Some of the most energetic astrophysical phenomena are connected to the activity of the most compact stars in the Universe: neutron stars and black holes. These objects concentrate in their vicinity extremely large energy densities through multiple channels (gravitational, rotational, electromagnetic), which are then converted into structured magnetized plasma outflows (relativistic winds or jets) and energetic particles (electrons, positrons, ions, photons, neutrinos) with disconcerting efficiency. The non-trivial interplay between (general) relativistic effects, plasma physics and (quantum) electrodynamics leads to formidable challenges at the forefront of modern theoretical physics and high-performance computing.

By analogy to the Earth and other bodies in the Solar System endowed with an organized large-scale magnetic field, black holes and neutron stars can be surrounded by a “relativistic magnetosphere”. This region plays a pivotal role in the formation of plasma outflows and particle acceleration. Like their non-relativistic analog, relativistic magnetospheres are intrinsically dynamic and sometimes reorganize abruptly via various plasma processes such as magnetic reconnection which give rise to pronounced particle acceleration and bursts of energetic radiation directly observable from Earth.

The emission of short pulses of light shaped by spinning neutron stars (pulsars) is a famous example of an astrophysical phenomenon most certainly involving an active relativistic magnetosphere. Although pulsars were discovered more than fifty years ago by radio astronomers, the origin of this emission remains today an outstanding puzzle. Giant X-ray magnetar flares – and their possible connection to the mysterious Fast Radio Bursts – are far more extreme and less well understood phenomena that involve another flavor of neutron star magnetospheres. Supermassive black hole magnetospheres may also play a crucial role in launching extragalactic relativistic jets observed in active galaxies. The first images of the M87* and SgrA* black holes obtained by the Event Horizon Telescope bring for the first time direct constraints on the innermost regions close to the black hole horizon possibly hosting a jet-launching magnetosphere. The discovery of hotspots orbiting at a few gravitational radii away from the SgrA* hole thanks to the exquisite astrometric measurements of the Gravity collaboration at the VLTI and ALMA provides other possible hints of an active magnetosphere.

The aim of this workshop is to bring together experts in the field of relativistic plasma astrophysics to highlight recent progress but also major shortcomings in our understanding of plasma flows and dissipative processes within the magnetospheres of compact objects from an observational, theoretical and computational perspective.