Bart Ripperda, Canadian Institute for Theoretical Astrophysics

Bart Ripperda received his master's degree in plasma physics from Eindhoven University of Technology in 2013. He then moved to the University of Cambridge for part III of the mathematical tripos, before starting his PhD in mathematics at Katholieke Universiteit (KU) Leuven in Belgium in 2014. After his PhD, he joined the Event Horizon Telescope and he became a joint postdoctoral fellow at the Flatiron Institute in New York City and at Princeton University. In 2022 he joined the Institute for Advanced Study in Princeton as a NASA Hubble Fellow and he started as an assistant professor at the Canadian Institute for Theoretical Astrophysics at the University of Toronto.

Ripperda is a theoretical astrophysicist trying to connect fundamental plasma physics with observations of high-energy emission from black holes and neutron stars. His research centers around large-scale numerical simulations of plasma dynamics in the vicinity of compact objects.

Black hole accretion disks, magnetospheres, coronae and jets typically consist of magnetized, collisionless, relativistic plasma. They can produce multiwavelength radiation due emitting electrons and protons, and multimessenger signals in the form of neutrinos, cosmic rays, and gravitational waves. Modeling these systems necessitates a deep understanding of the kinetic plasma dynamics that is responsible for energy dissipation and particle acceleration. Ripperda aims to study multimessenger emission from regions close to black hole event horizons. By using a novel combination of first-principles general-relativistic kinetic simulations and large-scale magnetohydrodynamics models he aims at capturing microscopic plasma physics and global dynamics.

How Black Holes Shine: From Fluid Dynamics to Kinetic Physics

High-energy astrophysical systems like black hole accretion disks, jets, and coronae consist of magnetized, often (near)-collisionless relativistic gas of charged particles. They produce observable high-energy radiation in the form of short, intense flares and bursts. It is currently unclear where and how this emission is produced. The radiation is typically non-thermal, implying a power-law distribution of emitting relativistic electrons. Turbulence and magnetic reconnection are viable mechanisms to tap the large reservoir of magnetic energy in these systems and accelerate electrons to extreme energies. The accelerated electrons can then emit high-energy photons that themselves may strongly interact with the plasma, rendering a highly nonlinear system. Modeling these systems necessitates a combination of magnetohydrodynamic fluid models to capture the global dynamics of the formation of dissipation regions, and a kinetic treatment of plasma processes that are responsible for energy dissipation, particle acceleration, and radiation. I will present novel studies of flaring and high-energy emission signatures from regions close to black hole event horizons, using both first-principles general relativistic kinetic particle-in-cell simulations and global large-scale three-dimensional magnetohydrodynamics models. With this combination I determine whether turbulence and magnetic reconnection are viable dissipation mechanisms, where they occur, and what kind of emission signatures are typically produced. In the end, I will outline how the approach of global magnetohydrodynamics and kinetic models will enable quantitative comparisons with observations of multiwavelength observations of radio, X-ray, and gamma-ray emission from accreting black holes, and how well a fluid model captures the physics of black hole accretion disks.