Relativistic Electrons in Blazars: A Hadronic Origin?

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Abstract
Recent observations of blazars have established that their γ-ray emission is associated, as a rule, with very fast variability (as short as ~15 minutes for the TeV photons of Mrk 421); as such, these observations push the theoretical models for the production of the required relativistic electrons to their limits. Herein we investigate the possibility that "blobs" loaded with relativistic protons could produce such an activity. We show that, if the proton number density in a blob exceeds a certain critical value, then reflection of its own synchrotron produced photons on some external "mirror," such as a line-emitting cloud, can initiate a feedback process in which the protons can lose most of their energy content in a blob crossing time, resulting in a flare of the same duration. By performing a dimensional analysis, we find the necessary conditions for such an instability to occur, and we show that the conditions required are consistent with those usually assumed to prevail within the relativistic jets of this class of active galactic nuclei.