Astroparticle Physics

168 7 Secondary Cosmic Raysactive galactic nucleiblazarsBL-Lacertae objects, quasarsenergy conversion efficiencyparticle jets from blazarssupergalactic origin?Active galactic nuclei (AGN’s) are frequently discussedas possible sources for the highest cosmic-ray energies. Inthis group of galaxies blazars play an outstanding rôle.Blazar is a short for sources belonging to the class of BL-Lacertae objects and quasars. BL-Lacertae objects, equallyas quasars, are Milky Way-like sources, whose nuclei outshinethe whole galaxy making them to appear like stars.While the optical spectra of quasars exhibit emission andabsorption lines, the spectra of BL-Lacertae objects showno structures at all. This is interpreted in such a way thatthe galactic nuclei of quasars are surrounded by dense gas,while BL-Lacertae objects reside in low-gas-density ellipticalgalaxies.A characteristic feature of blazars is their high variability.Considerable brightness excursions have been observedon time scales as short as a few days. Therefore, these objectsmust be extremely compact, because the size of thesources can hardly be larger than the time required for lightto travel across the diameter of the source. It is generally assumedthat blazars are powered by black holes at their center.The matter falling into a black hole liberates enormousamounts of energy. While in nuclear fission only 1‰ andin nuclear fusion still only 0.7% of the mass is transformedinto energy, an object of mass m can practically liberate allits rest energy mc 2 if it is swallowed by a black hole.Many high-energy γ -ray sources which were found bythe CGRO (Compton Gamma Ray Observatory) satellite,could be correlated with blazars. This led to the conjecturethat these blazars could also be responsible for the accelerationof the highest-energy particles. The particle jetsproduced by blazars exhibit magnetic fields of more than10 Gauss and extend over 10 −2 pc and more. Therefore,according to (7.20), particles could be accelerated to energiesexceeding 10 20 eV. If protons are accelerated in suchsources they could easily escape from these galaxies, becausetheir interaction strength is smaller than that of theelectrons which must certainly be accelerated as well. Ifthese arguments are correct, blazars should also be a richsource of high-energy neutrinos. This prediction can betested with the large water (or ice) Cherenkov counters.It has already been mentioned before that for protonsto arrive at Earth the sources must not be at too large distances.The best candidates for sources should therefore liein the supergalactic plane. The local supergalaxy is a kind of