Rony Keppens
Oliver Porth
Lorenzo Sironi
Sasha Philippov
Thomas Hertog
Bert Vercnocke
Daniel Mayerson
Fabio Bacchini
Jordy Davelaar
Elias Most
Hector Olivares
Matthew Liska
Jim Stone

My research is centered around the study of black holes and neutron stars. These compact objects provide us with an astrophysical laboratory to study exceptional physical conditions. In a single process, the merger of binary neutron stars combines extreme gravity, gravitational wave emission, complex particle physics and electromagnetic processes. Such mergers can result in the formation of a massive, highly magnetised black hole as a remnant. These black holes are often surrounded by an accretion disk, consisting of orbiting material ejecting relativistic outflows. The accretion disk and its outflows consist of a tenuous gas of charged particles, called a plasma. The macroscopic dynamics of this plasma is typically governed by an interplay between extremely strong electromagnetic and gravitational fields, as described by the general relativistic magnetohydrodynamics (GRMHD) equations. However, non-thermal emission from highly energetic particles in the plasma is an observed feature of such systems that cannot be explained by macroscopic models and kinetic physics is needed to understand these phenomena. 

particles orbiting in the accretion disk and the jet of a black hole

Observable electromagnetic signals from accretion disks and their outflows, provide our main information source on compact objects. The recently developed Event Horizon Telescope (EHT) represents a big leap in astronomy, providing us with extremely detailed, horizon-scaled images of Sgr A*, the supermassive black hole in the centre of our own Galaxy, and M87 in the Virgo A galaxy. Additionally, the GRAVITY collaboration employs a very large telescope interferometer (VLTI) specifically designed to observe highly variable emission from close to the event horizon of Sgr A*. The discovery of gravitational waves from the binary neutron star merger GW170817 by LIGO/Virgo yields a completely new window into the Universe. The Fermi space telescope has detected the subsequent electromagnetic counterpart from the merger, as well as highly variable non-thermal emission from supermassive black holes, on timescales as short as hours and even minutes. The era of “multi-messenger astronomy” has been initiated, and neutron star inspirals and their remnant compact objects demonstrate to play a prominent role in astrophysics, gravity and particle physics.

A comparison between the 2017 EHT observations and a GRMHD simulation of an accreting black hole.

It is currently still a mystery how particles accelerate up to the non-thermal energies required to explain the observed highly variable radiation. Therefore, this is the perfect time to investigate signatures of non-thermal particles, that can be observed in both the post-merger accretion disk and the electromagnetic signals from the merger. The conditions in these astrophysical plasma environments probe energy, length and time scales that are inaccessible for terrestrial laboratories. The interaction between the particles and the strong gravitational fields close to the black hole also provides a testbed for fundamental theories of quantum gravity. Advanced numerical methods are essential to study plasma dynamics in the extreme gravitational and electromagnetic fields at the edge of a black hole or a neutron star and compare directly to observations. To capture plasma physics on both macroscopic and microscopic particle scales and understand the fundamental nature of compact objects, it is essential to develop novel computational methods that are beyond the current state-of-the-art. I am developing innovative combinations of numerical particle techniques interacting with macroscopic plasma methods to fundamentally understand the origin of variable high-energy emission coming from compact objects.

The adaptive mesh refinement captures microscopic features like plasmoids in a reconnecting current sheet in resistive GRMHD simulations.