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Abstract In certain scenarios, the accreted angular momentum of plasma onto a black hole could be low; however, how the accretion dynamics depend on the angular momentum content of the plasma is still not fully understood. We present three-dimensional, general relativistic magnetohydrodynamic simulations of low angular momentum accretion flows around rapidly spinning black holes (with spina = +0.9). The initial condition is a Fishbone–Moncrief (FM) torus threaded by a large amount of poloidal magnetic flux, where the angular velocity is a fractionfof the standard value. Forf= 0, the accretion flow becomes magnetically arrested and launches relativistic jets but only for a very short duration. After that, free-falling plasma breaks through the magnetic barrier, loading the jet with mass and destroying the jet–disk structure. Meanwhile, magnetic flux is lost via giant, asymmetrical magnetic bubbles that float away from the black hole. The accretion then exits the magnetically arrested state. Forf= 0.1, the dimensionless magnetic flux threading the black hole oscillates quasiperiodically. The jet–disk structure shows concurrent revival and destruction while the gas outflow efficiency at the event horizon changes accordingly. Forf≥ 0.3, we find that the dynamical behavior of the system starts to approach that of a standard accreting FM torus. Our results thus suggest that the accreted angular momentum is an important parameter that governs the maintenance of a magnetically arrested flow and launching of relativistic jets around black holes. 

Abstract The presence of a strong, large-scale magnetic field in an accretion flow leads to the extraction of the rotational energy of the black hole (BH) through the Blandford–Znajek (BZ) process, believed to power relativistic jets in various astrophysical sources. We study rotational energy extraction from a BH surrounded by a highly magnetized thin disk by performing a set of 3D global GRMHD simulations. We find that the saturated flux threading the BH has a weaker dependence on BH spin, compared to highly magnetized hot (geometrically thick) accretion flows. Also, we find that only a fraction (10%–70%) of the extracted BZ power is channeled into the jet, depending on the spin parameter. The remaining energy is potentially used to launch winds or contribute to the radiative output of the disk or corona. Our simulations reveal that the presence of a strong magnetic field enhances the radiative efficiency of the disk, making it more luminous than its weakly magnetized counterpart or the standard disk model. We attribute this excess luminosity primarily to the enhanced magnetic dissipation in the intra-ISCO region. Our findings have implications for understanding X-ray corona formation and BH spin measurements, and interpreting BH transient phenomena. 

The Faraday rotation measure (RM) is a commonly used tool to trace electron number density and magnetic fields in hot accretion flows, particularly in low-luminosity accreting supermassive black holes. We focus on the nuclear region of M87, which was observed at 230 GHz (1.3 mm) by the Event Horizon Telescope in 2019. It remains unclear whether this emission originates from the accretion flow, the jet base, or both. To probe the presence of an accretion flow, we explore the scenario where the linearly polarized emission from the counter jet, visible at 43 GHz (7 mm), is Faraday-rotated by the accretion flow. We calculate theoretical predictions for counter-jet polarization using analytical and numerical models. In all cases, we find a Faraday-thick flow at 43 GHz (7 mm), with RM ∼ 10⁶rad m⁻², and a polarization angle that follows a linear relationship with wavelength squared, consistent with external Faraday rotation. The more realistic model, which includes turbulence and magnetic field fluctuations, predicts that the polarization pattern should be time-dependent, and that the counter-jet emission is depolarized due to Faraday depth fluctuations across the accretion flow. Despite the Faraday thick regime and strong depolarization, the linear relationship persists, enabling us to constrain the flow’s physical properties. Comparing the counter-jet and forward-jet linear polarization states should enable detection of M87’s accretion flow and provide lower limits on electron density, magnetic field strength, and mass accretion rate. 

Sagittarius A* (Sgr A*), the supermassive black hole at the centre of the Milky Way, undergoes large-amplitude near-infrared (NIR) flares that can coincide with the continuous rotation of the NIR emission region. One promising explanation for this observed NIR behaviour is a magnetic flux eruption, which occurs in three-dimensional General Relativistic Magneto-Hydrodynamic (3D GRMHD) simulations of magnetically arrested accretion flows. After running two-temperature 3D GRMHD simulations, where the electron temperature is evolved self-consistently along with the gas temperature, it is possible to calculate ray-traced images of the synchotron emission from thermal electrons in the accretion flow. Changes in the gas-dominated (σ = b2/2ρ < 1) regions of the accretion flow during a magnetic flux eruption reproduce the NIR flaring and NIR emission region rotation of Sgr A* with durations consistent with observation. In this paper, we demonstrate that these models also predict that large (1.5x – 2x) size increases of the sub-millimeter (sub-mm) and millimeter (mm) emission region follow most NIR flares by 20–50 min. These size increases occur across a wide parameter space of black hole spin (a = 0.3, 0.5, −0.5, and 0.9375) and initial tilt angle between the accretion flow and black hole spin axes θ0 (θ0 = 0°, 16°, and 30°). We also calculate the sub-mm polarization angle rotation and the shift of the sub-mm spectral index from zero to –0.8 during a prominent NIR flare in our high spin (a = 0.9375) simulation. We show that, during a magnetic flux eruption, a large (∼10rg), magnetically dominated (σ > 1), low-density, and high-temperature ‘bubble’ forms in the accretion flow. The drop in density inside the bubble and additional electron heating in accretion flow between 15rg and 25rg leads to a sub-mm size increase in corresponding images.