Spectral Features from Ultrarelativistic Ions in Gamma-Ray Bursts?
  Gamma ray burst outflows may entrain small blobs or filaments of dense, highly ionized metal rich material. Such inhomogeneities, accelerated by the flow to Lorentz factors in the range 10-100, could have a significant coverage factor, and give rise to broad features, especially due to Fe K-edges, which influence the spectrum below the MeV range, leading to a progressively decreasing hardness ratio. 
P. Meszaros (1) & M.J. Rees (2) ((1) Pennsylvania State University;   (2) Inst. of Astronomy, Cambridge University)