The Contribution of Fermi-2lac Blazars to 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. Blazars are one class of extragalactic sources which may produce such high-energy neutrinos. We present a likelihood analysis searching for cumulative neutrino emission from blazars in the 2nd Fermi-LAT AGN catalog (2LAC) using IceCube neutrino data set 2009-12, which was optimized for the detection of individual sources. In contrast to those in previous searches with IceCube, the populations investigated contain up to hundreds of sources, the largest one being the entire blazar sample in the 2LAC catalog. No significant excess is observed, and upper limits for the cumulative flux from these populations are obtained. These constrain the maximum contribution of 2LAC blazars to the observed astrophysical neutrino flux to 27% or less between around 10 TeV and 2 PeV, assuming the equipartition of flavors on Earth and a single power-law spectrum with a spectral index of −2.5. We can still exclude the fact that 2LAC blazars (and their subpopulations) 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 the fact that the neutrino source count distribution is unknown, and it does not assume strict proportionality of the neutrino flux to the measured 2LAC γ-ray signal for each source. Additionally, we constrain recent models for neutrino emission by blazars.

The Blazar TXS 0506+056 Associated with a High-energy Neutrino: Insights into Extragalactic Jets and Cosmic-Ray Acceleration

“…An interesting question concerns the potential contribution of the entire BL Lac population to the diffuse neutrino emission detected by IceCube (Aartsen et al 2017). In principle, BL Lac objects and FSRQs could account for a sizable fraction of the observed intensity (Tavecchio et al 2014;Righi et al 2017).…”

Constraints on Galactic Neutrino Emission with Seven Years of IceCube Data

“…Several extragalactic candidates have been shown to have a sub-dominant contribution to the flux. Notably, blazars are constrained to contribute less than 27% of the flux for -E 2.5 or 50% if the spectrum is as hard as -E 2.2 (Aartsen et al 2017). Prompt emission from triggered gamma-ray bursts are strongly constrained to <1% contribution to the astrophysical flux (Aartsen et al 2017b).…”

A Search for Neutrino Emission from Fast Radio Bursts with Six Years of IceCube Data

“…While the astrophysical neutrino flux has been detected in multiple channels with high significance, neither clustering in space or time nor cross correlations to catalogs have been found (Aartsen et al 2017a). The once promising sources for highenergy neutrinos such as GRBs (Abbasi et al 2012;Aartsen et al 2015e, 2017b and blazars (Aartsen et al 2017c) have been disfavored as the major contributors to the observed flux. To date, the origin of the astrophysical neutrinos remains a mystery.…”