  We investigate particle creation and entanglement structure in a dispersive
model with subliminal dispersion relation. Assuming the step function spatial
velocity profile of the background flow, mode functions for a massless scalar
field is exactly obtained by the matching method. Power spectrums of created
particles are calculated for the subsonic and the transsonic flow cases. For
the transsonic case, the sonic horizon exists and created particles show the
Planckian distribution for low frequency region but the thermal property
disappears for high frequency region near the cutoff frequency introduced by
the non-linear dispersion. For the subsonic case, although the sonic horizon
does not exist, the effective group velocity horizon appears due to the
non-linear dispersion for high frequency region and approximate thermal
property of the power spectrum arises. Relation between particle creation and
entanglement between each mode is also discussed.

General Relativity and Quantum Cosmology;High Energy Physics - Theory;Quantum Physics