Comptonization mechanisms in hot coronae in AGN. The NuSTAR view

Abstract

The local universe is made up mainly of galaxies. The radiation emitted by a galaxy can be considered, in first approximation, as the sum of the radiation of their stars. However, for a small percentage of galaxies, called active galaxies, or Active Galactic Nuclei (AGN for short), this is not true. AGN emit through all the electromagnetic spectrum. Much of the energy output of AGN has non-stellar origin, with many AGN being strong emitters of X-rays, radio and ultraviolet radiation, as well as optical radiation. The radiation from an AGN is believed to be a result of accretion of matter onto a supermassive black hole (106 −109 solar masses) at the center of its host galaxy. The accretion of matter occurs through an accretion disc in which gravitational energy is partially transformed into radiation. The spectral energy distribution (SED) of active galactic nuclei is very broad and ranges from the radio band up to X-rays and gamma-rays. The SED of an AGN can be decomposed into four main components: the primary emission in the optical/UV band; the infrared continuum; the high-energy continuum; the radio emission, which can be relatively strong (radio-loud AGN) or weak (radio quiet AGN). In addition to the broad, non- stellar energy distribution, AGN also exhibit strong emission lines in their spectra. The primary X-ray emission in AGN is believed to be produced in the so-called corona, a compact region located close to the supermassive black hole and composed by a plasma possibly in thermal equilibrium. The accretion disc produces optical/UV seed photons in a quasi-black body spectral shape; in the corona these photons are up scattered to the X-ray band due to the inverse Compton effect. Since this effect cools the corona, there must be some other mechanism that heats the corona, in order to maintain a high enough temperature. This energy source could be the dissipation of magnetic flux through reconnection. The inverse Compton scattering by the hot electrons of the UV seed photons emitted by the accretion disc produces a X-ray power law spectrum, extending to energies deter- mined by the coronal electrons temperature, with spectral index that typically t ranges from 1.5 to 2.0. The power law often shows a cutoff at high energies, around 100-200 keV. Both the energy of the cutoff and the photon index are related to the temperature and the optical depth of the corona. Comptonization models imply that the cutoffs energies are typically 2-3 times the temperature of the corona. The present work fits in this complex scenario. It will be based mainly on the study of the X-ray broad-band spectrum of AGN, to constrain the coronal parameters and start to look for correlations between these parameters and other physical parameters, such as the geometry and the position, with the aim of better understanding the complex environment present in AGNs. In fact the geometry of the disc/corona system is still un- known, and we also still lack good constraints on the coronal temperature and optical depth for most sources. The size and the location of the corona is still matter of debate. There are open questions like: is the corona spherical, or a slab, or it has a more complex shape? Is it compact, as assumed in the lamp-post geometry, or is it extended? Is the corona a continuous or a patchy medium made up of several blobs? To answer the questions raised above we need to study the broad-band X-ray spectrum and variability of AGN in details, on adequate time-scales in order to model all the spectral components and to investigate the shape of the nuclear continuum. It is very important to disentangle all the complex spectral features in this energy range, to remove all the degeneracies between the primary continuum features and other physical observables in order to constrain the coronal parameters and to have an overview of the physics and the structure of the hot corona. In the past, several cutoff energies in nearby Seyfert galaxies have been measured with hard X-ray satellites, such as BeppoSAX and INTEGRAL. These measurements ranged between 50 and 300 keV but the lack of focusing instruments at high energies resulted in large uncertainties and degeneracies between the cutoff energies and other physical observables (in particular the slope of the primary power law and the amount of radiation Compton scattered by circumnuclear matter). NuSTAR has been an observational breakthrough in X-ray astronomy with its unprecedented sensitivity at high energies, operating in the 3-79 keV energy range. Simultaneous observations with other X-ray observatories operating below 10 keV, such as XMM-Newton, Suzaku and Swift allowed to measure cutoff energies with great accuracy in a number of sources. This work is based both on new observations of NuSTAR, XMM-Newton and Swift X-ray satellites and on archival data. The detailed analysis of single sources allows to build and constrain physical models while the analysis of a large sample gives us insights into the average properties of AGN.