  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 genuine tripartite
entanglement is 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 is equally shared by two pairs of three modes and the partner
picture of the Hawking radiation does not hold.
