  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.

Astrophysics