  (abr.) The spectral energy distribution (SED) of blazars typically has a
double-humped appearance usually interpreted in terms of synchrotron
self-Compton models. In proton blazar models, the SED is instead explained in
terms of acceleration of protons and subsequent cascading. We discuss a
variation of the Synchrotron Proton Blazar model, first proposed by M\"ucke &
Protheroe (1999), in which the low energy part of the SED is mainly synchrotron
radiation by electrons co-accelerated with protons which produce the high
energy part of the SED mainly asproton synchrotron radiation. Using a Monte
Carlo/numerical technique to simulate the interactions and subsequent cascading
of the accelerated protons, we are able to fit the observed SED of Markarian
501 during the April 1997 flare. We find that the emerging cascade spectra
initiated by gamma-rays from $\pi^0$ decay and by $e^\pm$ from $\mu^\pm$ decay
turn out to be relatively featureless. Synchrotron radiation produced by
$\mu^\pm$ from $\pi^\pm$ decay, and even more importantly by protons, and
subsequent synchrotron-pair cascading, is able to reproduce well the high
energy part of the SED. For this fit we find that synchrotron radiation by
protons dominates the TeV emission, pion photoproduction being less important
with the consequence that we predict a lower neutrino flux than in other proton
blazar models.
