On the magnetization of jet-launching discs

Abstract

Magnetohydrodynamic models of collimated outflows produced by accretion discs around compact objects can be used for interpreting the phenomenology of active astrophysical objects as young stellar objects, microquasars, X-ray binaries, gamma-ray bursts, extended radio galaxies and active galactic nuclei. In the present work, we discuss how the strength of magnetic fields determines the characteristics of solutions in models where the collimated outflow and the accretion disc are treated consistently. We perform an extensive analysis of the magnetic field's strength by non-relativistic axisymmetric numerical simulations using the pluto code. We discuss in detail the characteristics of the numerical solutions with specific reference to the efficiency of transforming accretion inflows into collimated superfast magnetosonic outflows. The relevance of the resistivity parameter used in numerical simulations is analysed. The main results are that magnetic fields around and below equipartition with plasma pressure allow for steady superfast magnetosonic collimated jet solutions; for even lower magnetization, the solutions found are unsteady, with small velocities and matter-dominated magnetic field lines that behave kinematically; magnetic fields above equipartition lead to unsteady sub-Alfvénic winds. These results allow to conclude that stationary super-Alfvénic and superfast magnetosonic outflow solutions are found only for equipartition and weaker magnetic fields, for the range studied in this article.