Launching Jets from Magnetized Bondi-like Accretion

Gas accreting onto black hole usually carries angular momentum and forms a disk, the accretion process powers luminous emission, and energetic jet. It is common for analytic models to assume the accreting gas carries enough angular momentum to form a self-supported disk, e.g. standard thin α-disk,

However, some astrophysical systems may only carry very low angular momentum accreting gas, such as long gamma-ray bursts (LGRBs), Sagittarius A* (Sgr A*), tidal disruption events (TDEs) and high-mass X-ray binaries (HMXBs). Can such accretion flow launch a powerful jet?

To understand the dynamical evolution and the production of jets from low angular momentum accreting gas, I perform state-of-the-art 3D general relativistic magneto-hydrodynamics (GRMHD) simulations, run on supercomputer. We have simulated magnetized Bondi-like accretion flows with initially no or very low angular momentum around rapidly spinning black holes.

Simulations

Zero net angular momentum Rc = 0

After the simulation starts, gas follows the typical behaviour of Bondi flow, falls in radially. Soon after that, an accretion structure starts to form near the equatorial plane due to the frame-dragging effect by the fast spin of the black hole. Also, the magnetic flux accumulates around, and feed on the black hole, relativistic jets are launched. The gas in the inner region becomes magnetically arrested for a short time.

However, we can see the gas does not form a steady disk. While the gas flows inwards in certain directions, at other angles gas can flow out and carry away a lot of magnetic flux. Since the gas in the outer region lacks angular momentum, the "magnetic bubbles'' just leak all the way out, the magnetic flux around the horizon depletes, the gas is no longer magnetically arrested, and the jet becomes less powerful and stochastic.

In this closer look, we can see the inner disk and the jet are sometimes slightly tilted to the BH spin axis, due to the chaotic behaviour of the magnetic flux in the inner accretion region.

Slightly more net angular momentum Rc = 10 rg

Similarly, in this run, after the simulation starts, gas falls in almost radially. Soon after that, an accretion structure starts to form due to the frame-dragging effect and the tiny initial gas angular momentum. The magnetic flux accumulates around the black hole, relativistic jets are launched. The gas in the inner region also becomes magnetically arrested for a short time.

Again, the gas does not form a steady disk structure, the dragging of black hole rotation itself is not sufficient to form a stable, typical accretion disk. The magnetic flux depletes so the jet eventually becomes less powerful.

In this closer look, we can see the gas sometimes fall in the black hole along the polar direction. The jet evolution is not symmetric in the north and south hemisphere.

Even more (but still low) net angular momentum Rc = 50 rg

This run has more angular momentum to support an accretion disk structure near the equatorial plane. We can see that the gas steadily form a more axisymmetric disk structure, the gas flow inwards in all direction. The magnetic flux slowly accumulates around, and feeds on the black hole, eventually relativistic jets are launched. The gas in the inner region then becomes magnetically arrested, result in the patchy outflow in the equatorial plane close to the black hole, however, it is not strong enough to disrupt the inflow.

Therefore, the over-magnetized gas from MAD can only temporarily flow out, it will later get mixed with the rotating inflow in the outer region. The magnetic fluxes carried by the magnetized outflowing bubbles are also shredded by the inflow and then brought back to around the black hole. This allows the inner accretion flows to sustain the MAD state, the jet is also powerful all the time.