  A strong light pulse propagating in a nonlinear medium causes an effective
change in the local refractive index. With a suitable tuning of the pulse
velocity, the leading and trailing edge of the pulse were predicted to behave
as analogue black and white horizons in the limit of a dispersionless medium.
In this paper, we study a more realistic situation where the frequency
dispersion of the medium is fully taken into account. As soon as positive
frequency modes with negative norm are present in the comoving frame,
spontaneous emission of quantum vacuum radiation is expected to arise
independently of the presence of horizons. We finally investigate the kinematic
constraints put on the emission and we show that the optimal directions to
observe Hawking-like emission form a narrow angle with the direction of
propagation of the pulse.
