THE X-RAY EMISSION OF AGN: INVESTIGATING THE SURROUNDINGS OF BLACK HOLES

Abstract

Active galactic nuclei (AGN) are variable sources overshining their host galaxies and enlightening the sky at all redshifts (up to z∼7) and frequen cies. From an observational point of view, AGN are characterised by a complex and widespread spectral energy distribution and flux variations occurring at all wavelengths. Such a broadband emission is a by product of the AGN composite nature resulting off the interplay between the various components. The main physical process acting in AGN is accretion of matter onto supermassive black holes (SMBH) which explains their optical/UV emission. A consistent X-ray emission is a hallmark of these sources and accretion can not direclty account for the large energy output in this band. It is widely believed that X-rays emerge from the innermost region of AGN, in the proximity of black holes resulting off an inverse-Compton process involving seed optical/UV photons and a thermal distribution of electrons, the so-called hot corona. Therefore, the analysis of X-ray spectra provides a powerful tool to investi gate the physical properties of the complex environment around supermassive black holes (SMBH). This dissertation is addressed to some open questions in the field: i) What are the geometry and physical properties of the hot corona? In most sources, we still lack good constraints on the coronal temperature and optical depth. The geometry of the disc/corona system is also unknown, the size and location of the corona being a matter of debate; ii) What is the origin of the soft X-ray excess? This component is commonly interpreted as either relativistically blurred reflection or Comptonised emission by a warm, optically thick plasma in the upper layer of the accretion disc; iii) What is the origin of the reflected component? AGN spectra may display features of relativistically blurred reflection from the very inner part of the accretion disc. However, these features, likely produced within a few gravitational radii from the black hole, are not ubiquitous and in various cases the reflection component seems to orig inate only from distant material; iv) which is the driving process behind long term X-ray variability? Does warm Comptonisation leave a track of a typical timescale in AGN flux variations? To answer the addressed questions a series of different techiniques and ap proaches will be adopted. In particular we will report on: i) X-ray spectral analyses of individual sources based on long and repeated X-ray observations performed with XMM-Newton and/or NuSTAR. Data reduction and analyses are discussed in depth and, for all the spectral components (i.e. primary continuum, soft excess and reflection component) a physical interpreta tion is given. Special emphasis is deserved to the interpretation of the soft-excess and the two-corona model that explains the broadband X-ray emission of AGN as being the result of the mutual presence of a warm and a hot Comptoni sation process. ii) Timing analyses of a large sample of quasars with multiple serendipitous X-ray observations obtained from the XMM-Newton Point Source Catalog. Flux variations are therefore described from a statistical point of view using advanced techniques applied to the timing analysis and revealing an unex pected component of quasar variability on time scales of 20 years. iii) Equations to quickly obtain physical parameters from phenomenological fits will be pre sented and discussed. These are based on the advanced Comptonisation code MoCA that is well suited to reproduce the Comptonisation spectrum of accret ing objects accounting for a variety of physical conditions and geometries of the AGN corona. Moreover, a direct application of the results of the simulations on a sample of AGN observed by Nustar will be presented and discussed.