Reflection in Seyfert galaxies and the unified model of AGN

Abstract

We present a deep study of the average hard X-ray spectra of Seyfert galaxies. We aim to test the unified model of active galactic nuclei, and constrain differences and similarities between different classes of objects. We analyzed all public INTEGRAL IBIS/ISGRI data available on all the 165 Seyfert galaxies detected at z < 0.2. Our final sample consists of 44 Seyfert 1s, 29 Seyfert 1.5s, 78 Seyfert 2s, and 14 narrow-line Seyfert 1s. For each subsample, we stacked all the images, and derived their average hard X-ray spectra in the 17-250 keV energy range. We performed a detailed spectral analysis using both a model-independent and a model-dependent approach. All classes of Seyfert galaxies show on average the same nuclear continuum, as foreseen by the zeroth order unified model, with a cutoff energy of E C 200 keV, and a photon index of Γ ≃ 1.8. The average optical depth of the Comptonizing medium is consistent for the different classes (τ ≃ 0.8). Compton-thin Seyfert 2s show a reflection component stronger than Seyfert 1s and Seyfert 1.5s. Most of this reflection is due to mildly obscured (10 23 cm −2 ≤ N H < 10 24 cm −2 ) Seyfert 2s, which have a significantly stronger reflection component (R = 2.2 +4.5 −1.1 ) than Seyfert 1s (R ≤ 0.4), Seyfert 1.5s (R ≤ 0.4) and lightly obscured (N H < 10 23 cm −2 ) Seyfert 2s (R ≤ 0.5). This cannot be explained easily by the unified model. The absorber/reflector in mildly obscured Seyfert 2s might cover a large fraction of the X-ray source, and contain clumps of Compton-thick material. The large reflection found in the spectrum of mildly obscured Seyfert 2s reduces the amount of Compton-thick objects needed to explain the peak of the cosmic X-ray background. Our results are consistent with the fraction of Compton-thick sources being ∼ 10%. The spectra of Seyfert 2s with and without polarized broad lines do not show significant differences, the only difference between the two samples being the higher hard X-ray and bolometric luminosity of Seyfert 2s with polarized broad lines. The average hard X-ray spectrum of narrow-line Seyfert 1s is steeper than those of Seyfert 1s and Seyfert 1.5s, probably due to a lower energy of the cutoff.

The NuSTAR Extragalactic Surveys: X-Ray Spectroscopic Analysis of the Bright Hard-band Selected Sample

“…Our findings are also consistent with previous results showing low levels of reflection for high-redshift quasars (Vignali et al 1999;Reeves & Turner 2000;Page et al 2005), although we note that Ricci et al (2011) found higher levels of reflection from Seyfert2s than from Seyfert1s. Figure 8 (bottom panels) shows unobscured and obscured sources as empty and filled data points, respectively, and does not allow a firm conclusion on this point for Seyfert-like luminosity sources.…”

“…Unfortunately, with our small sample, we cannot test this hypothesis at high significance. Furthermore, we must mention that local studies find mildly obscured Seyferts to have stronger or equally strong reflections than unobscured counterparts (Ricci et al 2011;Esposito & Walter 2016). We have insufficient statistics to infer a similar trend at comparable low luminosities (i.e., L log erg s 44…”

X-ray spectral parameters for a sample of 95 active galactic nuclei

“…The last three observational parameters depend on the state of the accretion disk and of the presence of a gas-dusty torus on the line of sight (Awaki 1991;Cappi et al 2006;Petrucci et al 2001;Ueda et al 2007). The torus optical thickness and its geometry shape reveal themselves in the anisotropy of the absorption and scattering (Ricci et al 2011;Ueda et al 2007). Note that the photoelectric cross-section sharply declines above 20-30 keV, and have a significant influence only in soft and middle energy <2-3 keV.…”

“…This radiation can have different spectral properties depending on the geometry of the corona, its condition and state of the accretion (Beloborodov 1999;Perola et al 2002;Petrucci et al 2001;Ricci et al 2011;Vol'vach et al 2011). The presence of the gas-dust torus around the nucleus also strongly affects the absorption and emission lines.…”

NuSTAR UNVEILS A HEAVILY OBSCURED LOW-LUMINOSITY ACTIVE GALACTIC NUCLEUS IN THE LUMINOUS INFRARED GALAXY NGC 6286

“…To infer the value of the line of sight column density (N H ) we fitted the joint Swift/XRT, Chandra ACIS-S, XMM-Newton EPIC/PN and MOS, and NuSTAR FPMA and FPMB data with a model that consists of (a) an absorbed power-law with a photon index fixed to Γ=1.9, consistent with the average value of AGNs (e.g., Nandra & Pounds 1994a;Piconcelli et al 2005;Ricci et al 2011); (b) unabsorbed reprocessed X-ray emission from a slab; (c) a Gaussian to reproduce the fluorescent Fe Kα emission line (with the rest-frame energy fixed to E Kα = 6.4 keV); (d) a second power-law to reproduce the scattered component; and (e) an emission from a collisionally ionized plasma. To reproduce the effect of obscuration we included both Compton scattering and photoelectric absorption.…”