Gamma rays from interactions of stars with AGN jets
  We have developed a model for gamma ray emission in jets of active galactic nuclei in which particle acceleration takes place at a shock in the relativistic jet plasma due to a massive star in the central region of the host galaxy moving through the jet. The gamma rays are produced in a pair-Compton cascade in the radiation field of the star initiated by accelerated electrons. Our model may account for the observed GeV to TeV gamma ray spectrum and variability of Markarian 421 and other blazars detected by the EGRET instrument on the Compton Gamma Ray Observatory. 
W. Bednarek and R.J. Protheroe