In September 2017, the IceCube Neutrino Observatory recorded a very-highenergy neutrino in directional coincidence with a blazar in an unusually bright gamma-ray state, TXS0506+056 (1,2). Blazars are prominent photon sources in the universe because they harbor a relativistic jet whose radiation is strongly collimated and amplified. High-energy atomic nuclei known as cosmic rays can produce neutrinos; thus the recent detection may help identifying the sources of the diffuse neutrino flux (3) and the energetic cosmic rays. Here we report on a self-consistent analysis of the physical relation between the observed neutrino and the blazar, in particular the time evolution and spectral behavior of neutrino and photon emission. We demonstrate that a moderate enhancement in the number of cosmic rays during the flare can yield a very strong increase of the neutrino flux which is limited by co-produced hard Xrays and TeV gamma rays. We also test typical radiation models (4, 5) for compatibility and identify several model classes (6, 7) as incompatible with the observations. We investigate to what degree the findings can be generalized to 1 arXiv:1807.04275v3 [astro-ph.HE] 10 May 2019 Accretion disk Supermassive black hole Relativistic jet, Doppler factor = 20-30 Emission region Accelerated protons Accelerated e + /e -Observer at earth 10 pc 1.35 Gpc 0.05 pc Figure 1: Illustration of the emission region of TXS0506+056 traveling at relativistic speed. The distance between radiation zone and the central black hole is not an explicit model parameter and given here only for illustration. Note that the physical sizes of various objects are not drawn to scale. the entire population of blazars, to determine the relation between their output in photons, neutrinos, and cosmic rays, and suggest how to optimize the strategy of future observations.The amplification of radiation from the relativistic jet spawned by the central supermassive black hole in an active galactic nucleus makes the jet the dominant source of emission, if the observer has a frontal view of it, as is the case in a blazar like TXS0506+056. It is therefore appropriate to place the locale of particle acceleration and neutrino emission in the jet of TXS0506+056. An illustration of the structure of a blazar and the location of the emission region is presented in Fig. 1.The emission of very-high-energy radiation requires that the radiating particles, electrons or cosmic nuclei, be accelerated to even higher energies. A widely favored acceleration process is Fermi (diffusive) shock acceleration: charged particles gain energy by the frequent and repeated crossing of a shock front, leading to a particle spectrum in the form of a power law (∝ E −α ) with α > 1; similar spectra are indeed observed in nature. Once accelerated, the energetic

We investigate whether the emission of neutrinos observed in 2014-15 from the direction of the blazar TXS 0506+056 can be accommodated with leptohadronic multi-wavelength models of the source commonly adopted for the 2017 flare. While multi-wavelength data during the neutrino flare are sparse, the large number of neutrino events (13 ± 5) challenges the missing activity in gamma rays. We illustrate that two to five neutrino events during the flare can be explained with leptohadronic models of different categories: a one-zone model, a compact core model, and an external radiation field model. If, however, significantly more events were to be accommodated, the predicted multi-wavelength emission levels would be in conflict with observational X-ray constraints, or with the high-energy gamma ray fluxes observed by the Fermi LAT, depending on the model. For example, while the external radiation field model can predict up to five neutrino events without violating X-ray constraints, the absorption of high-energy gamma rays is in minor tension with data. We therefore do not find any model that can simultaneously explain the high event number quoted by IceCube and the (sparse) electromagnetic data during the neutrino flare.

We discuss the production of ultra-high-energy cosmic ray (UHECR) nuclei and neutrinos from blazars. We compute the nuclear cascade in the jet for both BL Lac objects and flat-spectrum radio quasars (FSRQs), and in the ambient radiation zones for FSRQs as well. By modeling representative spectral energy distributions along the blazar sequence, two distinct regimes are identified, which we call "nuclear survival" -typically found in low-luminosity BL Lacs, and "nuclear cascade"typically found in high-luminosity FSRQs. We quantify how the neutrino and cosmic-ray (CR) emission efficiencies evolve over the blazar sequence, and demonstrate that neutrinos and CRs come from very different object classes. For example, high-frequency peaked BL Lacs (HBLs) tend to produce CRs, and HL-FSRQs are the more efficient neutrino emitters. This conclusion does not depend on the CR escape mechanism, for which we discuss two alternatives (diffusive and advective escape). Finally, the neutrino spectrum from blazars is shown to significantly depend on the injection composition into the jet, especially in the nuclear cascade case: Injection compositions heavier than protons lead to reduced neutrino production at the peak, which moves at the same time to lower energies. Thus, these sources will exhibit better compatibility with the observed IceCube and UHECR data.

We summarize the science opportunity, design elements, current and projected partner observatories, and anticipated science returns of the Astrophysical Multimessenger Observatory Network (AMON). AMON will link multiple current and future high-energy, multimessenger, and follow-up observatories together into a single network, enabling near real-time coincidence searches for multimessenger astrophysical transients and their electromagnetic counterparts. Candidate and high-confidence multimessenger transient events will be identified, characterized, and distributed as AMON alerts within the network and to interested external observers, leading to follow-up observations across the electromagnetic spectrum. In this way, AMON aims to evoke the discovery of multimessenger transients from within observatory subthreshold data streams and facilitate the exploitation of these transients for purposes of astronomy and fundamental physics. As a central hub of global multimessenger science, AMON will also enable cross-collaboration analyses of archival datasets in search of rare or exotic astrophysical phenomena.Comment: 32 pages, 4 figure

We study the frequently used assumption in multi-messenger astrophysics that the gamma-ray and neutrino fluxes are directly connected because they are assumed to be produced by the same photohadronic production chain. An interesting candidate source for this test is the flat-spectrum radio quasar PKS B1424-418, which recently called attention of a potential correlation between an IceCube PeV-neutrino event and its burst phase. We simulate both the multi-waveband photon and the neutrino emission from this source using a self-consistent radiation model. We demonstrate that a simple hadronic model cannot adequately describe the spectral energy distribution for this source, but a lepto-hadronic model with subdominant hadronic component can reproduce the multi-waveband photon spectrum observed during various activity phases of the blazar. As a conclusion, up to about 0.3 neutrino events may coincide with the burst, which implies that the leptonic contribution dominates in the relevant energy band. We also demonstrate that the time-wise correlation between the neutrino event and burst phase is weak.