Application of the synchrotron proton blazar model to BL Lac objects
  We apply the synchrotron proton blazar (SPB) model to the April 1997 flare of Markarian 501 and find we are able to fit the observed spectral energy distribution. We explore the effect of target photon density on the high energy part of the spectral energy distribution (SED) for fixed assumed magnetic field, emission region size and Doppler factor and find that the luminosity and peak frequency of the high energy part of the SED may depend on the luminosity of the low energy part of the SED in high-frequency peaked BL Lac objects (HBL). 
R.J. Protheroe and A. Muecke