Radiation flux and spectrum in the Vaidya collapse model

We consider the quantization of a massless scalar field, using the geometri c optics approximation, in the background spacetime of a collapsing spheric al self-similar Vaidya star, which forms a black hole or a naked singularit y. We show that the outgoing radiation flux of the quantized scalar field d iverges on the Cauchy horizon. The spectrum of the produced scalar partcles is non-thermal when the background develops a naked singularity. These res ults are analogous to those obtained for the scalar quantization on a self- similar dust cloud. (C) 2000 Elsevier Science B.V. All rights reserved.