Accretion rate vs. time in the MRI model (top panel), clump-infall model (second panel), and collision model (third and forth panels). The collision model includes the cases of prograde (third panel) and retrograde (forth panel) collisions. In particular, the red and blue lines present the mass accretion rates on the intruder and target, respectively. The time is reset to zero for each chosen interval of disk evolution. The arrows indicate the bursts that are investigated in more detail below.