Tripartite entanglement of Hawking radiation in dispersive model

We investigate entanglement of the Hawking radiation in a dispersive model with subluminal dispersion. In this model, feature of the Hawking radiation is represented by three mode Bogoliubov transformation connecting the in-vacuum state and the out-state. We obtain the exact form of the tripartite in-vacuum state which encodes structure of multipartite entanglement. Bogoliubov coefficients are computed by numerical calculation of the wave equation with subluminal dispersion and it is found that genuine tripartite entanglement persists in whole frequency range up to the cutoff arisen from the subluminal dispersion. In the low frequency region, amount of the tripartite entanglement is far small compared to bipartite entanglement between the Hawking particle and its partner mode, and the deviation from the thermal spectrum is negligible. On the other hand, in the high frequency region near the cutoff, entanglement of the system is equally shared by two pairs of three modes, and the thermal nature of the Hawking radiation is lost.