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There is mounting evidence that ultra-energetic neutrinos of astrophysicalorigin may be associated with blazars. Here we investigate a unique sample of47 blazars, $\sim 20$ of which could be new neutrino sources. In particular, wefocus on 17 objects of yet unknown redshift, for which we present opticalspectroscopy secured at the Gran Telescopio Canarias and the ESO Very LargeTelescope. We find all sources but one (a quasar) to be BL Lac objects. Fornine targets we are able to determine the redshift (0.09~$<$~z~$<$~1.6), whilefor the others we set a lower limit on it, based on either the robust detectionof intervening absorption systems or on an estimation derived from the absenceof spectral signatures of the host galaxy. In some spectra we detect forbiddenand semi-forbidden emission lines with luminosities in the range $10^{40} -10^{41}$ erg s$^{-1}$. We also report on the spectroscopy of seven blazarspossibly associated with energetic neutrinos that partially meet the criteriaof our sample and are discussed in the Appendix. These results represent thestarting point of our investigation into the real nature of these objects andtheir likelihood of being neutrino emitters.

The spectra of IceCube neutrino candidate sources -- I. Optical spectroscopy of blazars