The Penrose process: its variants and their role in astrophysics

• Zdeněk Stuchlík

The basic adea of the Penrose process is given and its variants are introduced along with describing the role in astrophysical phenomena. These phenomena illustrate interplay of the regular and chaotic motion allowed for ionized matter. We focus on demonstration of possible occurence of ionized Keplerian disks orbiting a magnetized black hole and the situations giving rise to interesting astrophysical phenomena, occuring due to relevant variants of the Penrose process. In dependence on the intensity of the electromagnetic interaction of slightly charged matter of the orbiting disk with the external magnetic field, there are three possible outcomes of the ionized disks, namely, creation of quasiperiodic epicyclic oscillations of the orbiting matter for weak intensity, creation of winds for mediate intensity, and, due to the strong Magnetic Penrose process, creation of jets and highly ultra-relativistic particles for large intensity of the interaction. In active galactic nuclei the accelerated particles can obtain, due to the Magnetic Penrose process, energy corresponding to the most energetic particles observed in cosmic rays. These processes can occur in the effective ergosphere of the magnetized black hole. Highly accelerated particles could be, however, created also due to the Electric Penrose process where no rotation of the black hole is involved. Of potentially high astrophysical interest is the Radiative Penrose process of acceleration of charged particles due to their synchrotron radiation inside the black hole ergosphere that can occur for counter-rotating radiating particles due to concentration of photons with negative energy relative to observers at infinity at the direction of their motion. Such an effect can be important at the ergosphere boundary, where the energy increase of the accelerated ionized particle can reach one order.

Public talk: M87* and Sgr A*: Imaging supermassive black holes

• Luciano Rezzolla (Goethe University of Frankfurt)

I will briefly discuss how the first images of the supermassive black holes M87* and Sgr A* were obtained by the EHT collaboration. In particular, I will describe the theoretical aspects that have allowed us to model the dynamics of the plasma accreting onto the black hole and how such dynamics was used to generate synthetic black-hole images. I will also illustrate how the comparison between the theoretical images and the observations on a broad range of frequencies has allowed us to deduce the presence of supermassive black holes and to extract information about the accretion process. Finally, I will describe the lessons we have learned about strong-field gravity and alternatives to black holes.