Suppression of spiral density waves in collisionless accretion flows
Abstract
Spiral density waves 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 optically thin, low-luminosity black-hole accretion flows, the excitation of such waves leads to spiral patterns in the plasma density that influence the morphology of the emission region. As such, the observed variabilities of such accretion flows are expected to be affected by the behavior of spiral density waves. Thus far, investigations of spiral density waves in accretion flows have exclusively used the assumption that the accretion-flow plasmas are in local thermodynamic equilibrium (LTE) at all times. In reality, hot and diffuse accretion-flow plasmas are expected to deviate far from LTE, and should be modeled accordingly. Here, we employ a combination of kinetic and fluid models to describe spiral density wave propagation, damping, and excitation in a collisionless shearing sheet, which more accurately represents the plasma conditions of low-luminosity accretion flows than do models that assume LTE. We find that micro-physical plasma instabilities are especially important in determining how spiral density waves behave, because the weak particle scattering that they induce leads to appreciable viscous damping. Observational implications of these results include reduced emission-region variability, which may improve agreement between Event Horizon Telescope (EHT) observations and simulation/theory, as well as a reduction in emission-region pattern speeds compared to simulations employing LTE, which bears on the interpretation of eventual EHT movies.
Related
- The generation of spiral density waves by MRI in accretion discs
- Two sets of potential-density basis pairs for the study of radial perturbations in collisionless spherical stellar systems
- Primary and Secondary Star Formation in the Spiral Waves of NGC 628
- The protostar FU Orionis may not be bursting: dramatic accretion suppression in embedded young eccentric binaries
- Odd-parity ringdown gravitational waves of a spherically symmetric black hole with perfect fluid accretion
BibTeX
@misc{majeski2026suppression,
title = {{Suppression of spiral density waves in collisionless accretion flows}},
author = {Stephen Majeski and Matthew W. Kunz and Jason Dexter},
howpublished = {arXiv},
year = {2026},
month = oct,
doi = {10.48550/arxiv.2610.02475},
eprint = {2610.02475},
archivePrefix = {arXiv},
url = {https://arxiv.org/abs/2610.02475},
note = {CC BY-NC-SA 4.0},
}
RIS
TY - UNPB
TI - Suppression of spiral density waves in collisionless accretion flows
AU - Stephen Majeski
AU - Matthew W. Kunz
AU - Jason Dexter
PY - 2026
DA - 2026/10/01
PB - arXiv
DO - 10.48550/arxiv.2610.02475
UR - https://arxiv.org/abs/2610.02475
AB - Spiral density waves 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 optically thin, low-luminosity black-hole accretion flows, the excitation of such waves leads to spiral patterns in the plasma density that influence the morphology of the emission region. As such, the observed variabilities of such accretion flows are expected to be affected by the behavior of spiral density waves. Thus far, investigations of spiral density waves in accretion flows have exclusively used the assumption that the accretion-flow plasmas are in local thermodynamic equilibrium (LTE) at all times. In reality, hot and diffuse accretion-flow plasmas are expected to deviate far from LTE, and should be modeled accordingly. Here, we employ a combination of kinetic and fluid models to describe spiral density wave propagation, damping, and excitation in a collisionless shearing sheet, which more accurately represents the plasma conditions of low-luminosity accretion flows than do models that assume LTE. We find that micro-physical plasma instabilities are especially important in determining how spiral density waves behave, because the weak particle scattering that they induce leads to appreciable viscous damping. Observational implications of these results include reduced emission-region variability, which may improve agreement between Event Horizon Telescope (EHT) observations and simulation/theory, as well as a reduction in emission-region pattern speeds compared to simulations employing LTE, which bears on the interpretation of eventual EHT movies.
ER -