![]() Both models predict a substantial neutrino flux, which is correlated with the gamma-ray and soft X-ray fluxes. We find two hadronic models that can both describe the multi-wavelength emission during all three states: a leptohadronic model with a contribution from photo-hadronic processes to X-rays and high-energy gamma rays, and a proton synchrotron model, where the emission from keV to 10 GeV comes from proton synchrotron radiation. ![]() We identify three different activity states of the blazar: the quiescent state, and two distinct flaring states with hard and soft gamma-ray spectra. We perform multi-wavelength and multi-messenger modeling of this source, using a fully self-consistent one-zone model, which includes the contribution of external radiation fields typical of flat-spectrum radio quasars (FSRQs). In July of 2019, the IceCube experiment detected a high-energy neutrino from the direction of the powerful blazar PKS 1502 106. Remarkably, if cosmic ray halos, as the one observed around M31, are a common feature of galaxies, including our own, the interactions between cosmic ray protons and the Milky Way circumgalactic gas could also explain the isotropic diffuse flux of neutrinos observed by Icecube. It would imply the existence of a giant cosmic ray halo surrounding M31, possibly powered by the galaxy nuclear activity, or by accretion of intergalactic gas. In this paper, we argue that a cosmic ray origin (either leptonic or hadronic) of the $\gamma$-ray emission is possible in the framework of non standard cosmic ray propagation scenarios or in the case of particle acceleration taking place in the galaxy's halo. Explaining the extended $\gamma-$ray emission within the framework of standard scenarios for the escape of cosmic rays injected in the galactic disk or in the galactic center is problematic. The emission is centered on the galaxy, and extends for $\sim 100-200$ kpc away from its center. Recently, a diffuse emission of 1-100 GeV $\gamma$-rays has been detected from the direction of Andromeda. Other possible scenarios are also discussed. It is demonstrated that the observational data may be explained if the flux of astrophysical neutrinos includes the contribution of extragalactic sources, dominating at the highest energies, and the Galactic component, significant only at neutrino energies <~100 TeV. This review summarizes the experimental results with emphasis on those important for constraining theoretical models, discusses various scenarios for the origin of high-energy neutrinos and briefly lists particualr classes of their potential astrophysical sources. The origin of these neutrinos has not been conclusively established, and simple theoretical models, popular for decades, cannot explain all observational data. The observational results do not fully agree with what was expected before the start of these experiments. The existence of astrophysical neutrinos with energies of tens of TeV and higher has been reliably established by the IceCube experiment the first confirmations of this discovery are being obtained with the ANTARES and Baikal-GVD facilities.
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