A Comprehensive Analysis of the Hard X-Ray Spectra of Bright Seyfert Galaxies
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MNRAS 000, 1–24 (2015) Preprint 4 October 2018 Compiled using MNRAS LATEX style file v3.0 A comprehensive analysis of the hard X-ray spectra of bright Seyfert galaxies⋆ P. Lubi´nski1†, V. Beckmann2, L. Gibaud3‡, S. Paltani3, I. E. Papadakis4,5, C. Ricci6, S. Soldi2, M. Turler¨ 3, R. Walter3, A. A. Zdziarski7 1Institute of Physics, University of Zielona G´ora, Licealna 9, PL-65-417 Zielona G´ora, Poland 2Fran¸cois Arago Centre, APC, Universit´eParis Diderot, CNRS/IN2P3, CEA/Irfu, Observatoire de Paris, 13 rue Watt, 75013 Paris, France 3Department of Astronomy, University of Geneva, Chemin d’Ecogia 16, CH-1290 Versoix, Switzerland 4Physics Department, University of Crete, PO Box 2208, 710 03 Heraklion, Crete, Greece 5IESL, Foundation for Research and Technology, 711 10, Heraklion, Crete, Greece 6Pontificia Universidad Cat´olica de Chile, Instituto de Astrof´ısica, Casilla 306, Santiago 22, Chile; EMBIGGEN Anillo, Concepci´on, Chile 7Centrum Astronomiczne im. M. Kopernika, Bartycka 18, PL-00-716 Warszawa, Poland Accepted XXX. Received YYY; in original form ZZZ ABSTRACT Hard X-ray spectra of 28 bright Seyfert galaxies observed with INTEGRAL were analyzed together with the X-ray spectra from XMM-Newton, Suzaku and RXTE. These broad-band data were fitted with a model assuming a thermal Comptonization as a primary continuum component. We tested several model options through a fitting of the Comptonized continuum accompanied by a complex absorption and a Compton reflection. Both the large data set used and the model space explored allowed us to accurately determine a mean temperature kTe of the electron plasma, the Compton parameter y and the Compton reflection strength R for the majority of objects in the sample. Our main finding is that a vast majority of the sample (20 objects) is characterized by kTe < 100 keV, and only for two objects we found kTe > 200 keV. +57 The median kTe for entire sample is 48−14 keV. The distribution of the y parameter is bimodal, with a broad component centered at ≈ 0.8 and a narrow peak at ≈ 1.1. A complex, dual absorber model improved the fit for all data sets, compared to a simple absorption model, reducing the fitted strength of Compton reflection by a factor of about 2. Modest reflection (median R ≈ 0.32) together with a high ratio of Comptonized to seed photon fluxes point towards a geometry with a compact hard X-ray emitting region well separated from the accretion disc. Our results imply that the template Seyferts spectra used in AGN population synthesis models should be revised. Key words: galaxies: active – galaxies: Seyfert – galaxies: nuclei – X-rays: galaxies – gamma-rays: galaxies. arXiv:1602.08402v1 [astro-ph.GA] 26 Feb 2016 1 INTRODUCTION tion component (for a review see e.g., Beckmann & Shrader 2012). Evidence for that was collected already by the CGRO The spectra of Seyfert galaxies in the hard X-ray and soft (e.g., Zdziarski et al. 2000) and BeppoSAX satellites (e.g., γ-ray band can be well approximated by a power-law model Dadina 2007). Since the late 1990s there is a consensus with a high-energy cut-off accompanied by a Compton reflec- that the main mechanism responsible for this emission is the thermal Comptonization of seed photons in the plasma ⋆ Based on observations with INTEGRAL, an ESA project with cloud surrounding a black hole (BH) in the centre of the instruments and science data centre funded by ESA member AGN (e.g., Svensson 1999). Unfortunately, due to a lim- states (especially the PI countries: Denmark, France, Germany, ited sensitivity of the instruments a detailed quantitative Italy, Switzerland, and Spain), the Czech Republic, and Poland analysis using a physical Comptonization model instead of and with the participation of Russia and the US. a phenomenological one is not common. The most sensitive † E-mail: [email protected] soft γ-ray detector so far, OSSE onboard CGRO, excluded ‡ present address: Observatoire Fran¸cais des Tornades et des Or- the presence of a strong non-thermal emission in Seyferts ages Violents c 2015 The Authors 2 P. Lubi´nski et al. (Zdziarski et al. 1995). There remains, however, a rather Information about kTe and τ of the plasma and R of the large uncertainty about the typical temperature kTe of the reflecting medium can be affected by some limitations. First electrons forming the plasma cloud and the mean Thomson of all, it is quite common that the hard X-ray spectra from optical depth τ of this region. The studies based on the data BeppoSAX, RXTE, INTEGRAL, Suzaku and Swift used for from OSSE and other contemporary satellites point towards the spectral fit lack a clear detection above 