Sustained FU Orionis-type outbursts from colliding discs in stellar flybys

We perform 3D hydrodynamics simulations of disc-disc stellar flybys with on-the-fly Monte Carlo radiative transfer. We show that pre-existing circumstellar discs around both stars result in fast rising (∼yrs) outbursts lasting 2–5 times longer than for a star-disc flyby. The perturber always goes into outburst ($\dot{M}>10^{-5}~{\rm M_{\odot }~ yr^{-1}}$). Whereas we find that the primary goes into a decades long outburst only when the flyby is retrograde to the circumprimary disc rotation. High accretion rates during the outburst are triggered by angular momentum cancellation in misaligned material generated by the encounter. A large fraction of accreted material is alien.