.1 we illustrate the most commonly sion of MYTorus (Yaqoob 2012). This was done by: adopted techniques to detect AGN in U/LIRGs, and dis- i) allowing the values of the column density of the ab- cuss the case of NGC 6286 by exploiting the wealth of multi-wavelength data available for the GOALS sample. sorbing [N T(Z)] and reprocessing [N T(S,L)] material H H In § 4.2 we estimate the contribution of the AGN to the to have different values; ii) fixing the inclination angle of luminosity of NGC 6286, while in § 4.3 we discuss the op- mytorusL and mytorusS to θ (S,L)=0◦, and that i tical and radio properties of the galaxy, comparing them of mytorusZ to θ (Z)=90◦; iii) adding a second scat- i to those of other similar LIRGs. In § 4.4 we argue about tered component with θ (S,L) = 90◦; iv) leaving the i the origin of the extended soft X-ray component, and normalizations of the transmitted and scattered compo- finally in § 4.5 we discuss on the presence of heavily ob- nent (n and n ) free to vary. To this model we added po refl scured low-luminosity AGN in LIRGs. a scattered component and thermal emission. In xspec the model is: 4.1. Tracers of AGN activity in U/LIRGs ◦ tbabs Gal×{mytorusZ(90 )× zpower- ◦ ◦ AGN in U/LIRGs can be identified in the IR law + mytorusS(0 ) + mytorusS(90 ) + ◦ ◦ by several means. i) With the detection of high- gsmooth[mytorusL(0 ) + mytorusL(90 )] + excitation MIR emission lines (e.g., Sturm et al. apec + zpowerlaw}. 2002), and in particular of [Ne V] 14.32µm and Due to the low signal-to-noise ratio of the spectrum we [Ne V] 24.32µm (e.g., Weedman et al. 2005, Satyapal T could not constrain the different values of N H (S,L) and et al. 2008, Goulding & Alexander 2009). ii) Us- T N H (Z), so their values were tied. The same was done ing the ratios of high-to-low ionization fine-structure for the normalizations of the scattered and transmitted emission lines (e.g., [Ne V] 14.32µm/[Ne II] 12.8µm and components, while the photon index was left free to vary. [OIV] 25.89µm/[Ne II] 12.8µm; e.g., Lutz et al. 1999, Pet- Also this model yields a good fit and results in a line-of- ric et al. 2011). iii) With the EW of the PAH features, sight column density consistent with heavy obscuration which tend to be lower in the presence of a bright AGN, +51 22 −2 (N H = 88−38 × 10 cm ). since it can destroy PAH molecules (e.g., Imanishi et al. 2010). iv) Studying the slope of the 2.5–5µm contin- The parameters obtained from the spectral analysis are uum (Γ2.5−5, e.g., Imanishi et al. 2010) or the contin- reported in Table 3, while in Figure 4 we show the values uum 30µm/15µm flux density ratio (e.g., Stierwalt et al. 2 of ∆χ versus N H for the models described above. De- 2013), which tend to be red in the presence of an AGN. pending on the X-ray spectral model adopted, the intrin- v) Using the depth of absorption features (e.g., Imanishi sic (i.e. absorption and k-corrected) 2–10 keV luminosity & Dudley 2000; Risaliti et al. 2006), with large depths of NGC6286 is 3 − 20 × 1041 ergs−1. pointing towards AGN obscured by dust. vi) From de- NuSTAR unveils a low-luminosity heavily obscured AGN in the LIRG NGC 6286 7

Table 2 List of AGN tracers commonly used for U/LIRGs.

