A computer model of a black-hole accretion flow showed a stop-start pattern in its jets. High magnetic-flux states were associated with efficient outflows, while low-flux states with mixed or quadrupolar magnetic topology had strongly suppressed outflows. The model’s two hemispheric jets contributed roughly equally on average, but some phases were strongly asymmetric. The reported pattern tied the strongest jet activity to high-flux phases and the weakest or absent outflows to low-flux states.
The study asks which accretion-flow conditions are associated with transient or suppressed jets, and what total and polarized signatures could be compared with quiescent Sgr A⋆ emission. It combines a three-dimensional general-relativistic magnetohydrodynamic (GRMHD) simulation of an accreting magnetized flow with radiative-transfer post-processing, including Compton scattering, to produce synthetic emission. The comparison with Sgr A⋆ is therefore made through modelled light, not direct measurements of the source.
The flux did not settle quietly
The computational design is narrow. It focuses on one representative multi-loop model. The radiative calculations compare Rhigh = 1 and Rhigh = 160 variants of that same simulation, not independent samples. The GRMHD run used a rapidly spinning Kerr black hole with spin a = 0.9375 and a 192×192×192 grid, which the study describes as sufficient to sustain MRI-driven turbulence.
In the model’s time coordinate, the mass accretion rate saturated at about 6,000 M and stabilized from roughly 12,000 M. Magnetic flux built up, fell sharply near 10,000 M, then went through another accumulation-and-expulsion phase from about 12,000 to 16,000 M before stabilizing. Overall, the authors placed the behavior between SANE and MAD, shorthand for two different magnetic-flow regimes.
Jet power changed with the field
Jet outflow was intermittent across those flux phases. In the reported analysis, jet power moved between active and weak or absent states and was tied to high-magnetic-flux phases. The two hemispheric jets were roughly balanced on average, but the model also contained periods of marked asymmetry. That combination of average balance and temporary imbalance was part of the simulated signal.
The analysis also separated phases by magnetic topology—the arrangement of the magnetic field. High-flux, dipole-dominated states were associated with efficient jets and coherent outward Maxwell stress in the polar funnel. Low-flux mixed or quadrupolar states had strongly suppressed outflows. In high-efficiency phases, that stress marked coherent outward angular-momentum transport; in quenched phases, transport was dominated by turbulence.
From flow to synthetic light
Those changing flow states were then turned into synthetic observables. The radiative-transfer analysis compared Rhigh = 1 and Rhigh = 160 over the last 20 kM, an interval the paper describes as one of stable mass accretion. The post-processing generated total and polarized emission, allowing the model’s radiative prescriptions to be compared with quiescent Sgr A⋆ emission. It did not turn the calculation into a direct observation of the source.
For the high-energy transfer, the calculation used 105 simulated light packets, called super-photons, per snapshot. It used bias tuning to keep the scattered-to-direct ratio approximately at unity and generated at least 10 Monte Carlo realizations per snapshot. These settings describe the construction of the synthetic high-energy signal, not an additional observational dataset.
A detailed case, not a general rule
The broader conclusion remains conditional. This is a result from one representative computational setup, and the Rhigh cases are radiative variants of it rather than independent samples. The study therefore does not demonstrate that Sgr A⋆ itself has a transient jet or establish that the same switching pattern applies beyond this model. Its value is as a testable link between magnetic-flux states, field topology and synthetic jet signatures, with further simulations needed to see how broadly the pattern holds.
Paper data and sources
Original title: Dynamics and Spectra-Polarimetric Signatures of GRMHD Simulations with Multiple Magnetic Loops
Authors: Raoul Kinadeter, Christian M. Fromm, Yosuke Mizuno et al.
Journal/Repository: arXiv
Status: Preprint, not yet peer-reviewed
First online: 2026-08-26
DOI: Not available
Original paper · Full text