Magnetospheric dynamics around Kerr black holes: ab-initio simulations of eruptive episodes

The supermassive black hole Sgr A, nestled at the centre of our Galaxy, is known for its apparently low activity compared to its more distant and luminous counterparts, which are often associated with powerful extragalactic relativistic jets. Behind this apparent quiescence, however, lie recurrent outbursts of infrared and X-ray radiation, whose physical origin remains enigmatic. Recent observations by the Event Horizon Telescope and the GRAVITY instrument at the VLT suggest that Sgr A is embedded in a hot, strongly magnetised plasma. Furthermore, the detection of hot spots orbiting just a few gravitational radii from the black hole, temporally coinciding with X-ray flares, indicates that the source of this activity is located in the immediate vicinity of the event horizon.

A promising scenario to explain these phenomena involves the dynamics of a relativistic magnetosphere formed around the black hole. Similar to planetary or stellar magnetospheres, this magnetosphere can undergo sudden reconfigurations due to episodes of magnetic reconnection, during which a portion of the magnetic energy is released and efficiently accelerates the surrounding plasma. Recent simulations conducted by the team have shown that the spherical accretion of ambient plasma can transport and stretch magnetic field lines close to the horizon. When the magnetic tension exceeds gravitational attraction, the field is violently expelled, leading to particle acceleration before a new cycle of accretion and reconnection begins.

The goal of the PhD thesis is to extend this numerical model of magnetised spherical accretion to the case of a rapidly rotating black hole in three dimensions and to study the role of the plasma’s angular momentum on the frequency and intensity of magnetospheric flares. The expected radiative signatures will also be modelled and compared with available observational data, particularly from GRAVITY+. The thesis work will rely on exascale numerical simulations produced using the new particle-in-cell code Zeltron++ developed by the team.