Disentangling the Hadronic Components in NGC 1068

TL;DR

This paper analyzes NGC 1068's gamma-ray spectrum from 20 MeV to 1 TeV, revealing that starburst activity explains high-energy emissions above 500 MeV, while near the SMBH, additional hadronic processes dominate at lower energies, emphasizing the significance of the MeV band.

Contribution

It provides the first detailed spectrum of NGC 1068 across 20 MeV to 1 TeV, identifying different hadronic emission components near the SMBH and starburst regions.

Findings

Starburst hadronic component explains emission above 500 MeV.

Additional hadronic component is needed below 500 MeV near SMBH.

Highlights the importance of the MeV band for detecting hidden cosmic-ray accelerators.

Abstract

The recent detection of high-energy neutrinos by IceCube in the direction of the nearby Seyfert/starburst galaxy NGC 1068 implies that radio-quiet active galactic nuclei can accelerate cosmic-ray ions. Dedicated multi-messenger analyses suggest that the interaction of these high-energy ions { with ambient gas or photons} happens in a region of the galaxy that is highly opaque for GeV-TeV gamma rays. Otherwise, the GeV-TeV emission would violate existing constraints provided by {\it Fermi}-LAT and MAGIC. The conditions of high optical depth are realized near the central super-massive black hole (SMBH). At the same time, the GeV emission detected by the {\it Fermi}-Large Area Telescope (LAT) is likely related to the galaxy's sustained star-formation activity. In this work, we derive a 20\,MeV - 1\,TeV spectrum of NGC 1068 using 14\,yrs of {\it Fermi}-LAT observations. We find that the starburst hadronic component is responsible for NGC 1068's emission above $\sim$500\,MeV. However, below this energy an additional component is required. In the 20-500\,MeV range the {\it Fermi}-LAT data are consistent with hadronic emission {initiated by non-thermal ions interacting with gas or photons} in the vicinity of the central SMBH. This highlights the importance of the MeV band to discover hidden cosmic-ray accelerators.