Hadronic gamma-ray emission models
  The discovery of rapid and simultaneous TeV/optical (UV, X-ray) variability provides strong evidence against the interaction between a thermal radiation field produced by an accretion flow and relativistic electrons in a jet as the origin of the gamma-rays in BL Lacertae objects. Synchrotron-self-Compton scattering (SSC) or proton-initiated cascades (PIC) remain viable mechanisms. The mechanisms predict very different magnetic field strengths and upper cut-off energies in the gamma-ray sources, i.e. the field strengths in SSC models are more than two orders of magnitude lower than in PIC models and the PIC spectra do not cut-off at TeV. 
Karl Mannheim