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Abstract In the collisionless plasmas of radiatively inefficient accretion flows, heating and acceleration of ions and electrons are not well understood. Recent studies in the gyrokinetic limit revealed the importance of incorporating both the compressive and Alfvénic cascades when calculating the partition of dissipated energy between the plasma species. In this paper, we use a covariant analytic model of the accretion flow to explore the impact of compressive and Alfvénic heating, Coulomb collisions, compressional heating, and radiative cooling on the radial temperature profiles of ions and electrons. We show that, independent of the partition of heat between the plasma species, even a small fraction of turbulent energy dissipated to the electrons makes their temperature scale with a virial profile and the ion-to-electron temperature ratio smaller than in the case of pure Coulomb heating. In contrast, the presence of compressive cascades makes this ratio larger because compressive turbulent energy is channeled primarily into the ions. We calculate the ion-to-electron temperature in the inner accretion flow for a broad range of plasma properties, mass accretion rates, and black hole spins and show that it ranges between 5 ≲Ti/Te≲ 40. We provide a physically motivated expression for this ratio that can be used to calculate observables from simulations of black hole accretion flows for a wide range of conditions.
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This content will become publicly available on March 19, 2026
First-principles Measurement of Ion and Electron Energization in Collisionless Accretion Flows
We present the largest 3D particle-in-cell shearing-box simulations of turbulence driven by the magnetorotational instability, for the first time employing the realistic proton-to-electron mass ratio. We investigate the energy partition between relativistically hot electrons and subrelativistic ions in turbulent accreting plasma, a regime relevant to collisionless, radiatively inefficient accretion flows around supermassive black holes such as those targeted by the Event Horizon Telescope. We provide a simple empirical formula to describe the measured heating ratio between ions and electrons, which can be used for more accurate global modeling of accretion flows with standard fluid approaches such as general-relativistic magnetohydrodynamics.
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- Award ID(s):
- 2409316
- PAR ID:
- 10586049
- Publisher / Repository:
- American Astronomical Society
- Date Published:
- Journal Name:
- The Astrophysical Journal Letters
- Volume:
- 982
- Issue:
- 1
- ISSN:
- 2041-8205
- Page Range / eLocation ID:
- L28
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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