The Super-Massive Black Hole close environment in Active Galactic Nuclei

TL;DR

This paper reviews the physical processes and mechanisms of energy release near supermassive black holes in active galactic nuclei, highlighting current understanding, limitations, and future prospects.

Contribution

It provides a comprehensive overview of the high-energy emission mechanisms and the physical environment close to SMBHs in AGN, emphasizing recent insights and future directions.

Findings

High-energy emission originates within 100 gravitational radii of SMBH.

Accretion efficiency can reach up to 30-40%.

Understanding of emission mechanisms is advancing but still faces limitations.

Abstract

Active Galactic Nuclei are powered by accretion of matter onto a supermassive black hole (SMBH) of mass Mbh ~ 10^{5}-10^{9} Msun. The accretion process is indeed the most efficient mechanism for energy release we currently know of, with up to ~30-40% of the gravitational rest mass energy that can be converted into radiation. The vast majority of this energy is released at high energy (UV-X-rays) within the central $100$ gravitational radii from the central SMBH. This energy release occurs through a variety of emission and absorption mechanisms, spanning the entire electromagnetic spectrum. The UV emission being commonly explained by the presence of an optically thick accretion flow, while the X-rays generally require a hotter, optically thinner, plasma, the so-called X-ray corona. If outflows are present, they can also extract a significant part of the gravitational power. With an origin in the deep potential well of the SMBH, the study of the high-energy emission of AGN give a direct insight into the physical properties of the accretion, ejection and radiative mechanisms occurring in the SMBH close environment. While not exhaustive, we discuss in this chapter our present understanding of these mechanisms, the limitations we are currently facing and the expected advances in the future.