A proton synchrotron blazar model for flaring in Markarian 501

The spectral energy distribution (SED) of gamma-ray (gamma -ray) loud BL La c objects typically has a double-humped appearance usually interpreted in t erms of synchrotron self-Compton models. In proton blazar models, the SED i s instead explained in terms of acceleration of protons and subsequent casc ading. We discuss a variation of the synchrotron proton blazar model, first proposed by Mucke and Protheroe (Proc. Workshop GeV-TeV Astrophysics: Towa rd a Major Atmospheric Cherenkov Telescope VI, Snowbird, Utah, submitted fo r publication), in which the low energy part of the SED is mainly proton sy nchrotron radiation by electrons co-accelerated with protons, which produce the high energy part of the SED mainly as synchrotron radiation. As an approximation, we assume non-relativistic shock acceleration which co uld apply if the bulk of the plasma in the jet frame were non-relativistic. Our results may therefore change if a relativistic equation of state was u sed. We consider the case where the maximum energy of the accelerated proto ns is above the threshold for pion photoproduction interactions on the sync hrotron photons of the low energy part of the SED. Using a Monte Carlo/nume rical technique to simulate the interactions and subsequent cascading of th e accelerated protons, we are able to fit the high-energy gamma -ray portio n of 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) dec ay and by e(+/-) from mu (+/-) decay turn out to be relatively featureless. Synchrotron radiation produced by mu (+/-) from pi (+/-) decay, and even m ore importantly by protons, and subsequent synchrotron-pair cascading, is a ble to reproduce well the high energy part of the SED. For this fit, we fin d that synchrotron radiation by protons dominates the TeV emission, pion ph otoproduction being less important with the consequence that we predict a l ower neutrino flux than in other proton blazar models. (C) 2001 Elsevier Sc ience B.V. All rights reserved.