Electromagnetic Radiation from Cosmic-Ray Scatterings on Relic Neutrinos

TL;DR

This paper estimates gamma-ray and X-ray fluxes from cosmic-ray interactions with relic neutrinos, setting new constraints on neutrino background overdensity using Fermi-LAT data and discussing future detection prospects.

Contribution

First estimate of gamma-ray flux from cosmic-ray scatterings on relic neutrinos and new bounds on neutrino overdensity from gamma-ray observations.

Findings

Limit on neutrino overdensity: η ≲ 2×10^4 for m_ν ≳ 0.1 eV

Gamma-ray data constrains neutrino background more strongly than current lab experiments

X-ray emission provides complementary constraints

Abstract

Cosmic-ray scatterings on the cosmic neutrino background induce a flux of gamma-rays and X-rays from boosted meson decays and charged lepton processes. Here we present the first estimate of this flux and its cumulative cosmological contribution. Confronting expectations with Fermi-LAT diffuse gamma-ray data, we find a limit on the cosmic neutrino background overdensity at the level of $\eta \lesssim 2 \times 10^{4}$ for a lightest neutrino mass of $m_{\nu}\gtrsim0.1$ eV, orders of magnitude stronger than current direct laboratory probes, and comparable to constraints on the cosmic neutrino background obtained with IceCube. We further show that X-ray synchrotron emission from cascade electron-positron pairs in intergalactic magnetic fields provides a complementary, albeit weaker, constraint. We discuss how anisotropic signatures and future gamma-ray data from CTA could further improve bounds on the relic neutrino overdensity, approaching in sensitivity the $\Lambda$CDM expected density.