Cerenkov line-like radiation and origin of iron Kα line in GRBs

When thermal relativistic electrons with isotropic distribution of velocities move in a gas region, or impinge upon the surface of a dense gas slab, the Cerenkov effect produces peculiar atomic or ionic emission lines - the Cerenkov line-like radiation. This newly recognized emission mechanism may find wide applications in high energy astrophysics. In this paper, we give a clarified outline of the physics of the new emission mechanism, and argue that Cerenkov line mechanism could be responsible for the iron Kalpha line emission observed in some GRBs in the X-ray afterglow phase. If this new exploration of the origin of iron Kalpha line is reasonable, the arguments against the models with collimated jet and the mergers models become debatable because all objections arise from the assumption that the iron K lines of GRBs originates in the fluorescence-recombination mechanisms, which should be replaced by the Cerenkov line emission, as argued in this paper. (C) 2004 Elsevier B.V. All rights reserved.