From the death of a star to the death star On magnetic reconnection in black hole magnetospheres

High-energy phenomena (e.g., fast radio bursts, gamma-ray flares) are driven by magnetic reconnection, which converts magnetic energy into kinetic energy. Here, magnetic reconnection occurs in collisionless plasmas, requiring a kinetic description. However, large-scale astrophysical modeling typically uses a magnetohydrodynamic (MHD) description, which cannot accurately capture collisionless reconnection. To bridge this gap, the thesis proposes a non-uniform effective resistivity model based on findings from fully kinetic particle-in-cell simulations. This model aims to mimic essential relativistic collisionless reconnection properties within MHD frameworks to design physically grounded global models for reconnection-powered high-energy emission.
Nonaccreting BHs may have acquired magnetospheres from their magnetized progenitors or by merging with a magnetized NS. This thesis shows, using three-dimensional general relativistic MHD simulations, that an inclined split monopole magnetic field on a spinning BH aligns with the equatorial plane. The alignment rate is inversely proportional to the BH spin. This aligning and reconnecting current sheet may produce unique electromagnetic signatures coinciding with gravitational wave events.
This thesis reveals that complex initial magnetic field structures on nonaccreting BHs ultimately evolve into split monopole configurations. This evolution occurs in two phases: an initial phase where the magnetosphere settles into pressure equilibrium and a second phase characterized by reconnection-driven changes. This thesis presents an analytic model for the magnetospheric evolution of the second phase. Furthermore, during this phase the magnetic flux on the BH decays exponentially with a timescale depending on the BH spin.