Ojha, R.Zhang, H.Kadler, M.Neilson, N. K.Kreter, M.McEnery, J.Buson, S.Caputo, R.Coppi, P.D'Ammando, F.Angelis, A. DeFang, K.Giannios, D.Guiriec, S.Guo, F.Kopp, J.Krauss, F.Li, H.Meyer, M.Moiseev, A.Petropoulou, M.Prescod-Weinstein, C.Rani, B.Shrader, C.Venters, T.Wadiasingh, Z.2019-03-282019-03-282019-03-13R. Ojha, H. Zhang, et.al, Neutrinos, Cosmic Rays and the MeV Band , Astrophysics , High Energy Astrophysical Phenomena, 2019, https://arxiv.org/abs/1903.05765http://hdl.handle.net/11603/13226The 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.9 pagesen-USThis item is likely protected under Title 17 of the U.S. Copyright Law. Unless on a Creative Commons license, for uses protected by Copyright Law, contact the copyright holder or the author.high-energy neutrinoMeV gamma-ray bandmulti-messenger astronomyhigh-resolution telescopesText