100 keV or a low mean temperature kTe ≃ 70–80 keV and a large op- even below. This makes the analysis of a high-energy cut-off tical depth τ ≃ 1.7 (Zdziarski, Poutanen & Johnson 2000). present in both primary Comptonized and reflected compo- On the other hand, Petrucci et al. (2001) found high values nents less reliable. Thus, for example, any result placing kTe of kTe ≃ 170–320 keV and small τ ≃ 0.05–0.20 for 6 Seyfert well above the upper limit of the spectrum appears dubious. type 1 galaxies observed by BeppoSAX. Another limitation comes from the spectral models applied. The INTEGRAL (Winkler et al. 2003) and Swift Phenomenological models typically assume a power-law with (Gehrels et al. 2004) satellites, despite the detectors not be- an exponential cut-off, which is not as sharp as the cut-off ing as sensitive above 50 keV as OSSE, improved the situa- predicted by Comptonization models (Zdziarski et al. 2003). tion thanks to their large field of view resulting in a long This, in turn, can lead to biased results for the reflected total exposure time and a better spatial resolution than component, when the primary component has an unphysi- OSSE and BeppoSAX/PDS, important for more crowded cal form. fields. Two other contemporary hard X-ray satellites, Suzaku The cosmic X-ray background (CXB) is expected to be (Mitsuda et al. 2007) and NuSTAR (Harrison et al. 2013), the sum of the emission from the supermassive BH accreting are suited more for the shorter observations of single ob- systems (e.g., Mushotzky et al. 2000). Therefore, the CXB jects, due to their smaller field of view. NuSTAR hosts the spectrum, peaked at around 30 keV, provides a strong con- most sensitive detectors in the 10–80 keV band and provides straint on the cut-off energy of a typical AGN. The AGN high-quality spectra but cannot help with constraining a cut- population synthesis models reproduce the CXB emission off energy well above 100 keV. Therefore, a comprehensive assuming that the main constituent of the CXB spectrum analysis of average hard X-ray spectra for a larger sample of below 200 keV is the accumulated emission from Seyfert AGN can be performed predominantly with the INTEGRAL galaxies. Template spectra used in these models are in the and Swift data. form of the absorbed power-law model with the photon index Reliable estimates of the plasma temperature and opti- Γ in the 1.8–1.9 range and the exponential cut-off energy EC cal depth are crucial elements for drawing a picture of the in the 150–400 keV range (e.g., Gilli et al. 2007; Ueda et al. central engine of AGN. Another such element is the knowl- 2014), accompanied by a Compton reflection with R typi- edge of the system geometry, in particular the extent of the cally around 1. The Comptonization spectra can be approx- plasma region and its location relative to the accretion disc. imated by the power-law cut-off model with EC = 2–3 kTe, As it was demonstrated for NGC 4151, even in the case of thus the plasma temperature in Seyferts should not usually the bright state of this brightest Seyfert galaxy the quality exceed 200 keV. Nevertheless, due to a lack of detection of of the spectra does not allow to distinguish which geometry the emission above 200 keV for a large Seyfert sample the ac- option of the Comptonization model provides the best spec- tual EC remains quite uncertain and affects the predictions tral fit (Lubi´nski et al. 2010). Thus, until much better spec- of the synthesis models (Gilli et al. 2007). tra are provided by future satellites for the plasma geometry The main goal of this work is a comprehensive spec- studies (Petrucci 2008), additional information is needed to tral analysis of the high-quality medium/hard X-ray and constrain the structure of the system. The most promising is soft γ-ray spectra of a large sample of Seyfert galaxies us- the Compton reflection component, originating in the accre- ing a realistic, physical model. For this we use the hard tion disc and torus illuminated by the central X-ray source. X-ray/soft γ-ray data collected by the IBIS/ISGRI detec- Compton reflection was found to be a common fea- tor (Lebrun et al. 2003) onboard the INTEGRAL satellite ture of Seyfert spectra already by the GINGA satellite over many years for 28 Seyferts. These summed ISGRI spec- (Nandra & Pounds 1994). Further analysis of the AGN and tra are accompanied by all contemporary medium X-ray (3– Galactic BH data revealed that the strength of the Compton 18 keV) spectra from the XMM-Newton, Suzaku and RXTE reflection R relative to a reflection from an infinite slab is satellites, in order to model the spectral slope and absorp- quite strongly correlated with the photon index Γ of the X- tion that cannot be studied with the ISGRI spectrum alone.