The complex environment of AGN

Abstract

A few percent of bright galaxies host an active nucleus with emission due to non-stellar processes: they are called Active Galactic Nuclei (AGN) and are powered by accretion of matter onto a supermassive black hole (ranging from 106 􀀀 109 solar masses). Accretion occurs via an accretion disc that e ciently converts gravitational energy into radiation. Seyfert galaxies are a sub-class of the wide variety of AGN. A further classi ca- tion is based on their optical/UV spectra: broad and narrow emission lines are both present in Seyfert 1s while only the latters are visible in Seyfert 2s. The standard Uni cation Model for AGN assumes the same internal structure for both Seyfert 1 and Seyfert 2 galaxies (Antonucci, 1993), with all observatio- nal di erences ascribed to an axisymmetric distribution of gas, located between the Broad Line Region (BLR) and the Narrow Line Region (NLR), in order to obscure the former, but not the latter. A natural geometrical and physical scenario is that of a homogeneous torus on a parsec scale (Krolik & Begelman 1988). If the so-called torus intercepts our line of sight the primary emission and the BLR are obscured with a resulting lack of broad lines in the optical/UV spectrum and a classi cation as Seyfert 2. On the contrary, if the nucleus is unobscured, the source is classi ed as a Seyfert 1 and every component of the spectrum is visible. This scenario came into existence after the discovery of polarized broad permitted emission lines in the optical/UV spectrum of one of the brightest Seyfert 2 galaxies, NGC 1068 (Antonucci & Miller 1985; Miller & Antonucci 1983), suggesting a geometry in which: (a) BLRs are con ned in a relatively small region ( light-days) surrounding the central source, (b) their direct view (in Seyfert 2s) is obscured by a at distribution of distant material coplanar with the disc-plane, and (c) their line emission is Compton-scattered into the line of sight by a population of warm electrons extending at large radii above and below the accretion disc. A further discrimination among Seyfert 2 galaxies is based on the column den- sity of the absorbing circumnuclear material in the torus (NH): Compton thick (NH > 1024cm􀀀2) and Compton thin (NH < 1024cm􀀀2) AGN. The latter class of objects have an observed ux much higher than the former one because the circumnuclear material becomes transparent to the radiation in the 2-10 keV band. In the X-ray spectra the reprocessing of the primary emission from the circumnuclear gas gives origin to a cold neutral re ection of the nuclear radia- tion and emission lines, in particular the neutral Fe K . However in the past few years, exceptions to orientation-based uni cation mo- dels have been found, suggesting the possibility that not all Seyfert 2 galaxies host a Seyfert 1 nucleus. Indeed, spectropolarimetric surveys nd that only about 50% of the brightest Seyfert 2 galaxies show hidden broad-line regions (HBLR) in their optical-polarimetric spectra (Tran 2001, 2003) Several authors have suggested that the presence of BLRs in Seyfert 2s can be linked to the luminosity of the active nucleus, and may disappear at low luminosities (Lum- sden & Alexander 2001; Laor 2003; Elitzur & Ho 2009) , or low accretion rates (Nicastro 2000) . In both cases, the presence of the BLR is not an inescapable feature of all Seyfert galaxies, as postulated by the uni cation model, but it is tightly linked to a physical parameter of the AGN, either the luminosity or the accretion rate. Furthermore, recent studies on X-ray absorbing column density changes perfor- med with Chandra, XMM-Newton and Suzaku satellites ruled out a universal geometrical structure of the circumnuclear absorber, as postulated by the Uni- ed Model. Absorption variability has been found to be common (almost ubi- quitous) when we compare observations months to years apart (Risaliti 2002), and, most notably, on time scales of hours to days in several sources, such as NGC1365 (Risaliti et al. 2005, 2007, 2009), NGC 4388 (Elvis et al. 2004), NGC 4151 (Puccetti et al. 2007) and NGC 7582 (Bianchi et al. 2009). In the framework of this complex scenario, my work will be focused on the con- nection between the Hidden Broad Line Region and the accretion rate in Seyfert 2 galaxies and on the study of the geometrical structure of two galaxies that showed very peculiar characteristics in their spectral properties. In particular, I will address two di erent, complementary, approaches to study the circumnu- clear medium of AGN: a statistical analysis of a small sample of both Compton Thin and Compton Thick Seyfert 2 galaxies and via a detailed X-ray spectral investigation of two among the brightest Seyfert 2 galaxies known. With concern to the former topic we will use the observed X-ray (2-10 keV) continuum emission as a direct probe of the AGN activity. We select a sam- ple of type 2 Seyfert galaxies with good quality spectropolarimetric and X-ray observations, for which we can give a good estimate of the mass of the central supermassive black hole. By doing so, we nd evidence suggesting that accre- tion rate is the main parameter that sets the existence of HBLRs in Seyfert 2 galaxies, con rming the model presented by Nicastro (2000) and suggesting a revision of the Uni ed Model. Then we will investigate the geometrical structure of the circumnuclear envi- ronment in two nearby Seyfert 2 galaxies: NGC 4945 and NGC 4507. We will perform a time, spectral and imaging analysis of the X-ray inner structure of NGC 4945, which reveals its geometrical and physical properties with unpre- cedented detail. NGC 4945 hosts one of the brightest AGN in the sky above 10 keV, but it is only visible through its re ected/scattered emission below 10 keV, due to absorption by a column density of NH ' 4 1024cm􀀀2. A new Suzaku campaign of 5 observations spanning 6 months, together with past XMM-Newton and Chandra observations, show a remarkable constancy (within 10%) of the re ected component. Instead, Swift-BAT reveals strong intrinsic variability on time scales longer than one year. We show that the re ector is at a distance > 30-50 pc, well within the imaging capabilities of Chandra at the distance of NGC 4945. Indeed, the Chandra imaging reveals a resolved, at- tened, 150 pc-long clumpy structure, whose spectrum is fully consistent with cold re ection of the primary AGN emission. The clumpiness may explain the small covering factor derived from the spectral and variability properties. This study provides evidence for absorption variability at a parsec-scale distance and proves the fundamental importance of a broadband X-ray study of AGN. We will then present a spectral analysis of an XMM-Newton and Chandra campaign of the obscured AGN in the Seyfert 2 galaxy NGC 4507, consisting of six observations spanning a period of six months. Using the Epic and the ACIS-S CCD cameras, we can detect strong absorption variability on time sca- les between 1.5 and 4 months in the circumnuclear material through a very detailed spectral analysis. The variation of the absorbing column density is the rst one ever observed on these time scales for this source and it suggests that the obscuring material consists of gas clouds at parsec-scale distance. The lack of variability on shorter time scales rules out the possibility of absorption by broad line region clouds, which was instead found in other studies of similar sources. This shows that a single, universal structure of the absorber (either BLR clouds, or the parsec-scale torus) is not enough to reproduce the observed complexity of the X-ray absorption features of (at least) these two AGN.