We know about relativistic astrophysical sources through the radiation they emit. In cases of jets, pulsars and their nebula, the radiation is emitted by particles in a non-thermal distribution making these objects shine throughout the entire electromagnetic spectrum.

My interest in modeling radiation from such astrophysical sources is two-fold: on the one hand, it lets us extract important source parameters like the speed of the flow or its energy; on the other hand, the mechanisms transferring energy to the radiating particles are rich in physics themselves -- and often poorly understood!

Magnetic Reconnection

Currently, the most promising mechanism to explain rapid flares and plasma heating in astrophysical sources is magnetic reconnection. In this process, the magnetic energy is dissipated by topologically re-arranging the magnetic field via current sheets. The non-linear dynamics and effective dissipation of magnetic reconnection is subject to much speculation as its study requires an understanding of the formation of the current sheet (e.g. through turbulent processes) as well as a description of the microphysical parameters in the current sheet itself.

With collaborators, we have recently shown how plasma turbulence in a black hole accretion flow leads to copious magnetic reconnection along with magnetic islands.

Current density in a black hole accretion simulation showing formation of current sheets and reconnection. From Nathanail et al. (2020).

Sometimes, reconnection occurs in the highly magnetised regions above the black hole. Then plasmoids trapping emitting particles can be flung out as an episodic relativistic outflow. See the paper led by Dr. Antonios Nathanail for details.

Plasma density of a relativistic reconnection simulation with anti-parallel magnetic fields showing a plasmoid chain.

In his first thesis project, Sebastiaan investigates the reconnection process both from the fluid scale and from the microphysical kinetic scales with the aim to understand how reconnection proceeds and energizes particles in astrophysical sources.