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Where Are Compton-Thick Radio Galaxies? a Hard X-Ray View of Three Candidates

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Where Are Compton-Thick Radio Galaxies? a Hard X-Ray View of Three Candidates MNRAS 000, 000–000 (0000) Preprint 11 September 2018 Compiled using MNRAS LATEX style file v3.0 Where are Compton-thick radio galaxies? A hard X-ray view of three candidates F. Ursini,1 ? L. Bassani,1 F. Panessa,2 A. Bazzano,2 A. J. Bird,3 A. Malizia,1 and P. Ubertini2 1 INAF-IASF Bologna, Via Gobetti 101, I-40129 Bologna, Italy. 2 INAF/Istituto di Astrofisica e Planetologia Spaziali, via Fosso del Cavaliere, 00133 Roma, Italy. 3 School of Physics and Astronomy, University of Southampton, SO17 1BJ, UK. Released Xxxx Xxxxx XX ABSTRACT We present a broad-band X-ray spectral analysis of the radio-loud active galactic nuclei NGC 612, 4C 73.08 and 3C 452, exploiting archival data from NuSTAR, XMM-Newton, Swift and INTEGRAL. These Compton-thick candidates are the most absorbed sources among the hard X-ray selected radio galaxies studied in Panessa et al.(2016). We find an X-ray absorbing column density in every case below 1:5 × 1024 cm−2, and no evidence for a strong reflection continuum or iron K α line. Therefore, none of these sources is properly Compton-thick. We review other Compton-thick radio galaxies reported in the literature, arguing that we currently lack strong evidences for heavily absorbed radio-loud AGNs. Key words: galaxies: active – galaxies: Seyfert – X-rays: galaxies – X-rays: individual: NGC 612, 4C 73.08, 3C 452 1 INTRODUCTION 2000). A number of local CT AGNs have been detected thanks to recent hard X-ray surveys with INTEGRAL (Sazonov et al. 2008; A significant fraction of active galactic nuclei (AGNs) are known Malizia et al. 2009, 2012) and Swift (e.g. Burlon et al. 2011; Ricci to be intrinsically obscured by gas and dust surrounding their cen- et al. 2015). At high redshift, the rest-frame hard X-ray emission tral engine. According to the unified model, the presence of a torus becomes observable below 10 keV and CT AGNs have been de- at pc scales can explain the properties of obscured AGNs and the tected using deep surveys by XMM-Newton (e.g. Lanzuisi et al. dichotomy with unabsorbed sources, simply due to a different ori- 2015) and Chandra (e.g. Brightman et al. 2014). While the ob- entation with respect to the line of sight (e.g. Antonucci & Miller served fraction of CT AGNs is generally found to be of a few per- 1985; Antonucci 1993). cent, their intrinsic fraction could be as high as 20–30 per cent. Obscured AGNs are considered an important class of sources This estimate remains highly uncertain, owing to the difficulties in for two main reasons. First of all, most AGNs in the local universe detecting and classifying such heavily obscured sources. are absorbed (N > 1022 cm−2; e.g. Risaliti et al. 1999; Bassani H It is a matter of debate whether differences exist in the ab- et al. 1999). As shown by different X-ray surveys, the observed sorption properties between different classes of AGNs, depending fraction of absorbed AGNs generally ranges between 40 and 60 on their physical parameters (like the accretion rate or the lumi- per cent (e.g. Burlon et al. 2011, and references therein). However, nosity, e.g. Elitzur & Shlosman 2006). For example, the fraction of their intrinsic fraction, i.e. corrected for selection biases, is around arXiv:1712.01300v1 [astro-ph.HE] 4 Dec 2017 obscured AGN is observed to decrease with increasing X-ray lumi- 80 per cent (e.g. Malizia et al. 2012). Moreover, obscured AGNs nosity (e.g. Ueda et al. 2003; La Franca et al. 2005; Sazonov et al. are an important ingredient of both the cosmic X-ray background 2015), although with some uncertainties related to luminous but (e.g. Fabian & Iwasawa 1999; Gilli et al. 2007) and of the infrared heavily obscured sources (Mateos et al. 2017), and possibly with background, where most of the absorbed high-energy radiation is decreasing redshift (e.g. La Franca et al. 2005). Radio galaxies are re-emitted (e.g. Risaliti et al. 2002). particularly interesting in this respect, because the presence of a In particular, Seyfert 2 galaxies in which the absorbing ma- jet could have an impact on the circumnuclear environment and terial has N > σ−1 = 1:5 × 1024 cm−2, where σ is the Thom- H T T on the obscuring medium. Large fractions of X-ray obscured ra- son cross-section, are called Compton-thick (CT). Such high col- dio galaxies are generally found (e.g. Sambruna et al. 1999; Evans umn densities produce a photoelectric cut-off at energies of 10 keV et al. 2006; Hardcastle et al. 2009; Wilkes et al. 2013). In partic- or more. Hard X-rays data are thus needed to properly study CT ular, Wilkes et al.