(1) (2) (3) (4) (5) (6) Indicator NGC 6286 Reference Mean GOALS Threshold AGN

[NeV]14.32µm [10−17 Wm−2] 0.33 ± 0.11 Dudik et al. (2009) 2.27A ··· ? [NeV]24.32µm [10−17 Wm−2] 0.99 ± 0.20 Dudik et al. (2009) ······ ? [Ne V]/[Ne II] 0.02 Dudik et al. (2009) 0.07B ≥ 0.1C ✕ [OIV]/[Ne II] 0.05 Dudik et al. (2009) 0.03D/0.24E ≥ 1.75F ✕ G Γ2.5−5µm −0.1 Imanishi et al. (2010) ··· ≥ 1 ✕ +2 H ✕ Fν (30µm)/Fν (15µm) 5.97 Stierwalt et al. (2013) 8−1.5 ··· EW (PAH 3.3µm)[nm] 48 Imanishi et al. (2010) ··· < 40G ✕ EW (PAH 6.2µ m) [µm] 0.59 Stierwalt et al. (2013) 0.55H ≤ 0.3C ✕ G τ3.1µm (3.1µm H2O ice) ND Imanishi et al. (2010) ··· > 0.3 ✕ G τ3.4µm (3.4µm bare carbonaceous) ND Imanishi et al. (2010) ··· > 0.2 ✕ H τ9.7µm −0.40 Stierwalt et al. (2013) −0.35 ··· ✕ Chandra hardness ratio −0.85 ± 0.07 This work −0.56I > −0.3J ✕ −1 K L Observed log L 2−10 [ergs ] 40.80 This work 41.3 > 42 ✕ Radio/FIR ﬂux ratio (q) 2 U et al. (2012) 2.41 ± 0.29M < 1.64N ✕

