Thinner Disks Struggle to Launch Jets: GRMHD Simulations of Magnetic Field Generation and Transport
Abstract
Relativistic jets are observed in some accreting black-hole systems but not others. In X-ray binaries, jet production is closely associated with geometrically thicker accretion states, suggesting that disk thickness may influence the conditions for jet launching. We investigate this connection using three-dimensional GRMHD simulations of disks with h/r=0.1--$0.3$, initialized with a toroidal magnetic field with plasma β=5. We find that the magnetorotational instability (MRI) generates large-scale poloidal fields through non-axisymmetric shearing waves, with an additional contribution from axisymmetric induction through stretching by channel-like modes. The resulting mean fields become progressively weaker as the disk becomes thinner. Because these fields regulate MRI turbulence, thinner disks exhibit weaker angular-momentum transport and accrete more slowly than the standard vacc∝(h/r)2 thin-disk scaling would predict. The magnetic-flux transport velocity remains comparable to the accretion velocity, so weaker field generation and slower transport combine to strongly suppress black-hole flux accumulation at smaller h/r. Correspondingly, the thinnest disk (h/r=0.1) produces an electromagnetic jet efficiency of only ηEM∼1%, while the thickest disk reaches the magnetically arrested disk (MAD) state and attains ηEM≥100%. Using the measured large-scale magnetic flux distribution and transport velocities, we estimate the time required to accumulate MAD-level magnetic flux. Only the h/r∼0.3 disk reaches MAD in the duration of our simulations, at t≃2×104 rg/c, whereas for h/r=0.1 we estimate a timescale of ∼108 rg/c. These results link disk geometry to magnetic-flux accumulation and jet launching, with direct implications for the timescales over which jets can emerge in transient accreting systems.
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BibTeX
@misc{jacqueminide2026thinner,
title = {{Thinner Disks Struggle to Launch Jets: GRMHD Simulations of Magnetic Field Generation and Transport}},
author = {Jonatan Jacquemin-Ide and Mitchell C. Begelman and Henrik Latter and Jason Dexter and Alexander Tchekhovskoy},
howpublished = {arXiv},
year = {2026},
month = oct,
doi = {10.48550/arxiv.2610.37758},
eprint = {2610.37758},
archivePrefix = {arXiv},
url = {https://arxiv.org/abs/2610.37758},
note = {CC BY 4.0},
}
RIS
TY - UNPB
TI - Thinner Disks Struggle to Launch Jets: GRMHD Simulations of Magnetic Field Generation and Transport
AU - Jonatan Jacquemin-Ide
AU - Mitchell C. Begelman
AU - Henrik Latter
AU - Jason Dexter
AU - Alexander Tchekhovskoy
PY - 2026
DA - 2026/10/27
PB - arXiv
DO - 10.48550/arxiv.2610.37758
UR - https://arxiv.org/abs/2610.37758
AB - Relativistic jets are observed in some accreting black-hole systems but not others. In X-ray binaries, jet production is closely associated with geometrically thicker accretion states, suggesting that disk thickness may influence the conditions for jet launching. We investigate this connection using three-dimensional GRMHD simulations of disks with h/r=0.1--$0.3$, initialized with a toroidal magnetic field with plasma β=5. We find that the magnetorotational instability (MRI) generates large-scale poloidal fields through non-axisymmetric shearing waves, with an additional contribution from axisymmetric induction through stretching by channel-like modes. The resulting mean fields become progressively weaker as the disk becomes thinner. Because these fields regulate MRI turbulence, thinner disks exhibit weaker angular-momentum transport and accrete more slowly than the standard v_acc∝(h/r)^2 thin-disk scaling would predict. The magnetic-flux transport velocity remains comparable to the accretion velocity, so weaker field generation and slower transport combine to strongly suppress black-hole flux accumulation at smaller h/r. Correspondingly, the thinnest disk (h/r=0.1) produces an electromagnetic jet efficiency of only η_EM∼1%, while the thickest disk reaches the magnetically arrested disk (MAD) state and attains η_EM≥100%. Using the measured large-scale magnetic flux distribution and transport velocities, we estimate the time required to accumulate MAD-level magnetic flux. Only the h/r∼0.3 disk reaches MAD in the duration of our simulations, at t≃2×10^4 r_g/c, whereas for h/r=0.1 we estimate a timescale of ∼10^8 r_g/c. These results link disk geometry to magnetic-flux accumulation and jet launching, with direct implications for the timescales over which jets can emerge in transient accreting systems.
ER -