The role of the disk magnetization on the hysteresis behavior 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 disk with an amplitude measured by the disk magnetization $\mu(r,t) =B_z^2/(\mu_o P_{tot})$, where $P_{tot}$ is the total, gas and radiation, pressure. Below a transition radius $r_J$, a jet emitting disk (the JED) is settled and drives self-collimated non relativistic jets. Beyond $r_J$, no jet is produced despite the presence of the magnetic field and a standard accretion disc (the SAD) is established. The radial distribution of the disk magnetization $\mu$ adjusts itself to any change of the disk accretion rate $\dot m$, thereby modifying the transition radius $r_J$. We propose that a SAD-to-JED transition occurs locally, at a given radius, in a SAD when $\mu=\mu_{max} \simeq 1$ while the reverse transition occurs in a JED only when $\mu=\mu_{min}\simeq 0.1$. This bimodal behavior of the accretion disk provides a promising way to explain the hysteresis cycles followed by X-ray binaries during outbursts.