L’origine de l’émission électromagnétique de haute énergie dans l’environnement des trous noirs supermassifs. Phénomènes cosmiques de haute energie

Abstract

Supermassive black holes of several hundred million solar masses lie at the centre of most massive galaxies. In 90% of cases, these black holes are in quiescent, very low luminous states. Nevertheless, in the remaining 10%, extremely violent processes are seen, with the liberation of huge amounts of energy especially in the UV, X-ray and gamma-ray bands. We also sometimes observe powerful jets, extending up to several hundred kpc scales. The cores of these galaxies are called Active Galactic Nuclei (AGNs). These are among the most luminous objects in the Universe. The accretion of surrounding matter onto the central supermassive black hole is generally considered as the most likely energy source to explain the extraordinary observed luminosity. The gravitational energy would be partly liberated into an accretion disc as thermal radiation peaking in the optical/UV band, and partly radiated in the X-ray/gamma-ray band by a corona of hot plasma lying in the environment close to the black hole. However, several phenomena are still poorly understood and a number of questions lack satisfactory answers: what are the dynamics and the structure of the accretion and ejection flows in AGNs? What are the radiative processes producing the UV/X-ray radiation? What is the origin of the different spectral components present in those energy bands? The goal of this thesis is to derive new observational constraints to better answer to these questions. Its original ity resides in the development and application of realistic models of thermal Comptonization, allowing us to better constrain the physical and geometrical properties of the UV and X-ray-emitting regions, and to better understand the origin of the different observed spectral components. In particular, we studied the excess of the soft (< 2 keV) X-ray emission, seen in a great number of AGNs, and whose origin is still unknown. This work is structured along two main branches. One is the detailed spec tral analysis of long, multiwavelength observational campaigns on three Seyfert galaxies (NGC 5548, NGC 7213 and NGC 4593). The quality of the data permitted to reveal the geometrical and physical parameters (in particular the temperature and optical depth) of the thermal corona producing the X-ray con tinuum. The second branch is based on the analysis of archival data (from the XMM-Newton satellite) of a large sample of Seyfert galaxies. This allowed us to derive more general constraints on the high-energy emission processes observed in these objects. These two approaches have shown, in particular, that the soft X-ray emission excess may arise in the warm upper layers of the accretion disc, suggesting a more effective heating of the surface rather than the inner regions.