On the decay of strong magnetization in global disc simulations with toroidal fields

Abstract

Strong magnetization in accretion discs could resolve a number of outstanding issues related to stability and state transitions in low-mass X-ray binaries. However, it is unclear how real discs become strongly magnetized and, even if they do, whether they can remain in such a state. In this paper, we address the latter issue through a pair of global disc simulations. Here, we only consider cases of initially purely toroidal magnetic fields contained entirely within a compact torus. We find that over only a few tens of orbital periods, the magnetization of an initially strongly magnetized disc, Pmag/Pgas ≥ 10, drops to ≲ 0.1, similar to the steady-state value reached in initially weakly magnetized discs. This is consistent with recent shearing box simulations with initially strong toroidal fields, the robust conclusion being that strongly magnetized toroidal fields cannot be locally self-sustaining. These results appear to leave net poloidal flux or extended radial fields as the only avenues for establishing strongly magnetized discs, ruling out the thermal collapse scenario.