The Neutrino-BL LAC Connection
Despite intense inquiry, the sources at the origin of the high-energy neutrinos revealed by IceCube are still unknown. The potential correlation of an IceCube event with a flaring gamma-ray source has recently focused the attention on BL Lac objects, active galactic nuclei with a relativistic jet pointing to the Earth. Since the nuclei of these sources lack bright thermal components, it is generally expected that, for acceptable values of the high-energy proton luminosity, the neutrino output (mediated by photopion reactions with photons produced within the jet) is low. I will discuss how this problem can be overcome considering alternative sources of soft photons external to the jet, as a slow sheath surrounding the jet or the (likely radiatively ineffcient) accretion ow.