The table lists (1) the indicator used, (2) the value and (3) reference for NGC 6286, (4) the mean value for the GOALS sample, (5) the threshold used to infer the presence of AGN, and (6) whether an AGN was found or not. Notes. ND: not detected; A median of the 43 detections (18% of the sample) from Petric et al. (2011); B median (Petric et al. 2011); C threshold used by Inami et al. (2013) to establish a significant contribution of the AGN to the MIR flux. D median and E mean from Petric et al. (2011); F Value indicating if the AGN contributes to more than 50% of the MIR flux (Petric et al. 2011); G value used by Imanishi et al. (2010) H mean value for objects in the same merger stage (B) as NGC 6286 (Stierwalt et al. 2013), the 30µm/15µm flux density ratio of NGC 6286 is only marginally lower than the average value for the B merger stage, and has a value consistent with 63% of GOALS LIRGs; I median of the 44 objects reported in Iwasawa et al. (2011); J value used to establish the presence of an AGN and K median value (Iwasawa et al. 2011); L Values commonly used to separate AGN from starbursts in the 2–10 keV band (e.g., Szokoly et al. 2004, Kartaltepe et al. 2010); M mean obtained for the 64 objects studied by U et al. (2012); N threshold for radio-excess defined by Yun et al. (2001). viations of the well known correlation between the far- Sources with HR > −0.3 are reported as candidate IR (FIR) and the radio luminosity (Helou et al. 1985, AGN . This value was chosen because ULIRGs which Condon et al. 1991, Condon 1992), by using the radio- are known to host AGNs, such as Mrk231, Mrk273, and FIR flux ratio q (e.g., Yun et al. 2001). We find that UGC 5101, tend to cluster just above this limit (Iwa- all these proxies (Table 2) fail to detect the AGN in sawa et al. 2009). NGC6286 has a hardness ratio of NGC6286, with the exception of the NeV lines. These HR = −0.85 ± 0.07, which would exclude the presence lines can however also be produced by a young starburst of an AGN. However, as discussed by Iwasawa et al. with a large population of Wolf-Rayet and O stars (e.g., (2011) this threshold could become less reliable for some Abel & Satyapal 2008), so their detection does not al- CT AGN, since mostly reprocessed radiation is observed ways provide conclusive evidence of an AGN. This is in the hard X-ray band. Another criteria commonly especially true for NGC 6286, since the Ne V lines are used to identify AGN is the observed 2–10 keV X-ray −1 −1 weak [log(L [Ne V]/ergs ) ∼ 38.8]. Spitzer/IRAC selec- luminosity. Using log(L2−10/ ergs ) > 42 as threshold tion provides another important tool for identifying AGN (e.g., Szokoly et al. 2004, Kartaltepe et al. 2010) one (e.g., Lacy et al. 2004, Stern et al. 2005). Using the AGN would also miss identifying NGC 6286 as a buried AGN −1 selection criteria proposed by Donley et al. (2012) (see [log(L2−10/ ergs )=40.80]. Eq.1 and 2 in their paper), and considering the fluxes Spectral decomposition (e.g., Nardini et al. 2008, reported by U et al. (2012), we find that NGC6286 does Alonso-Herrero et al. 2012) is another powerful method not satisfy the conditions for the presence of an AGN. to constrain the contribution of AGN to the multi- The fact that the IR proxies fail to identify the AGN wavelength spectral energy distribution (SED). Vega emission in NGC 6286 is due to the problematic identifi- et al. (2008) found that a pure starburst model fails to cation of low-luminosity AGN with IR spectra dominated well reproduce the near-IR to radio SED of NGC6286, by the host, as in the case of NGC6286. and a buried AGN accounting for 5% of the IR luminos- Iwasawa et al. (2011) have studied 44 LIRGs of the ity is required by the data. GOALS sample with Chandra, and assessed the pres- 4.2. ence of an AGN by using the hardness ratio HR=(H- AGN contribution to the IR luminosity S)/(H+S), where H and S are the background-corrected The IR luminosity of NGC 6286 is 8.8 × 1044 ergs−1, counts in the 2–8 and 0.5–2 keV range, respectively. which would imply that, depending on the X-ray spec- 8 Ricci et al. tral model used, we obtain a ratio L2−10/L IR ≃ 4 × 10−4 − 2.3 × 10−3. Considering the observed 2–10keV obs luminosity, the ratio is log(L2−10/L IR) ≃ −4.14, which is consistent with the average value found for the GOALS obs sample [log(L2−10/L IR) = −4.40 ± 0.63, Iwasawa et al. 2011]. By using the relationship of Mullaney et al. (2011) it is possible convert the 2–10 keV luminosity into the expected IR luminosity emitted by the dust around the AGN: AGN log L IR, 43 = (0.53±0.26)+(1.11±0.07) logL2−10,43. (1) AGN In the above equation L IR, 43 and L2−10,43 are the 8– 1000µm and 2–10 keV luminosities of the AGN in units − of 1043 ergs 1. Considering the range of values obtained Figure 5. Intrinsic X-ray luminosity of the AGN in the 2–10 keV band versus the total IR luminosity of the system (in the 8–100µm for the 2–10 keV intrinsic luminosity, the IR luminosity of − AGN band). Both luminosities are in units of 1043 erg s 1. The contin- the AGN is L IR = 41.91−42.75. Comparing this to the −1 uous black line represents the values for which AGN and starburst IR luminosity of the system [log(L IR/ ergs ) = 44.96] contribute in the same amount to the IR flux, while the dashed we find that the IR luminosity of the AGN is between lines show contributions of the AGN of 20, 10 and 1%. The values 0.1 and 0.6% of the total IR luminosity. This value is in of the IR luminosity expected