The baryon content of magnetically arrested black hole disks and jets

Abstract We study the transport of baryons in magnetically arrested accretion flows and relativistic jets using general relativistic magnetohydrodynamic simulations that incorporate a passive Eulerian tracer. The tracer allows us to reconstruct a proxy for the physical baryon density supplied by the accretion disk while excluding the mass injected numerically to maintain stability in highly magnetized, low-density regions. Applying this method to axisymmetric black hole simulations with varying spin, we show that baryon loading of the jet is intrinsically episodic and regulated by magnetic flux eruption cycles occurring in the inner accretion flow. Each eruption evacuates baryons from the innermost equatorial region, drives reconnection in extended current sheets, and expels moderately magnetized disk material along the funnel wall, establishing a recurrent mass-loading channel. In spinning black holes, shear-driven waves along the jet boundary further enhance baryon entrainment, whereas this mechanism is suppressed in the non-spinning case. For parameters representative of the black hole accretion flow in M87, we map the global structure and time evolution of the Goldreich-Julian screening boundary, defined as the surface separating regions where the plasma density is sufficient to supply the charges required to screen electric fields parallel to the magnetic field from regions that are charge starved. For spinning black holes, we find that the electromagnetic power of the jet is predominantly carried by baryon-poor plasma, with extended time intervals of charge starvation. Our results provide a framework for diagnosing jet composition, charge starvation, and reconnection-driven mass loading in magnetically arrested black hole systems, with direct implications for particle acceleration and non-thermal emission in low-luminosity accretion flows.