The contribution of Fermi-2LAC blazars to the diffuse TeV-PeV neutrino flux

Abstract: The recent discovery of a diffuse cosmic neutrino flux extending up to PeV energies raises the question of which astrophysical sources generate this signal. One class of extragalactic sources which may produce such high-energy neutrinos are blazars. We present a likelihood analysis searching for cumulative neutrino emission from blazars in the 2nd Fermi-LAT AGN catalogue (2LAC) using an IceCube neutrino dataset 2009-12 which was optimised for the detection of individual sources. In contrast to previous searches with IceCube, the populations investigated contain up to hundreds of sources, the largest one being the entire blazar sample in the 2LAC catalogue. No significant excess is observed and upper limits for the cumulative flux from these populations are obtained. These constrain the maximum contribution of the 2LAC blazars to the observed astrophysical neutrino flux to be $27 \%$ or less between around 10 TeV and 2 PeV, assuming equipartition of flavours at Earth and a single power-law spectrum with a spectral index of $-2.5$. We can still exclude that the 2LAC blazars (and sub-populations) emit more than $50 \%$ of the observed neutrinos up to a spectral index as hard as $-2.2$ in the same energy range. Our result takes into account that the neutrino source count distribution is unknown, and it does not assume strict proportionality of the neutrino flux to the measured 2LAC $\gamma$-ray signal for each source. Additionally, we constrain recent models for neutrino emission by blazars.

• The paper uses IceCube's 2009–2012 neutrino data and an advanced likelihood analysis to assess Fermi-2LAC blazars' potential neutrino production.
• It establishes upper limits on the neutrino flux, constraining the blazars' cumulative contribution to less than 27% for a spectral index of −2.5.
• The results challenge existing models linking gamma-ray and neutrino emissions, prompting further multi-messenger investigations of cosmic accelerators.

Analysis of the Contribution of Fermi-2LAC Blazars to the Diffuse TeV-PeV Neutrino Flux

The study by M.G. Aartsen et al., conducted by the IceCube Collaboration, presents a comprehensive analysis of the potential contributions of high-energy neutrinos from blazars recorded in the Fermi Second LAT AGN Catalog (2LAC) to the diffuse neutrino flux observed between TeV and PeV energies. Through this research, the authors aimed to rigorously assess whether observed neutrino signals could be associated with blazar populations, given the assumptions about such astrophysical objects being prolific cosmic ray accelerators capable of producing neutrinos via hadronic interactions.

Key Methodologies and Analytical Approach

The research utilizes an advanced likelihood analysis approach, leveraging IceCube's neutrino dataset from 2009-2012. This dataset was specifically optimized for gigantic detection environments like blazars, characterized by relativistic jets that potentially produce high-energy neutrinos through interactions such as $pp$ or $p\gamma$. The authors strategically analyze cumulative neutrino emissions based on the Fermi-2LAC data, categorizing different blazar subtypes for scrutiny: Flat Spectrum Radio Quasars (FSRQs), Low, Intermediate, and High Synchrotron Peak blazars (LSP, ISP, HSP), and BL Lac objects aggregated in various configurations.

Innovatively, this analysis incorporates not only standard $\gamma-$ray weighting proportional methods, presupposing a connection between observed $\gamma-$ray and neutrino outputs, but also a model-independent analysis through equal-weighting schemes. These methodological advancements facilitate the mitigation of potential biases linked to variations in the correlation between the $\gamma$ luminosity and neutrino production.

Results and Implications

The findings illustrate that no significant neutrino excess directly aligns with the examined blazar positions, with the derived upper limits on neutrino flux challenging several existing hypotheses about blazar contributions to the neutrino flux. Notably, it constrains the cumulative contribution of the 2LAC blazars to the observed astrophysical neutrino flux to less than 27% over the entire analyzed range under the equipartition assumption and a spectral index of $-2.5$. Furthermore, it excludes scenarios where these blazars collectively emit more than 50% of observed neutrinos for a spectral index of $-2.2$.

From a theoretical viewpoint, these results significantly narrow down the viable parameter space for models predicting diffuse neutrino emissions from blazars, adding stringent constraints especially to those that do not differentiate among blazar classes. The implications suggest that explanations for the diffuse neutrino flux might need to shift towards considering other source populations or distributions of extragalactic objects, reassessing potential neutrino production mechanisms within blazars.

Speculative Outlook and Future Pathways

The outcome places the role of blazars, particularly those identifiable in the 2LAC catalog, within a broader cosmic ray acceleration and neutrino production framework, inviting further observational and theoretical scrutiny. Future efforts, armed with enhanced observational capabilities and additional multi-messenger data, could aim to explore unresolved blazar components or explore precise detection of individual or clustered events around known blazar locations during high-activity periods. These investigations are essential in refining our understanding of neutrino astrophysics and in probing deeper connections within the dynamic cosmic-ray environment. Overall, while the current study delineates existing limitations, it also opens pathways to new avenues in identifying astrophysical accelerators responsible for ultra-high-energy cosmic phenomena.