  The onset of most microquasar outbursts is characterized by a state
transition between a Low/Hard State (LHS) and a High/Soft State (HSS). Besides
drastic spectral and timing changes, this transition often shows a discrete
ejection event detectable in the radio range. However, the exact nature of the
ejected material and the mechanisms that give birth to these phenomena are yet
to be unraveled. Recent simultaneous radio and X-ray observations on several
sources point to a coronal nature of the ejected material. In the cases of GRS
1915+105, XTE J1550-564, and the 2002 outburst of GX 339-4, the flux of the
Compton component decreases sharply just before an ejection is detected in the
radio range. Finally, in the case of H1743-322, drastic physical changes
occurred in the corona just before the state transition, compatible with the
disappearance of part of this medium. Thus, the behaviour of at least 4
microquasars points in the direction of an ejection of the corona at the state
transition, feature that is yet to be confirmed (or infirmed) in the case of
other available sources.
