Galactic outflow driven by the active nucleus and the origin of the gamma-ray emission in NGC 1068

TL;DR

This paper models gamma-ray and radio emissions in NGC 1068 as driven by AGN shocks, comparing predictions with observations and exploring particle acceleration efficiencies and neutrino fluxes.

Contribution

It introduces a detailed model of AGN-driven shocks as a source of gamma-ray and radio emissions, comparing it with other sources like SNRs and jets.

Findings

AGN shocks contribute significantly to gamma-ray emission.

Standard acceleration efficiencies can explain observed gamma-rays.

Predicted neutrino fluxes are within reach of upcoming detectors.

Abstract

We compute the non-thermal emissions produced by relativistic particles accelerated by the AGN-driven shocks in NGC 1068, and we compare the model predictions with the observed gamma-ray and radio spectra . The former is contributed by pion decay, inverse Compton scattering, and bremsstrahlung, while the latter is produced by synchrotron radiation. We derive the gamma-ray and radio emissions by assuming the standard acceleration theory, and we discuss how our results compare with those corresponding to other commonly assumed sources of gamma-ray and radio emissions, like Supernova remnants (SNR) or AGN jets. We find that the AGN-driven shocks observed in the circumnuclear molecular disk of such a galaxy provide a contribution to the gamma-ray emission comparable to that provided by the starburst activity when standard particle acceleration efficiencies are assumed, while they can yield the whole gamma-ray emission only when the parameters describing the acceleration efficiency and the proton coupling with the molecular gas are tuned to values larger than those assumed in standard, SNR-driven shocks. We discuss the range of acceleration efficiencies (for protons and electrons) and of proton calorimetric fractions required to account for the observed gamma-ray emission in the AGN outflow model. We further compare the neutrino flux expected in our model with constraints from current experiments, and we provide predictions for the detections by the upcoming KM3NeT neutrino telescope. This analysis strongly motivates observations of NGC 1068 at >TeV energies with current and future Cherenkov telescopes in order to gain insight into the nature of the gamma-rays source.