TeV Gamma Rays from BL Lac Objects due to Synchrotron Radiation of   Extremely High Energy Protons
  One of remarkable features of the gamma ray blazar Markarian 501 is the reported shape of the TeV spectrum, which during strong flares of the source remains essentially stable despite dramatic variations of the absolute gamma-ray flux. I argue that this (to a large extent unexpected) behavior of the source could be explained assuming that the TeV gamma-ray emission is a result of synchrotron radiation of extremely high energy protons in highly magnetized compact regions of the jet. 
F.A. Aharonian