  The possible association of the blazar TXS 0506+056 with a high-energy
neutrino detected by IceCube holds the tantalizing potential to answer three
astrophysical questions: 1. Where do high-energy neutrinos originate? 2. Where
are cosmic rays produced and accelerated? 3. What radiation mechanisms produce
the high-energy {\gamma}-rays in blazars? The MeV gamma-ray band holds the key
to these questions, because it is an excellent proxy for photo-hadronic
processes in blazar jets, which also produce neutrino counterparts. Variability
in MeV gamma-rays sheds light on the physical conditions and mechanisms that
take place in the particle acceleration sites in blazar jets. In addition,
hadronic blazar models also predict a high level of polarization fraction in
the MeV band, which can unambiguously distinguish the radiation mechanism.
Future MeV missions with a large field of view, high sensitivity, and
polarization capabilities will play a central role in multi-messenger
astronomy, since pointed, high-resolution telescopes will follow neutrino
alerts only when triggered by an all-sky instrument.
