Quantum entanglement in analogue Hawking radiation: When is the final state nonseparable?

We study the quantum entanglement of the quasiparticle pairs emitted by analog black holes. We use a phenomenological description of the spectra in dispersive media to study the domains in parameter space where the final state is nonseparable. In stationary flows, three modes are involved in each sector of fixed frequency, and not two as in homogeneous situations. The third spectator mode acts as an environment for the pairs, and the strength of the coupling significantly reduces the quantum coherence. The non-separability of the pairs emitted by white holes is also considered and compared with that of black holes.