Magnetic support, wind-driven accretion, coronal heating, and fast outflows in a thin magnetically arrested disc

Author:

Scepi Nicolas12ORCID,Begelman Mitchell C13ORCID,Dexter Jason13ORCID

Affiliation:

1. JILA, University of Colorado and National Institute of Standards and Technology , 440 UCB, Boulder, CO 80309-0440 , USA

2. School of Physics and Astronomy, University of Southampton , Highfield, Southampton SO17 1BJ , UK

3. Department of Astrophysical and Planetary Sciences, University of Colorado , 391 UCB, Boulder, CO 80309-0391 , USA

Abstract

ABSTRACT Accretion discs properties should deviate from standard theory when magnetic pressure exceeds the thermal pressure. To quantify these deviations, we present a systematic study of the dynamical properties of magnetically arrested discs (MADs), the most magnetized type of accretion disc. Using an artificial cooling function to regulate the gas temperature, we study MADs of three different thermal thicknesses, hth/r = 0.3, 0.1, and 0.03. We find that the radial structure of the disc is never mostly supported by the magnetic field. In fact, thin MADs are very near Keplerian. However, as discs gets colder, they become more magnetized and the largest deviations from standard theory appear in our thinnest disc with hth/r = 0.03. In this case, the disc is much more extended vertically and much less dense than in standard theory because of vertical support from the turbulent magnetic pressure and wind-driven angular momentum transport that enhances the inflow speed. The thin disc also dissipates a lot of thermal energy outside of z/r = ±0.03 and a significant fraction of this dissipation happens in mildly relativistic winds. The enhanced dissipation in low-density regions could possibly feed coronae in X-ray binaries (XRBs) and active galactic nuclei (AGNs). Wind-driven accretion will also impact the dynamical evolution of accretion discs and could provide a mechanism to explain the rapid evolution of changing-look AGN and the secular evolution of XRBs. Finally, our MAD winds have terminal velocities and mass-loss rates in good agreement with the properties of ultrafast outflows observed in AGN.

Funder

NASA

National Science Foundation

Publisher

Oxford University Press (OUP)

Subject

Space and Planetary Science,Astronomy and Astrophysics

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