Reconnection-driven flares in 3D black hole magnetospheres -- A scenario for hot spots around Sagittarius A*

Abstract: Low-luminosity supermassive and stellar-mass black holes (BHs) may be embedded in a collisionless and highly magnetized plasma. They show non-thermal flares indicative of efficient dissipative processes in the vicinity of the BH. During NIR flares from the supermassive BH Sagittarius A* (Sgr A*), GRAVITY detected circular motion and polarization evolution which suggest the presence of transient synchrotron-emitting hot spots moving around the BH. We study 3D reconnecting current layers in the magnetosphere of spinning BHs to determine whether plasma-loaded flux ropes formed near the event horizon could reproduce the hot spot observations and help constraining the BH spin. We perform global 3D particle-in-cell simulations in Kerr spacetime of a pair plasma embedded in a strong large-scale magnetic field originating in a disk in prograde Keplerian rotation. A cone-shaped current layer develops which surrounds the twisted open magnetic field lines threading the event horizon. Magnetic field lines coupling the disk to the BH inflate and reconnect a few gravitational radii above the disk. Particles accelerate and accumulate in a few rotating macroscopic flux ropes. Once flux ropes detach, they propagate in the current layer following what appears as a rapidly opening spiral when seen face-on. A single flux rope carries enough relativistic particles to emit synchrotron radiation at levels suitable to reproduce the flares' peak-luminosity of Sgr A* but it quickly fades away as it flows away. Our kinematic analysis of flux ropes' motion favors a BH spin of 0.65 to 0.8 for Sgr A*. The flares' duration require that the underlying magnetic loop seeded in the disk mid-plane has a finite lifetime and azimuthal extent. In this scenario, the hot spot corresponds to a spinning arc along which multiple reconnection sites power the net emission as flux ropes episodically detach.