  Activated Random Walk is a system of interacting particles which presents a
phase transition and a conjectured phenomenon of self-organized criticality. In
this note, we prove that, in dimension 1, in the supercritical case, when a
segment is stabilized with particles being killed when they jump out of the
segment, a positive fraction of the particles leaves the segment with positive
probability.
  This was already known to be a sufficient condition for being in the active
phase of the model, and the result of this paper shows that this condition is
also necessary, except maybe precisely at the critical point. This result can
also be seen as a partial answer to some of the many conjectures which connect
the different points of view on the phase transition of the model.