to be due to the AGN are calculated from the 2–10 keV luminosity following Eq. 1. The two values of the disagreement with that obtained by Vega et al. (2008) by 2–10 keV luminosity of NGC 6286 represent the minimum and max- using spectral decomposition, who found that the contri- imum value obtained with the different models discussed in Sect 3.2 bution of the AGN to the total IR luminosity is about one (see also Table 3). The figure shows that the AGN in NGC 6286 order of magnitude larger. A 5% of contribution to the contributes to < 1% of the total IR luminosity. AGN total IR luminosity would imply that L IR = 43.66 and the intrinsic 2–10 keV luminosity of the AGN would be (2010) using the classification scheme of Veilleux & Os- −1 log(L2−10/ ergs ) = 43.12, also an order of magnitude terbrock (1987), based on the diagram first proposed by larger than predicted by our X-ray spectral analysis. To Baldwin, Phillips & Terlevich (1981). Yuan et al. (2010) have such a luminosity, the AGN should be obscured by have recently used a new semi-empirical optical spec- −2 log(N H/cm ) > 25, which is inconsistent with the re- tral classification to classify IR-selected galaxies based sults obtained here. An alternative explanation for this on three diagrams: [OIII]/Hβ versus [NII]/Hα, [SII]/Hα discrepancy is that the AGN is intrinsically weak at X- and [OI]/Hα line ratios. This is based on the work of ray wavelengths, as recently found by NuSTAR for the Kewley et al. (2006) to separate starburst galaxies, star- AGN in Mrk 231 (Teng et al. 2014, see also Teng et al. burst/AGN composite galaxies, Seyfert2, and LINERs. 2015). In the scheme of Kewley et al. (2006) objects that were Assuming a 2–10 keV bolometric correction of 20 (e.g., classified as LINERs according to Veilleux & Osterbrock Vasudevan & Fabian 2007), the bolometric output of the (1987) would be either true LINERs, Seyfert2s, compos- AGN would be 7−40×1042 ergs−1. This implies that the ite HII-AGN galaxies, or high metallicity star-forming ratio between the IR luminosity and the total output of galaxies. Yuan et al. (2010) found that true LINERs are Bol the AGN is L AGN/L IR ≃ 0.8−4.5%. The AGN bolomet- rare in IR-selected samples (< 5%), and most of the ob- ric output can also be inferred from the [Ne V] 14.32µm jects would be either classified as star-forming galaxies or luminosity, following the relation obtained by Satyapal starburst/AGN composites. Yuan et al. (2010) classified et al. (2007): NGC 6286 as a composite using [NII], a HII region using [SII] and a LINER using [OI]. Therefore they adopted a AGN log L Bol =0.938 log L [Ne V] +6.317, (2) composite classification for the source, which might im- AGN −1 ply the presence of an AGN. Yuan et al. (2010) found that and is log(L Bol / ergs ) ∼ 42.7. This would give a 11 12 Bol in the IR luminosity bin L IR = 10 −10 L⊙ about 37% ratio L AGN/L IR ≃ 0.6%. of the objects in the IRAS Bright Galaxy Sample (BGS, The lack of a significant AGN contribution to the to- Sanders et al. 1995, Veilleux et al. 1995) are classified as tal IR flux is also confirmed considering the [Ne V]/[Ne II] composites. ratio versus the EW of the 6.2µm PAH feature [see Fig. 1 To characterise the relative AGN contribution to the and 2 of Petric et al. (2011)], which shows that the ra- extreme ultraviolet (EUV) radiation field, Yuan et al. AGN tio between L IR and L IR is below 1% for this object. (2010) use D AGN, which is the normalised distance from This disfavours the intrinsically X-ray weak AGN sce- the outer boundary of the star-forming sequence. While nario, and we can therefore conclude that the energetics this quantity does not provide information on the frac- of NGC 6286 are clearly dominated by the host galaxy, tion of emission due to the AGN, it can be used to com- with the low-luminosity AGN providing only a minor pare the amount of EUV radiation due to the AGN in contribution to the total flux. The contribution of the different objects. For NGC 6286 they found D AGN =0.5 AGN to the IR flux of the system is shown in Fig. 5. using both the [OI]/Hα and the [NII]/Hα diagram. Yuan et al. (2010) found a statistically significant increase of 4.3. Optical and radio emission D AGN with L IR, consistent with the idea that the frac- NGC 6286 has been classified as a low-ionization nu- tion of AGN increases for increasing values of the 8- clear emission-line region (LINER) by Imanishi et al. 100µm luminosity (e.g., Veilleux et al. 1995). The value NuSTAR unveils a low-luminosity heavily obscured AGN in the LIRG NGC 6286 9 obtained for NGC 6286 is marginally larger than the av- AGN if it is highly obscured, since the gas and dust would erage value obtained by Yuan et al. (2010) for the BGS shield the PAH-emitting molecules, or if it is not very lu- sample for 11 < log(L IR/L⊙) < 12 (D AGN ≃ 0.35). minous. A low-luminosity AGN would also be difficult A radio core is rather common in low-luminosity AGN, to find by studying the 2.5–5µm slope, since the IR emis- as shown by the work of Nagar et al. (2005), who sion would be dominated by the starburst, and the AGN found evidence of radio emission in ≥ 50% of the Low- emission can still be self-absorbed. Absorption features luminosity AGN of the Palomar Spectroscopic sample also would not be able to help if the AGN is not very (see also Ho 2008). The flux of NGC6286 at 1.4GHz is luminous. A more reliable tracer is [Ne V], but while its f1.4 GHz = 157.4 ± 5.6 mJy, which