Similarities and Differences between Relativistic Electron-Photon   Cascades Developed in Matter, Photon Gas and Magnetic Field
  We investigate properties of astrophysical electromagnetic cascades developed in matter, photon gas and magnetic fields, and discuss similarities and differences between characteristics of electron-photon showers developed in these 3 substances. 
F.A. Aharonian and A.V. Plyasheshnikov