Black Hole Spin Evolution in Magnetized Accretion Flows
Abstract
Black hole (BH) spin grows as accretion supplies angular momentum and decreases as magnetically driven jets remove it, driving the BH toward an equilibrium spin, a⋆; eq. Existing estimates of a⋆; eq rely on general-relativistic magnetohydrodynamic (GRMHD) simulations with fixed BH mass and spin, and cover only the extreme limits of horizon magnetic flux ϕ→0 and magnetically arrested disks (MADs), ϕ≈ϕMAD. We evolve the BH mass and spin in long 3D GRMHD simulations of radiatively inefficient accretion flows aligned or anti-aligned with the BH spin. We find the spin-up parameter as a function of BH spin and ϕ/ϕMAD, and derive the equilibrium spin as a continuous function of flux, a⋆; eq(ϕ/ϕMAD). The equilibrium spin falls steeply with flux, from a⋆; eq≈0.9 in weakly magnetized flows through a⋆; eq(ϕ/ϕMAD=0.5)≈0.5 to a⋆; eq(ϕ/ϕMAD=1)≈0.06 in sustained MADs. In weakly magnetized flows, a slowly spinning BH must accrete at least its initial mass to reach equilibrium, whereas MADs spin down rapidly rotating BHs after accreting less than half of it. However, as the BH grows, the growing gravitational radius and enhanced inflow reduce ϕ/ϕMAD, driving disks whose magnetic-flux supply cannot keep pace out of the MAD state and allowing the BH to transition from spin-down to spin-up. The final spin thus depends on both the magnetic flux history and the accreted mass. We discuss implications for a variety of BH-powered engines and provide a subgrid prescription for galaxy-scale simulations and population studies.
Related
- Magnetized Accretion Flows with Low Angular Momentum Around Supermassive Black Holes
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- Radiative Efficiency Enhancement by Electromagnetic Energy Dissipation in Strongly Magnetized Supercritical Accretion Flows around Kerr Black Holes
- Strong Black Hole Natal Kicks in Magnetized Accretion-Powered Explosions
BibTeX
@misc{issa2026black,
title = {{Black Hole Spin Evolution in Magnetized Accretion Flows}},
author = {Danat Issa and Ore Gottlieb and Jonatan Jacquemin-Ide and Matthew Liska},
howpublished = {arXiv},
year = {2026},
month = oct,
doi = {10.48550/arxiv.2610.38673},
eprint = {2610.38673},
archivePrefix = {arXiv},
url = {https://arxiv.org/abs/2610.38673},
note = {CC BY 4.0},
}
RIS
TY - UNPB
TI - Black Hole Spin Evolution in Magnetized Accretion Flows
AU - Danat Issa
AU - Ore Gottlieb
AU - Jonatan Jacquemin-Ide
AU - Matthew Liska
PY - 2026
DA - 2026/10/28
PB - arXiv
DO - 10.48550/arxiv.2610.38673
UR - https://arxiv.org/abs/2610.38673
AB - Black hole (BH) spin grows as accretion supplies angular momentum and decreases as magnetically driven jets remove it, driving the BH toward an equilibrium spin, a_⋆; eq. Existing estimates of a_⋆; eq rely on general-relativistic magnetohydrodynamic (GRMHD) simulations with fixed BH mass and spin, and cover only the extreme limits of horizon magnetic flux ϕ→0 and magnetically arrested disks (MADs), ϕ≈ϕ_MAD. We evolve the BH mass and spin in long 3D GRMHD simulations of radiatively inefficient accretion flows aligned or anti-aligned with the BH spin. We find the spin-up parameter as a function of BH spin and ϕ/ϕ_MAD, and derive the equilibrium spin as a continuous function of flux, a_⋆; eq(ϕ/ϕ_MAD). The equilibrium spin falls steeply with flux, from a_⋆; eq≈0.9 in weakly magnetized flows through a_⋆; eq(ϕ/ϕ_MAD=0.5)≈0.5 to a_⋆; eq(ϕ/ϕ_MAD=1)≈0.06 in sustained MADs. In weakly magnetized flows, a slowly spinning BH must accrete at least its initial mass to reach equilibrium, whereas MADs spin down rapidly rotating BHs after accreting less than half of it. However, as the BH grows, the growing gravitational radius and enhanced inflow reduce ϕ/ϕ_MAD, driving disks whose magnetic-flux supply cannot keep pace out of the MAD state and allowing the BH to transition from spin-down to spin-up. The final spin thus depends on both the magnetic flux history and the accreted mass. We discuss implications for a variety of BH-powered engines and provide a subgrid prescription for galaxy-scale simulations and population studies.
ER -