Interpretation of the Diffuse Astrophysical Neutrino Flux in Terms of the Blazar Sequence

Abstract

We study if the diffuse astrophysical neutrino flux can come from blazar jets -a subclass of Active Galactic Nuclei (AGNs) -while it, at the same time, respects the blazar stacking limit based on source catalogs and is consistent with the observation from TXS 0506+056. We compute the neutrino flux from resolved and unresolved sources using an averaged, empirical relationship between electromagnetic spectrum and luminosity, known as the blazar sequence, for two populations of blazars (BL Lacs and FSRQs). Using a source model with realistic neutrino flux computations, we demonstrate that blazars can indeed power the diffuse neutrino flux at the highest energies and obey the stacking limit at the same time, and we derive the conditions for the baryonic loading (proton versus γ-ray luminosity) evolving over the blazar sequence. Under the hypothesis that low-luminosity blazars power the diffuse astrophysical neutrino flux, we find that the dominant contribution of the diffuse flux up to PeV energies must come from unresolved BL Lacs with baryonic loadings larger than about 10 5 -while only a very small contribution may come from resolved high-luminosity BL Lacs or FSRQs, which can be directly tested by the stacking limit. We find that the blazar TXS 0506+056 is on the verge of these populations in our baseline scenario, at a relatively high luminosity and redshift; as a consequence we predict about 0.3 γ-ray-neutrino associations per year from the whole population, dominated by BL Lacs with L γ 10 45 erg/s and z ∼ 0.1. arXiv:1806.04769v4 [astro-ph.HE]

Multi-epoch Modeling of TXS 0506+056 and Implications for Long-term High-energy Neutrino Emission

“…Nonetheless, our results can constrain some extreme models. For reference, we also show the baryon loading factor and its uncertainty (solid blue line and shaded region) invoked to explain the diffuse astrophysical neutrino flux at energies 1 PeV with blazars (for details, see Palladino et al 2019). Although there is no physically motivated scenario to predict a negative correlation between the baryon loading factor and gamma-ray luminosity, our results demonstrate that multi-epoch modeling of even a single source at different gamma-ray luminosity levels can be a powerful method for constraining models of diffuse neutrino emission from blazars.…”

“…Maximal baryon loading factor ξ of TXS 0506+056 (filled symbols with arrows) as a function of the Fermi-LAT (0.1-300 GeV) gamma-ray luminosity for different epochs (see colorbar). For comparison, we show the baryon loading factor (solid blue line) with its uncertainty (shaded region) obtained from a model for the diffuse astrophysical neutrino flux at energies 1 PeV from blazars (see scenario 3 in Palladino et al 2019). Note-Lγ is computed using the best-fit power-law model reported in Table 6, except for the 2017 flare, for which we adopted the value from Keivani et al (2018).…”

The opacity of the extragalactic medium to gamma radiation

“…Using equation 3.8 for BL Lacs we get 8033 sources and for FSRQ we obtain 1138, values similar to those found in [64] (8019 for BL Lacs and 1167 for FSRQ), where the authors use these GLF models to study the diuse astrophysical neutrino emission. Figure 3.7 shows the redshift, luminosity and photon spectral index distribution for blazars.…”

On the sources of high energy neutrinos

“…Therefore L ν ∝ L 2 γ for low luminosity BL Lacs and L ν ∝ L γ for high luminosity ones. In [8] it has been shown that low luminosity BL Lacs (mainly unresolved objects) may give an important contribution to the neutrino flux at sub-PeV energy, under the assumption that these objects are rich of protons. In that model the contribution of high luminosity sources, that would violate the IceCube stacking limit, is suppressed reducing the amount of protons in high luminosity objects.…”

“…On the sources of high energy neutrinos Andrea Palladino Figure 2: On the left panel: interpretation of the most energetic IceCube events in terms of blazars (pγ source, yellow band) [8]. On the right panel: interpretation of the most energetic IceCube data in terms of pp sources (blue curve) [11].…”