implies that the radio detection might indicate the presence of an AGN, its non- loudness is log R X = log(f1.4 GHz/f2−10)= −2.6 to −3.1, detection does not exclude it. Moreover, [Ne V] could be depending on the X-ray spectral model assumed. These created in young starbursts, and for low-luminosity AGN values were obtained taking into account only the nu- it could be too faint to be detected (see Eq.2). Radio clear emission in the computation of the 2–10 keV flux. studies can also give important insights, but since not all Considering the threshold suggested by La Franca et al. AGN are very strong in this band they are not always (2010) (see also Panessa et al. 2007, Terashima & Wilson conclusive. Hard X-ray studies are possibly the best way 2003), log R X = −4.3, NGC 6286 would be classified as to unveil obscured AGN in U/LIRGs, although they can −2 radio-loud AGN. Murphy (2013) report that the radio also be limited by absorption for log(N H/cm ) ≫ 24. 12 spectral index of NGC6286 is α low = −0.73 ± 0.03, By using multi-wavelength indicators of AGN for a α mid = −0.89 ± 0.03 and α high = −1.02 ± 0.12 for subsample of 53 U/LIRGs within the GOALS sample, ν < 5 GHz, 1 <ν < 10 GHz and ν > 10GHz, respec- U et al. (2012) found that ∼ 60% and ∼ 25% of ULIRGs tively. This, together with the radio-loud classification, and LIRGs host AGN. Studying the whole GOALS sam- would point towards a significant contribution of syn- ple, Petric et al. (2011) found that 18% of the LIRGs chrotron emission. show evidence of [NeV]14.32µm, and hence might host an AGN. By means of optical spectroscopy, Yuan et al. 4.4. The soft X-ray emission (2010) found that 58.9% of the 51 single nuclei galaxies with 11 < log(L IR/L⊙) < 12 in the BGS sample host While the hard X-ray emission is due to a buried 13 low-luminosity AGN, the diffuse soft X-ray emission in an AGN . The fraction of AGN is larger (76.5%) if one NGC6286 could be due to thermal plasma associated to considers only two of the three diagrams for the spectral a starburst, to hot gas heated in shocks created by out- classification. A significant fraction of the composite sys- flows from the AGN, or to photo-ionised emission created tems might hide buried low-luminosity AGN, as in the by the AGN. For region A and C the current data do not case of NGC6286. Treister et al. (2010b) have shown, by stacking Chandra spectra of LIRGs in the Chandra Deep allow to discern amongst these three scenarios, the three 11 spectral models being able to fit well the spectra in all Field-South, that 15% of the objects with L IR > 10 L⊙ cases. The X-ray spectrum of regionB seems however to contain heavily obscured AGN. By stacking X-ray spec- privilege our phenomenological photo-ionised model, al- tra in different bins of stellar mass, they found a significant excess at E = 6 − 7 keV in the stacked spectrum though we cannot draw a firm conclusion because of the 11 low signal-to-noise ratio of the spectrum. Given the prox- of sources with mass M > 10 M⊙, very likely related imity to the accreting SMBH, it would seem plausible to to a prominent Fe Kα line, while no clear evidence of have a significant contribution from photo-ionisation in AGN activity was found in less-massive galaxies. Treis- this region. ter et al. (2010b) concluded that there might be a large population of heavily obscured AGN in high mass galax- The fraction of the scattered component we found for 11 ies. NGC 6286, with a stellar mass of 1.26 × 10 M⊙ NGC 6286 (f scatt ∼ 1−9%) is consistent with the average value obtained for the ∼ 730 non-blazar AGN reported (Howell et al. 2010), fits extremely well into this scenario in the local Universe. in the 70-months Swift/BAT catalog (f scatt ∼2.5%, Ricci et al. in prep.), but is significantly higher than the We have shown in § 4.1 and 4.3 that NGC6286 has op- value found for AGN with log(N /cm−2) ∼ 24 − 25 tical and IR characteristics quite typical of LIRGs, and H consistent with other galaxies of the GOALS sample for (f scatt ∼0.6%). Since the fraction of scattered radiation depends both on the covering factor and the density the same merger stage. It is interesting to notice that also of the material responsible for Thomson scattering (e.g., the hardness ratio and the observed 2–10 keV luminosity Ueda et al. 2007, Eguchi et al. 2009), this might imply inferred by Chandra are consistent with a large fraction that in NGC6286 there is a larger amount of gas and dust of the objects of the sample of Iwasawa et al. (2011) (see surrounding the AGN. This would be consistent with the Fig. 5 and 6 of their paper, respectively), which might in- idea that greater quantities of gas are channeled towards dicate that several more heavily obscured low-luminosity the centre of galaxies during mergers. AGN are present in LIRGs of the GOALS sample. More- over, we have shown that in the low-count regime it is 4.5. Heavily obscured low-luminosity AGN in U/LIRGs possible to miss obscured AGN by adopting a simple phenomenological model to reproduce their X-ray spectra. As discussed above for the case of NGC 6286, the iden- Therefore there might be a significant population of low- tification of heavily obscured AGN in LIRGs can be luminosity heavily obscured AGN in LIRGs that we are rather difficult if the AGN has a low-luminosity. The missing due to the lack of sensitive hard X-ray observa- EW of PAH features would not be highly affected by the tions. Our on-going campaign of NuSTAR observations

