Friday, February 20, 2026 at 12:15 pm

JILA Foothills Room

Stephen Majeski, JILA

"Collisionless shwaves in black-hole accretion flows: A path to reduced variability"

Abstract:

Shearing waves, or "shwaves", are ubiquitous phenomena in differentially rotating flows. They are non-modal, non-axisymmetric perturbations that, due to shear in the background flow, possess time- evolving wavenumbers, frequencies, and amplitudes. In global simulations of black-hole accretion, such as those performed by the EHT to model Sgr A*, the excitation of acoustic shwaves is thought to lead to spiral patterns that affect the morphology of the emission region. As such, the variability of EHT observations is expected to be heavily influenced by the presence of such shwaves. Unfortunately, GRMHD models of Sgr A* regularly exhibit too much variability to be compatible with observations, indicating that simulated shwave amplitudes may be too large. In this talk, I argue that this "variability crisis" may be resolved by modeling shwave excitation and propagation in a collisionless plasma, which more accurately represents the conditions of the Sgr A* accretion flow than does collisional MHD. I will place particular emphasis on the role of micro-physical plasma instabilities and their tendency to affect large-scale material properties of the flow, finding here that they enhance the damping of acoustic shwaves through viscous stresses that are otherwise limited in influence. With EHT movies of Sgr A* being imminent, these results advocate for updated modeling techniques that can better incorporate the collisionless nature of black-hole accretion flows, hopefully reducing reliance on tunable parameters that do not adequately capture the underlying plasma physics inherent to the problem.

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