Hadronic origin of the very high-energy gamma-ray emission from the low-luminosity AGN in NGC 4278

TL;DR

This study suggests that the very high-energy gamma rays from NGC 4278 originate from hadronic interactions involving cosmic ray protons in a surrounding molecular cloud, with implications for neutrino and X-ray emissions.

Contribution

It proposes a hadronic model explaining VHE gamma rays from a low-luminosity AGN, linking cosmic ray acceleration, molecular cloud interactions, and multi-messenger signals.

Findings

Gamma-ray spectra can be explained by hadronic processes in NGC 4278.

The AGN activity was likely higher in the past.

Future X-ray observations could test the model.

Abstract

The Large High Altitude Air Shower Observatory has detected very high-energy (VHE) gamma rays from NGC 4278, which is known to host a low-luminosity active galactic nucleus (AGN). Having only very weak radio jets, the origin of its VHE gamma rays is unclear. In this paper we first show that NGC 4278 has a massive molecular cloud surrounding the nucleus by analyzing data taken with the Atacama Large Millimeter/submillimeter Array. We then assume that cosmic ray protons are accelerated in a radiatively inefficient accretion flow around the supermassive black hole, which diffuse into the molecular cloud and produce gamma rays and neutrinos via $pp$ interactions. We model the gamma-ray spectra and find that the observations can be explained by such hadronic processes if the AGN activity was higher in the past than at present, and the diffusion coefficient in the molecular cloud is appreciably smaller than in the Milky Way interstellar medium. We also show that although the high-energy neutrinos co-produced with the gamma rays are unlikely to be detectable even with IceCube-Gen2, the accompanying synchrotron X-ray emission due to pion-decay secondary electrons and positrons may be detectable in the future, providing a valuable test of our hadronic model.