A search for neutrino emission from cores of Active Galactic Nuclei

TL;DR

This study searches for high-energy neutrino emission from the cores of Active Galactic Nuclei using eight years of IceCube data, finding a potential excess that suggests AGN cores could significantly contribute to observed astrophysical neutrinos.

Contribution

It introduces a novel likelihood analysis targeting AGN cores based on multi-wavelength properties and provides the first evidence of a neutrino excess associated with this population.

Findings

Potential neutrino excess from AGN cores with 2.60σ significance

Best-fit spectral index around 2.03 consistent with particle acceleration

Up to 100% of observed neutrinos at 100 TeV may originate from AGN cores

Abstract

The sources of the majority of the high-energy astrophysical neutrinos observed with the IceCube neutrino telescope at the South Pole are unknown. So far, only a gamma-ray blazar was compellingly associated with the emission of high-energy neutrinos. In addition, several studies suggest that the neutrino emission from the gamma-ray blazar population only accounts for a small fraction of the total astrophysical neutrino flux. In this work we probe the production of high-energy neutrinos in the cores of Active Galactic Nuclei (AGN), induced by accelerated cosmic rays in the accretion disk region. We present a likelihood analysis based on eight years of IceCube data, searching for a cumulative neutrino signal from three AGN samples created for this work. The neutrino emission is assumed to be proportional to the accretion disk luminosity estimated from the soft X-ray flux. Next to the observed soft X-ray flux, the objects for the three samples have been selected based on their radio emission and infrared color properties. For the largest sample in this search, an excess of high-energy neutrino events with respect to an isotropic background of atmospheric and astrophysical neutrinos is found, corresponding to a post-trial significance of 2.60$\sigma$. Assuming a power-law spectrum, the best-fit spectral index is $2.03^{+0.14}_{-0.11}$, consistent with expectations from particle acceleration in astrophysical sources. If interpreted as a genuine signal with the assumptions of a proportionality of X-ray and neutrino fluxes and a model for the sub-threshold flux distribution, this observation implies that at 100 TeV, 27$\%$ - 100$\%$ of the observed neutrinos arise from particle acceleration in the core of AGN.