  We consider a simple realization of an event horizon in the flow of a
one-dimensional two-component Bose-Einstein condensate. Such a condensate has
two types of quasiparticles; In the system we study, one corresponds to density
fluctuations and the other to polarization fluctuations. We treat the case
where a horizon occurs only for one type of quasiparticles (the polarization
ones). We study the one- and two-body signal associated to the analog of
spontaneous Hawking radiation and demonstrate by explicit computation that it
consists only in the emission of polarization waves. We discuss the
experimental consequences of the present results in the domain of atomic
Bose-Einstein condensates and also for the physics of exciton-polaritons in
semiconductor microcavities.
