Disentangling the Hadronic Components in NGC 1068
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 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 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 Fermi-LAT observations. We find that the starburst hadronic component is responsible for NGC 1068's emission above 500\,MeV. However, below this energy an additional component is required. In the 20-500\,MeV range the 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.
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