Neutrino oscillations and blazars

Three independent predictions follow from postulating the existence of prot ons co-accelerated with electrons in extragalactic jets (i) multi-TeV gamma ray emission from nearby blazars, (ii) extragalactic cosmic ray protons up to similar to 10(20) eV, and (iii) extragalactic neutrinos up to similar t o 5 x 10(18) eV. Recent gamma ray observations of Mrk 421 and Mrk 501 emplo ying the air-Cerenkov technique are consistent with the predicted gamma ray spectrum, if one corrects for pair attenuation on the infrared background. Prediction (ii) is consistent with cosmic ray data, if one requires that j ets are responsible for a at least a sizable fraction of the extragalactic gamma ray background. With cubic kilometer neutrino telescopes, it will be possible to test (iii), although the muon event rates are rather low. Neutr ino oscillations can increase the event rate by inducing tau-cascades remov ing the so-called Earth shadowing effect. (C) 1999 Elsevier Science B.V. Al l rights reserved.