Hawking spectrum and high frequency dispersion

We study the spectrum of created particles in two-dimensional black hole geometries for a linear, Hermitian scalar field satisfying a Lorentz noninvariant field equation with higher spatial derivative terms that are suppressed by powers of a fundamental momentum scale k(0). The preferred frame is the ''free-fall frame'' of the black hole. This model is a variation of Unruh's sonic black hole analogy. We find that there are two qualitatively different types of particle production in this model: a thermal Hawking flux generated by ''mode conversion'' at the black hole horizon, and a nonthermal spectrum generated via scattering off the background into negative free-fall frequency modes. This second process has nothing to do with black holes and does not occur for the ordinary wave equation because such modes do not propagate outside the horizon with positive Killing frequency. The horizon component of the radiation is astonishingly close to a perfect thermal spectrum: for the smoothest metric studied, with Hawking temperature T-H similar or equal to 0.0008k(0), agreement is of order (T-H/k(0))(3) at frequency omega=T-H, and agreement to order T-H/k(0) persists out to omega/T-H similar or equal to 45 where the thermal number flux is similar to 10(-20). The flux from scattering dominates at large omega and becomes many orders of magnitude larger than the horizon component for metrics with a ''kink,'' i.e., a region of high curvature localized on a static world line outside the horizon. This nonthermal flux amounts to roughly 10% of the total luminosity for the kinkier metrics considered. The flur exhibits oscillations as a function of frequency which can be explained by interference between the various contributions to the flux.