12 We consider here the following definition of the spectral index: 13 37.3% are composite AGN/starburst, 13.7% Seyfert 2s, 2% α Fν ∝ ν . Sy1s and 5.9% LINERs. 10 Ricci et al. of ten LIRGs will allow to study the AGN fraction in This research has made use of the NuSTAR Data Anal- merging galaxies in the hard X-ray band across the whole ysis Software (NuSTARDAS) jointly developed by the merger sequence. ASI Science Data Center (ASDC, Italy) and the Califor- Another object showing similar characteristics to nia Institute of Technology (Caltech, USA), and of the NGC6286 is IC883, a LIRG in a late merger stage that NASA/ IPAC Infrared Science Archive and NASA/IPAC was found to host a low-luminosity AGN by exploiting Extragalactic Database (NED), which are operated by radio observations (Romero-Cañizales et al., in prep.; the Jet Propulsion Laboratory, California Institute of Romero-Cañizales et al. 2012b). As for NGC6286, the Technology, under contract with the National Aeronau- IR emission of IC 883 is dominated by star-formation and tics and Space Administration. We acknowledge fi- the AGN does not contribute significantly to the energet- nancial support from the CONICYT-Chile grants ”EM- ics of the system. Interestingly, similarly to NGC 6268 BIGGEN” Anillo ACT1101 (CR, FEB, ET), FONDE- also IC883 is reported as a composite AGN/starburst CYT 1141218 (CR, FEB), Basal-CATA PFB–06/2007 system by Yuan et al. (2010), along with more than one (CR, FEB, ET), and the Ministry of Economy, De- third of LIRGs from the BGS sample. velopment, and Tourism’s Millennium Science Initiative through grant IC120009, awarded to The Millennium In- stitute of Astrophysics, MAS (FEB). Facilities: Nustar, Chandra, Swift, XMM-Newton. 5. 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ApJ, 596, L167 NuSTAR unveils a low-luminosity heavily obscured AGN in the LIRG NGC 6286 11

