Constraints on the Cosmic Neutrino Background from NGC 1068

We use recent evidence of TeV neutrino events from NGC 1068, detected by the IceCube experiment, to constrain the overdensity of relic neutrinos locally and globally. Since these high-energy neutrinos have travelled long distances through a sea of relic neutrinos, they could have undergone scattering, altering their observed flux on Earth. Considering only Standard Model interactions, we constrain the relic overdensity to be η≤ 3.85 × 10^8 (5.39 × 10^11) at the 95% confidence level for overdensities with a radius of 14 Mpc (10 kpc), assuming the sum of neutrino masses saturates the cosmological bound, ∑_i m_i = 0.13 eV. We demonstrate that this limit improves with larger neutrino masses and how it depends on the scale of the overdensity region.