Magnetic outflows from turbulent accretion disks

Abstract

Context. Astrophysical disks are likely embedded in an ambient vertical magnetic field generated by its environment. This ambient field is known to drive magneto-rotational turbulence in the disk bulk, but it is also responsible for launching magnetised outflows at the origin of astrophysical jets. Yet, the interplay between turbulence and outflows is not understood. In particular, the vertical structure and long-term (secular) evolution of such a system lack quantitative predictions. It is, nevertheless, this secular evolution which is proposed to explain time variability in many accreting systems such as FuOr, X-ray binaries, and novae like systems. Aims. We seek to constraint the structure and long-term evolution of turbulent astrophysical disks subject to magnetised outflows in the non-relativistic regime. More specifically we aim to characterise the mechanism driving accretion, the dynamics of the disk atmosphere, the role played by the outflow, and the long-term evolution of mass and magnetic flux distributions. Methods. We computed and analysed global 3D ideal magnetohydrynamic (MHD) simulations of an accretion disk threaded by a large-scale magnetic field. We measured the turbulent state of the system by Reynolds averaging the ideal MHD equations and evaluate the role of the turbulent terms in the equilibrium of the system. We then computed the transport of mass, angular momentum, and magnetic fields in the disk to characterise its secular evolution. Finally, we performed a parameter exploration survey in order to characterise how the transport properties depend on the disk properties. Results. We find that weakly magnetised disks drive jets that carry a small fraction of the disk angular momentum away. The mass-weighted accretion speed remains subsonic, although there is always an upper turbulent atmospheric region where transsonic accretion takes place. We show that this turbulence is driven by a strongly magnetised version of the magneto-rotational instability. The internal disk structure therefore appears drastically different from the conventional hydrostatic picture. We expect that the turbulent atmosphere region will lead to non-thermal features in the emission spectra from compact objects. In addition, we show that the disk is subject to a secular viscous-type instability, which leads to the formation of long-lived ring-like structures in the disk surface density distribution. This instability is likely connected to the magnetic field transport. Finally, we show that for all of the parameters explored, the ambient magnetic field is always dragged inward in the disk at a velocity which increases with the disk magnetisation. Beyond a threshold on the latter, the disk undergoes a profound radial readjustment. It leads to the formation of an inner accretion-ejection region with a supersonic mass-weighted accretion speed and where the magnetic field distribution becomes steady and reaches a magnitude near equipartition with the thermal pressure. This inner structure shares many properties with the jet emitting disk model. Overall, these results pave the way for quantitative self-consistent secular disk models.

Synthetic gamma-ray light curves of Kerr black hole magnetospheric activity from particle-in-cell simulations

“…This setup can mimic inward magnetic flux transport in accretion flows (Lubow et al 1994). The latter value of V 0 that we used is consistent with ideal MHD simulations of accretion disks (Jacquemin-Ide et al 2021).…”

Tracking the evolution of the accretion flow in MAXI J1820+070 during its hard state with the JED-SAD model

“…For µ < 10 −3 or less, magnetic winds can be launched, possibly carrying away a significant fraction of the mass. However, these winds exert a negligible torque on the underlying disk (Zhu & Stone 2018;Jacquemin-Ide et al 2021). At these distances from the BH, particles are only subject to the torque due to the internal turbulent viscosity, so that the disk can be well described with the classic Shakura & Sunyaev (1973) model.…”

“…This argument would exclude that the optical winds detected in MAXI J1820 in hard state (Muñoz-Darias et al 2019; Sánchez-Sierras & Muñoz-Darias 2020) could play the role of shadowing reflection from intermediate regions. However, the presence of a large scale vertical magnetic field make real-life accretion disks more layered and "puffed up" (Zhu & Stone 2018;Jacquemin-Ide et al 2021). It is therefore possible that the boundary radius at which the disk is geometrically thickened by irradiation may be located even closer and reasonably impacts the reflection spectrum observed from the disk at high inclinations.…”

Clues on jet behavior from simultaneous radio-X-ray fits of GX 339-4

“…Magnetized accretion-ejection solutions that selfconsistently treat both the accretion disk and the jets have been in development for more than 20 years (Ferreira & Pelletier 1995;Ferreira 1997) and have since been validated through numerical simulations (e.g., Zanni et al 2007, Jacquemin-Ide et al 2021. In these works the accretion disk is assumed to be threaded by a large-scale magnetic field.…”

“…On the contrary, the inner part has an important mid-plane magnetization and the accretion flow has a JED structure. This radial distribution of the magnetization appears to be a natural outcome of the presence of large-scale magnetic fields in the accretion flow, the magnetic flux accumulating toward the center to produce a magnetized disk with a fast accretion timescale (Scepi et al 2020, Jacquemin-Ide et al 2021. Marcel et al (2018a) developed a two-temperature plasma code to compute the spectral energy distribution (SED) of any JED-SAD configuration.…”