Table 3 Summary of the X-ray spectral analysis for the spatially integrated X-ray spectrum of NGC 6286.

Pexrav

22 −2 +82 Column density (N H)[10 cm ] 132−54 Reflection parameter (R) ≤ 0.4 Plasma temperature (kT )[ keV] 0.79 A +1.9 B Scattered fraction (f scatt)[%] 1.3−0.7 Fe Kα EW [ eV] ≤ 2318 obs −13 −1 −2 Observed 2–10 keV flux (F2−10)[10 erg s cm ] 0.9 obs −13 −1 −2 Observed 10–50 keV flux (F10−50)[10 erg s cm ] 9.6 −12 −1 −2 Intrinsic 2–10 keV flux (F2−10)[10 erg s cm ] 2.6 −12 −1 −2 intrinsic 10–50 keV flux (F10−50)[10 erg s cm ] 3.1 −1 42 2–10 keV luminosity (L2−10)[ erg s ] 2.01 × 10 −1 42 10–50 keV luminosity (L10−50)[ erg s ] 2.34 × 10 χ2/DOF 47.2/44

Torus ◦ ◦ ◦ θ OA = 40 θ OA = 60 θ OA = 80

Plasma temperature (kT )[ keV] 0.79A 0.79A 0.79A +2.0B +2.0B +1.9 B Scattered fraction (f scatt)[%] 3.0−1.3 2.4−1.3 2.0−1.3 22 −2 +89 +109 +101 Column density (N H)[10 cm ] 110−39 111−41 106−38 −13 −1 −2 Intrinsic 2–10 keV ﬂux (F2−10)[10 erg s cm ] 10.7 13.0 16.0 −13 −1 −2 intrinsic 10–50 keV ﬂux (F10−50)[10 erg s cm ] 13.1 15.8 19.5 −1 41 41 42 2–10 keV luminosity (L2−10)[ erg s ] 8.14 × 10 9.84 × 10 1.21 × 10 −1 41 42 42 10–50 keV luminosity (L10−50)[ erg s ] 9.90 × 10 1.20 × 10 1.48 × 10 χ2/DOF 47.8/46 48.1/46 48.5/46

Sphere

Plasma temperature (kT )[ keV] 0.79 A +2.1B Scattered fraction (f scatt)[%] 3.6−1.2 22 −2 +46 Column density (N H)[10 cm ] 101−32 −13 −1 −2 Intrinsic 2–10 keV ﬂux (F2−10)[10 erg s cm ] 9.2 −13 −1 −2 intrinsic 10–50 keV ﬂux (F10−50)[10 erg s cm ] 11.2 −1 41 2–10 keV luminosity (L2−10)[ erg s ] 6.98 × 10 −1 41 10–50 keV luminosity (L10−50)[ erg s ] 8.50 × 10 χ2/DOF 47.2/46

MyTORUS

+0.10C Photon index (Γ) 1.53−NC Plasma temperature (kT )[ keV] 0.79 A +4.9B Scattered fraction (f scatt)[%] 8.8−2.6 22 −2 +61 Column density (N H)[10 cm ] 95−39 −13 −1 −2 Intrinsic 2–10 keV ﬂux (F2−10)[10 erg s cm ] 4.7 −13 −1 −2 intrinsic 10–50 keV ﬂux (F10−50)[10 erg s cm ] 11.1 −1 41 2–10 keV luminosity (L2−10)[ erg s ] 3.49 × 10 −1 41 10–50 keV luminosity (L10−50)[ erg s ] 8.35 × 10 χ2/DOF 52.5/45

Notes. A parameter left free to vary within the uncertainties of the value obtained ﬁtting B scatt the extended emission (see § 3.2 for details). value calculated from the ratio of n po and n po . C the photon index in MyTORUS is calculated only in the range Γ = 1.4 − 2.6. 12 Ricci et al.

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