General relativistic magnetohydrodynamic simulations of magnetically choked accretion flows around black holes

Abstract

Black hole (BH) accretion flows and jets are qualitatively affected by the presence of ordered magnetic fields. We study fully 3D global general relativistic magnetohydrodynamic (MHD) simulations of radially extended and thick (height H‐to‐cylindrical radius R ratio of |H/R| ∼ 0.2–1) accretion flows around BHs with various dimensionless spins (a/M, with BH mass M) and with initially toroidally dominated (φ‐directed) and poloidally dominated (R−z directed) magnetic fields. First, for toroidal field models and BHs with high enough |a/M|, coherent large‐scale (i.e. ≫H) dipolar poloidal magnetic flux patches emerge, thread the BH, and generate transient relativistic jets. Second, for poloidal field models, poloidal magnetic flux readily accretes through the disc from large radii and builds up to a natural saturation point near the BH. While models with |H/R| ∼ 1 and |a/M| ≤ 0.5 do not launch jets due to quenching by mass infall, for sufficiently high |a/M| or low |H/R| the polar magnetic field compresses the inflow into a geometrically thin highly non‐axisymmetric ‘magnetically choked accretion flow’ (MCAF) within which the standard linear magnetorotational instability is suppressed. The condition of a highly magnetized state over most of the horizon is optimal for the Blandford–Znajek mechanism which generates persistent relativistic jets with ≳100 per cent efficiency for |a/M| ≳ 0.9. A magnetic Rayleigh–Taylor and Kelvin–Helmholtz unstable magnetospheric interface forms between the compressed inflow and bulging jet magnetosphere, which drives a new jet–disc quasi‐periodic oscillation (JD‐QPO) mechanism. The high‐frequency QPO has spherical harmonic |m| = 1 mode period of τ∼ 70GM/c3 for a/M∼ 0.9 with coherence quality factors Q≳ 10. Overall, our models are qualitatively distinct from most prior MHD simulations (typically, |H/R| ≪ 1 and poloidal flux is limited by initial conditions), so they should prove useful for testing accretion‐jet theories and measuring a/M in systems such as SgrA* and M87.

OJ 287 as a Rotating Helix

“…Theoretical studies [11,12] and numerical simulations [13,14] show that the relativistic jets in AGN are likely to be driven by the rotating magnetic field trapped in the disk of a giant black hole. This can produce a jet of relativistically moving rotating plasma, threaded by a helical magnetic field.…”

Jet Signatures in the Spectra of Accreting Black Holes

“…For comparison, we checked our results against the A0.99N100 model from McKinney et al (2012). This model is a MAD RIAF and is qualitatively similar to the fiducial model, however, the disk is geometrically thinner.…”

“…The jet power in the BZ model is given by (Blandford & Znajek 1977;Tchekhovskoy et al 2010;Yuan & Narayan 2014) = , and M is the mass of the black hole. Thus, the highly magnetized state over most of the horizon (see Figure 1), and large black hole spin (a=0.9375), are optimal for the BZ mechanism to generate powerful, relativistic jets (Tchekhovskoy et al 2011;McKinney et al 2012).…”

“…3.2.2. Jet-disk Quasi-periodic Oscillations (QPOs) McKinney et al (2012) found that the black hole magnetosphere and disk exhibit significant QPOs in dynamical quantities including the mass density and magnetic energy density. These QPOs result from instabilities at the jet-disk interface and strongly affect the jet dynamics.…”

“…Avara et al (2015), with the inclusion of results from McKinney et al (2012), showed that the BZ mechanism produces much more powerful jets in MAD ADAFs than in MAD thin disks.…”