Observational Properties of Near-Maximally Spinning Supermassive Black Holes

TL;DR

This study uses simulations and ray-tracing to compare near-maximally spinning black holes, finding that their observable properties are similar for spins above 0.9375, which impacts how we interpret EHT data.

Contribution

The paper demonstrates that black hole images with spins of 0.9375 and 0.998 are remarkably similar, suggesting previous models are representative for high-spin black holes.

Findings

Simulations show minimal differences in properties for spins 0.9375 and 0.998.

Black hole images are similar despite rapid evolution near spin 1.

Shape and size of photon rings may be key to distinguishing high spins.

Abstract

Black holes described by the Kerr metric can have a theoretical maximum dimensionless spin parameter of $a_\bullet = 1$, but several effects may limit the maximum spin parameter in astrophysical systems. We perform general relativistic magnetohydrodynamics simulations of accretion flows around black holes with $a_\bullet = 0.9375$ and $a_\bullet = 0.998$, each corresponding to a proposed astrophysical limit in the literature. We then perform full polarized general relativistic ray-tracing to produce astrophysical movies of these simulations, as can be spatially resolved by the Event Horizon Telescope (EHT) and its extensions. Although many properties of black holes and accretion flows evolve rapidly as $a_\bullet \to 1$, we find that our $a_\bullet=0.9375$ and $a_\bullet=0.998$ simulations are remarkably similar, both in terms of their GRMHD fluid properties and their full-Stokes, time-variable images. This suggests that previous work using simulations with $a_\bullet \approx 0.9375$ may be representative of models with $a_\bullet \gtrsim 0.9375$ in most practical cases. Our calculations suggest that shape and size constraints on the photon ring, enabled by extensions of the EHT into space by missions such as the Black Hole Explorer (BHEX) may be the only practical way to distinguish between models with different spin parameters as $a\to 1$.