EXTRAGALACTIC ULTRA-HIGH ENERGY COSMIC-RAYS .1. CONTRIBUTION FROM HOT-SPOTS IN FR-II RADIO GALAXIES

The hot spots of Fanaroff-Riley class II radio galaxies, considered as working surfaces of highly collimated plasma jets, are proposed to be the dominant sources of the cosmic rays at energies above 1 EeV(a). W e apply the model of first order Fermi acceleration at strong, nonrela tivistic shock waves to the hot spot region. The strength of the model has been demonstrated by Biermann & Strittmatter (1987) and by Meisen heimer et al. (1989), who explain their radio-to-optical spectra and i nfer the physical conditions of the radiating plasma. Using synchrotro n radiating electrons as a trace, we can calculate the spectrum and th e maximum energy of protons accelerated under the same conditions. For simplicity, we disregard heavy nuclei, but their probable role is dis cussed. The normalization of proton flux injected in extragalactic spa ce is performed by using estimates from Rawlings & Saunders (1991) for the total energy stored in relativistic particles inside the jets and radio galaxy evolution models given by Peacock (1985). We calculate t he spectral modifications due to interactions of the protons with the microwave background photons in an evolving universe, following Berezi nsky & Grigor'eva (1988). Constraints on the extragalactic magnetic fi eld can be imposed, since it must permit an almost homogeneous filling of the universe with energetic protons. The observed ultra-high energ y cosmic ray spectrum is reproduced in slope and flux, limited at high energies by the Greisen-cutoff at about 80 EeV. The requirements on t he content of relativistic protons in jets and the constraints to the extragalactic magnetic field are consistent with common estimates. The data beyond the Greisen cutoff for protons may be explained by includ ing heavy nuclei in our model, since they can propagate over cosmologi cal distances up to more than 100 EeV.