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

TL;DR

This study presents optical spectroscopy of 17 blazars, determining redshifts and spectral features, to assess their potential as sources of astrophysical neutrinos, advancing understanding of blazar-neutrino associations.

Contribution

It provides new optical spectroscopic data for blazars with unknown redshifts, identifying potential neutrino sources and characterizing their spectral properties.

Findings

Most sources are BL Lac objects.

Redshifts determined for nine targets.

Detection of emission lines with luminosities of 10^{40}-10^{41} erg s^{-1}.

Abstract

There is mounting evidence that ultra-energetic neutrinos of astrophysical origin may be associated with blazars. Here we investigate a unique sample of 47 blazars, $\sim 20$ of which could be new neutrino sources. In particular, we focus on 17 objects of yet unknown redshift, for which we present optical spectroscopy secured at the Gran Telescopio Canarias and the ESO Very Large Telescope. We find all sources but one (a quasar) to be BL Lac objects. For nine targets we are able to determine the redshift (0.09~$<$~z~$<$~1.6), while for the others we set a lower limit on it, based on either the robust detection of intervening absorption systems or on an estimation derived from the absence of spectral signatures of the host galaxy. In some spectra we detect forbidden and semi-forbidden emission lines with luminosities in the range $10^{40} - 10^{41}$ erg s$^{-1}$. We also report on the spectroscopy of seven blazars possibly associated with energetic neutrinos that partially meet the criteria of our sample and are discussed in the Appendix. These results represent the starting point of our investigation into the real nature of these objects and their likelihood of being neutrino emitters.