A majority of Active Galactic Nuclei (AGN) shows the presence of a soft X-ray emission, below 2 keV, in excess with respect to extrapolation of the high- energy power law. The origin of this soft X-ray excess is a long-standing issue in our understanding of the AGN X-ray emission. Different models have been proposed to explain the soft X-ray excess but, presently, only two appear viable: either blurred ionized reflection in the accretion disk, or thermal Comptonization in an optically thick (τ >1) and warm (kT∼1 keV) plasma (the so-called warm corona). The reality can also be a mix of the two whose relative importance can vary from object to object. The thermal Comptonization modeling of the soft X-ray excess has been given new life by Petrucci et al. (2013, 2018) who suggested that the warm corona could be a powerful, extended, and optically thick plasma covering an almost non-dissipative accretion disk, i.e., all the accretion power would be released mainly in the warm corona. This result is at odds with the commonly accepted behavior of standard optical- ly thick, geometrically thin accretion flows (Shakura & Sunyaev, 1973) where the gravitational power is believed to be released in the deeper layers. Recent theoretical and numerical works (e.g. Różańska et al. 2015, Ballantyne 2020, Petrucci et al. 2020) confirm that warm Comptonization is a physically sound model that can explain the origin of the soft X-ray excess.
We propose a study project, carried out by an international team of experts on X-ray observations of AGNs and high energy radiative transfer modeling, aimed at testing and better understanding the warm corona model as an explanation for the soft X-ray excess. This ISSI project is three-fold: 1) we will discuss and compare the warm co- rona models recently developed in our team (e.g. Ballantyne 2020, Petrucci et al. 2020), 2) we will produce XSPEC tables to apply our models to UV-X-ray archival data (mainly from XMM-Newton, Swift and NuSTAR when possible) to constrain the soft X-ray excess in different types of AGNs and 3) we will discuss the observational results and their possible interpretations and consequences for the model, as well as define, coordinate and plan the writing of several scientific papers expected from the above analysis.