TeV gamma rays from BL Lac objects due to synchrotron radiation of extremely high energy protons

One of remarkable features of the gamma-ray blazar Markarian 501 is the reported shape of the TeV spectrum, which during strong flares of the source remains essentially stable despite dramatic variations of the absolute gamma -ray flux. I argue that this unusual behavior of the source could be explained assuming that the TeV emission is a result of synchrotron radiation of extremely high energy (E greater than or equal to 10(19) eV) protons in highly magnetized (B similar to 30-100 G) compact regions of the jet with typical size R similar to 10(15) - 10(16) cm and Doppler factor delta (j) similar or equal to 10-30. It is shown that if protons are accelerated at the maximum possible rate, ie t(acc) = eta (r(g)/c) with so-called gyro-factor eta similar to1, the synchroton cooling of protons could not only dominate over other radiative and non-radiative loses, but could also provide good fits (within uncertainties introduced by extragalactic gamma -ray extinction) to the gamma -radiation of two firmly established TeV blazars - Markarian 501 and Markarian 421. Remarkably, if the proton acceleration takes place in the regime dominated by synchrotron losses, the spectral shape of the Doppler-boosted gamma -radiation in the observer's frame is determined essentially by the self-regulated "synchrotron cutoff" at epsilon (o) similar or equal to 0.3 delta (j) eta (-1) TeV. The hypothesis of the proton- synchrotron origin of TeV flares of BL Lac objects inevitably implies that the energy contained in the form of magnetic field in the gamma -ray emitting region exceeds the kinetic energy of accelerated protons. (C) Elsevier Science B.V. All rights reserved.