Hot Coronae in Local AGN: Present Status and Future Perspectives

Andrea Marinucci,Giorgio Matt,Riccardo Middei,Alessia Tortosa,Francesco Tamborra,Stefano Bianchi,Michal Dovčiak

doi:10.3390/galaxies6020044

Abstract

The nuclear X-ray emission in radio-quiet Active Galactic Nuclei (AGN) is commonly believed to be due to inverse Compton scattering of soft UV photons in a hot corona. The radiation is expected to be polarized, the polarization degree depending mainly on the geometry and optical depth of the corona. Nuclear Spectroscopic Telescope Array (NuSTAR) observations are providing for the first time high quality measurements of the coronal physical parameters—temperature and optical depth. We hereby review the NuSTAR results on the coronal physical parameters (temperature and optical depth) and discuss their implications for future X-ray polarimetric studies.

Highlights

Active Galactic Nuclei (AGN) represent the perfect astrophysical laboratory to study accretion processes
Thanks to the recently approved Imaging X-ray Polarimetry Explorer (IXPE) satellite, X-ray polarimetry will be the new pair of eyes that will allow us to prefer one coronal geometry to another, at least in the brightest, unobscured AGN
In none of the objects observed with Nuclear Spectroscopic Telescope Array (NuSTAR), simultaneously to XMM, Chandra, Suzaku, Swift, and included in the catalogs mentioned above [3,28,41,42], X-ray spectroscopy has led to a clear discrimination between different coronal geometries

Summary

Introduction

Active Galactic Nuclei (AGN) represent the perfect astrophysical laboratory to study accretion processes. The common paradigm for the geometry of the innermost regions around the central black hole postulates the presence of a hot corona (the so called two phase model [1,2]). In this scenario, the UV radiation from the accretion disk (emitted as a multi-temperature black body spectrum) is Compton scattered by the hot electrons in the corona, up to X-ray wavelengths. A broad spectral coverage, from 0.5 up to ∼100 keV is fundamental to discriminate between the different components of an X-ray spectrum, and to measure the parameters of the intrinsic nuclear emission. AGN, discuss some applications of a new model for Comptonization in astrophysics (MoCA [19]), and present future perspectives in the framework of X-ray polarimetry