The Spectra of IceCube Neutrino (SIN) candidate sources -- II. Source
  Characterisation
  Eight years after the first detection of high-energy astrophysical neutrinos
by IceCube we are still almost clueless as regards to their origin, although
the case for blazars being neutrino sources is getting stronger. After the
first significant association at the $3 - 3.5\,\sigma$ level in time and space
with IceCube neutrinos, i.e. the blazar TXS 0506+056 at $z=0.3365$, some of us
have in fact selected a unique sample of 47 blazars, out of which $\sim 16$
could be associated with individual neutrino track events detected by IceCube.
Building upon our recent spectroscopy work on these objects, here we
characterise them to determine their real nature and check if they are
different from the rest of the blazar population. For the first time we also
present a systematic study of the frequency of masquerading BL Lacs, i.e.
flat-spectrum radio quasars with their broad lines swamped by non-thermal jet
emission, in a $\gamma$-ray- and IceCube-selected sample, finding a fraction
$>$ 24 per cent and possibly as high as 80 per cent. In terms of their
broad-band properties, our sources appear to be indistinguishable from the rest
of the blazar population. We also discuss two theoretical scenarios for
neutrino emission, one in which neutrinos are produced in interactions of
protons with jet photons and one in which the target photons are from the broad
line region. Both scenarios can equally account for the neutrino-blazar
correlation observed by some of us. Future observations with neutrino
telescopes and X-ray satellites will test them out.
