October 21
Weakly magnetised accretion within the innermost stable circular orbit: GRMHD simulations and analytic models
Speaker: Jake Rule, Oxford
Abstract: Black hole spin measurements constrain the formation and growth of supermassive black holes, their feedback on the surrounding environment, and the progenitors and core-collapse physics of stellar-mass black holes. Electromagnetic spin measurements inferred from the thermal continuum or iron K-alpha fluorescence rely on accurate modelling of the inner accretion disc, where emission is most sensitive to black hole spin. Canonical accretion disc models assume a vanishing-stress boundary condition at the ISCO, with thermodynamic profiles sharply truncated there and the plunging region between the ISCO and the event horizon excluded. However, GRMHD simulations consistently exhibit a non-zero magnetic stress at the ISCO, along with rich plunging region thermodynamics. Including the plunging fluid and accounting for a finite ISCO stress are critical for accurately estimating the spin: a low-spin black hole with a non-zero ISCO stress may be mistaken for a rapidly spinning black hole with no emission from the plunging region. This degeneracy may contribute to the apparent tension between the spin distributions inferred from gravitational wave-detected binary mergers and electromagnetic observations of high-mass X-ray binaries. To address these concerns, we require a model that incorporates both the plunging region and a finite ISCO stress. We show that our weakly magnetised (SANE) GRMHD simulations across a broad range of black hole spins are consistent with a gravity-dominated geodesic plunge within the ISCO. This result greatly simplifies the construction of an analytic thermodynamic framework, which we further show is consistent with our GRMHD simulations provided that non-adiabatic heating from grid-scale magnetic dissipation is accounted for. We also show that the ISCO stress increases strongly with black hole spin, suggesting that the apparent modelling degeneracy is not fundamental. Finally, we demonstrate that the magnetic fields can be modelled analytically, qualitatively explaining the spin-stress trend and further refining our thermodynamic framework.
Host: Prof. Chris Reynolds