  Spatially extended systems can support local transient excitations in which
just a part of the system is excited. The mechanisms reported so far are local
excitability and excitation of a localized structure. Here we introduce an
alternative mechanism based on the coexistence of two homogeneous stable states
and spatial coupling. We show the existence of a threshold for perturbations of
the homogeneous state. Sub-threshold perturbations decay exponentially.
Super-threshold perturbations induce the emergence of a long-lived structure
formed by two back to back fronts that join the two homogeneous states. While
in typical excitability the trajectory follows the remnants of a limit cycle,
here reinjection is provided by front interaction, such that fronts slowly
approach each other until eventually annihilating. This front-mediated
mechanism shows that extended systems with no oscillatory regimes can display
excitability.
