  To understand the onset of collective motion, we investigate active systems
where particles switch on and off their self-propulsion. We prove that even
when the only possible transition is off$\to$on, an active 2-state system
behaves as an effective 3-state system that exhibits a sharp phase transition
in 1D, and critical behavior in 2D, with scale-invariant activity avalanches.
The obtained results show how criticality can naturally emerge in active
systems, providing insight into the way collectives distribute, process, and
respond to local environmental cues.