(2013) reported a fraction of 20 per cent of CT sources (see, e.g., the analysis of BeppoSAX data by Matt et al. sources in a high-redshift sample (1 < z < 2). However, only a few radio galaxies in the local Universe have been reported as CT: ? e-mail: [email protected] Mrk 668 (Guainazzi et al. 2004), 3C 284 (Hardcastle et al. 2006), c 0000 The Authors 2 F. Ursini Table 1. Basic data of the three sources selected from P16. the neighbour galaxy NGC 619 (Emonts et al. 2008). The AGN in NGC 612 is optically classified as a Seyfert 2 (e.g. Véron-Cetty & NGC 612 4C 73.08 3C 452 Véron 2006; Parisi et al. 2009) and it is absorbed in the X-rays. Us- 24 −2 ing Suzaku data, Eguchi et al.(2009) found NH ' 1:1 × 10 cm Optical classification Sy2 Sy2 Sy2 and suggested a scenario where the X-ray source is obscured by a Radio morphology FRI/II FRII FRII torus with a half-opening angle of 60-70 deg, seen from a nearly Host galaxy type S0 E E edge-on angle. Redshift 0.029 771 0.058 100 0.081 100 4C 73.08 is a giant radio galaxy, with a linear size of around 1 Galactic N (1020 cm−2) 1.85 2.41 11.9 H Mpc (e.g. Bassani et al. 2016). It exhibits a complex morphology, but overall consistent with a FRII (e.g. Lara et al. 2001; Strom et al. 2013). The AGN is optically classified as a Seyfert 2 (e.g. Hewitt 3C 223 (LaMassa et al. 2014; Corral et al. 2014), 3C 321 (Evans & Burbidge 1991; Parisi et al. 2014). This source is poorly studied et al. 2008; Severgnini et al. 2012), 3C 452 (Fioretti et al. 2013), in the X-rays, however Evans et al.(2008) inferred an absorbing TXS 2021+614 (Siemiginowska et al. 2016) and PKS 1607+26 column density of around 9 × 1023 cm−2 for the nucleus, from the (Tengstrand et al. 2009; Siemiginowska et al. 2016). analysis of XMM-Newton data. In a recent work, Panessa et al.(2016, P16 hereafter) investi- 3C 452 is a radio galaxy with a symmetrical FRII morphol- gated the role of absorption in radio galaxies. The sample analysed ogy (e.g. Black et al. 1992), optically classified as a Seyfert 2 by P16 consisted of 64 AGNs showing extended radio emission, se- (e.g. Véron-Cetty & Véron 2006). From Chandra data, Isobe et al. lected from hard X-ray catalogues by Bassani et al.(2016). Being (2002) found the nucleus to be absorbed by a column density of hard X-ray selected, this sample is relatively unbiased towards ab- around 6 × 1023 cm−2, and detected a diffuse X-ray emission from sorption, at least up to a few ×1024 cm−2. P16 derived the column the lobes. Similar results were obtained by Evans et al.(2006) with density distribution, and found an observed fraction of absorbed the same data set. Using XMM-Newton data, Shelton et al.(2011) sources of 40 per cent (75 per cent among the type 2s). However, performed a detailed X-ray study of both the AGN and the lobes, the fraction of CT sources was found to be no more than 2–3 per constraining their inverse Compton emission and environmental cent, whereas 7–10 per cent would have been expected (e.g. Mal- impact. No significant variations were found from the comparison izia et al. 2012). with the Chandra observation, while the absorbing column den- In this paper, we focus on the most absorbed radio galaxies sity was found to be around 4 × 1023 cm−2 for the AGN. The same reported in P16, with the aim of studying their broad-band X-ray column density was reported by Fioretti et al.(2013), who anal- spectra and to well constrain their column density. We take advan- ysed Suzaku data. Finally, according to the results of Fioretti et al. tage of archival NuSTAR data, which are very helpful in constrain- (2013), the hard energy band of the X-ray spectrum is dominated ing the spectral shape of absorbed sources thanks to the high-energy by Compton reflection off cold matter, with a reflection fraction coverage (e.g. Balokovic´ et al. 2014). The structure of the paper is (as defined in Magdziarz & Zdziarski 1995) found to be R > 400. as follows. In Section2 we present the sources and their archival This unphysical value could be due to an inhomogeneous circum- X-ray data used. In Section3, we present the spectral analysis. We nuclear material, where a significant fraction of the solid angle is discuss the results in Section4, and summarize the conclusions in covered by a gas thicker than that along the line of sight (Fioretti Section5.
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