Limits on the source properties of FR-I galaxies from high-energy neutrino and gamma observations

TL;DR

This paper uses high-energy neutrino and gamma-ray observations to constrain the properties of FR-I galaxies, shedding light on cosmic ray acceleration sites and their physical conditions.

Contribution

It provides new limits on target densities in FR-I galaxies using IceCube neutrino data and models gamma-ray fluxes for Cen A and M87.

Findings

Target densities are constrained to be below 30-2000 cm^{-3}.

Some AGN core acceleration sites are excluded based on neutrino limits.

Neutrino detection could reveal details of particle acceleration processes.

Abstract

Active galactic nuclei (AGN) are believed to be the source of ultra high energy cosmic rays (UHECRs). Particles are assumed to be accelerated in the accretion disk and the plasma jets, produced due to conservation of angular momentum, to the highest energies, where they interact with each other and produce pions, which decay among others in neutrinos. For a known cosmic ray spectral behavior, the main parameters in the calcula- tion of the neutrino flux from proton-proton interactions are the target density nH and the ratio of electrons to protons fe . Using most recent neutrino flux limits from IceCube point source searches, we set limits on the target densities for 33 FR-I galaxies. The densities are shown to be smaller than 30 cm^{-3} to 2000 cm^{-3}, depending on the source and when using a fixed electron to proton ratio of f_{e} = 0.1. This implies that some cosmic ray acceleration sites, espe- cially those close to the core of the AGN, can already be excluded, or else that the ratio of electrons to protons deviates significantly from the commonly used value of 0.1. For Centaurus A (Cen A) and Messier 87 (M 87) we use Fermi observations to model the {\gamma}-flux, the neutrino flux and the resulting target density. The detec- tion of these neutrinos will help to find information about acceleration processes in the source.