The role of the disc magnetization on the hysteresis behaviour of X‐ray binaries

Abstract

We present a framework for understanding the dynamical and spectral properties of X-ray binaries, where the presence of an organized large-scale magnetic field plays a major role. Such a field is threading the whole accretion disc with an amplitude measured by the disc magnetization μ(r, t) =B2z/(μoPtot), where Ptot is the total, gas and radiation, pressure. Below a transition radius rJ, a jet emitting disc (JED) is settled and drives self-collimated non-relativistic jets. Beyond rJ, no jet is produced despite the presence of the magnetic field and a standard accretion disc (SAD) is established. The radial distribution of the disc magnetization μ adjusts itself to any change in the disc accretion rate , thereby modifying the transition radius rJ.We propose that a SAD-to-JED transition occurs locally, at a given radius, in a SAD when μ=μmax≃ 1 while the reverse transition occurs in a JED only when μ=μmin≃ 0.1. This bimodal behaviour of the accretion disc provides a promising way to explain the hysteresis cycles followed by X-ray binaries during outbursts.