Cerenkov Line Emission as a Possible Mechanism of X-ray Lines in   Gamma-ray Bursts
  The recent discoveries of X-ray lines in the afterglows of gamma-ray bursts (GRBs) provide significant clues to the nature of GRB progenitors and central environments. However, the iron line interpretation by fluorescence or recombination mechanism requires a large amount of iron material. We argue that the very strong iron line could be attributed to an alternative mechanism: Cerenkov line emission since relativistic electrons and dense medium exist near GRB sites. Therefore, the broad iron lines are expected, and line intensity will be nearly independent of the iron abundance, the medium with the anomalously high Fe abundance is not required. 
W. Wang (1), Y. Zhao (1) and J. H. You (2) ((1) National Astronomical   Observatories, Beijing; (2) Shanghai Jiao Tong Unive...