[Paper Review] IceCube: Neutrinos from Active Galaxies

F. Halzen|arXiv (Cornell University)|May 11, 2023

Astrophysics and Cosmic Phenomena4 citations

TL;DR

This paper presents strong evidence that active galaxies, particularly NGC 1068, are sources of high-energy cosmic neutrinos, with IceCube detecting a significant excess of neutrinos at the position of NGC 1068 after reanalyzing 10 years of data. The findings indicate that neutrinos originate in dense, gamma-ray-obscured regions near supermassive black holes, within 10–100 Schwarzschild radii, via proton-proton and proton-photon interactions, resolving a long-standing puzzle in cosmic ray origins.

ABSTRACT

The IceCube project transformed a cubic kilometer of transparent, natural Antarctic ice into a Cherenkov detector. It discovered neutrinos of TeV-PeV energy originating beyond our Galaxy with an energy flux that exceeds the one of high-energy gamma rays of extragalactic origin. Unlike at any other wavelength of light, extragalactic neutrinos outshine the nearby sources in our own Milky way. Updated measurements of the diffuse cosmic neutrino flux indicate that the high-energy gamma rays produced by the neutral pions that accompany cosmic neutrinos lose energy in the sources and are likely to be observed at MeV energy, or below. After the reanalysis of 10 years of archival data with an improved data selection and enhanced data analysis methods, the active galaxy NGC 1068 emerged as the hottest spot in the neutrino sky map. It is also the most significant source in a search at the positions of 110 preselected high-energy gamma-ray sources. Additionally, we find evidence for neutrino emission from the active galaxies PKS 1424+240 and TXS 0506+056. TXS 0506+056 had already been identified as a neutrino source in a multimessenger campaign triggered by a neutrino of 290 TeV energy and, by the independent observation of a neutrino burst in 2014 from this source in archival IceCube data. The observations point to active galaxies as the sources of cosmic neutrinos, and cosmic rays, with the gamma-ray-obscured dense cores near the supermassive black holes at their center as the sites where neutrinos originate, typically within $10\sim100$ Schwarzschild radii.

Motivation & Objective

• To identify the astrophysical sources of high-energy cosmic neutrinos detected by IceCube.
• To determine whether active galaxies are the dominant sources of cosmic rays and their associated neutrinos.
• To investigate the physical mechanisms and emission sites responsible for high-energy neutrino production in extragalactic sources.
• To reconcile the observed neutrino flux with theoretical models of particle acceleration and interaction in galactic nuclei.
• To assess the role of gamma-ray absorption and multiwavelength emission in constraining source models.

Proposed method

• Reanalysis of 10 years of archival IceCube data using improved data selection and enhanced analysis techniques to enhance sensitivity to point sources.
• Application of a directional neutrino search focused on 110 preselected high-energy gamma-ray sources, including active galaxies.
• Use of the IceCube detector’s ability to identify upgoing muon neutrinos and shower events from electron and tau neutrinos to isolate cosmic neutrino fluxes.
• Modeling of neutrino production via pγ and pp interactions in the dense, X-ray-emitting environments near supermassive black holes, with opacity calculations using line-of-sight proton density.
• Comparison of predicted neutrino and gamma-ray fluxes with observational limits from MAGIC and other instruments to constrain source models.
• Estimation of the collective contribution of active galaxies to the diffuse cosmic neutrino flux using source density and luminosity functions.

Figure 1: Flow diagram showing the production of charged and neutral pions in $p\gamma$ interactions. The circles indicate equal energy going into gamma rays and into pairs of muon neutrinos and antineutrinos, which IceCube cannot distinguish between. Because the charged pion energy is shared roughl

Experimental results

Research questions

• RQ1 Is NGC 1068 the first identified active galaxy as a source of high-energy cosmic neutrinos?
• RQ2 What physical processes in the vicinity of supermassive black holes produce high-energy neutrinos, and where are they located?
• RQ3 Why do the observed neutrino fluxes from active galaxies like TXS 0506+056 show burst-like behavior with a harder spectrum?
• RQ4 How do gamma-ray absorption and X-ray opacity in the corona affect the detectability of pionic gamma rays and the consistency of multiwavelength models?
• RQ5 Can the collective emission from a population of X-ray-loud active galaxies reproduce the observed diffuse cosmic neutrino flux?

Key findings

• NGC 1068 emerged as the most significant hot spot in the IceCube neutrino sky map after reanalysis, with a high significance detection of high-energy neutrinos.
• The detection of neutrinos from PKS 1424+240 and TXS 0506+056 confirms active galaxies as sources of cosmic neutrinos, with TXS 0506+056 previously identified via a 290 TeV neutrino event.
• The diffuse cosmic neutrino flux observed by IceCube is consistent with a population of active galaxies with X-ray luminosities exceeding 10^43 erg/s, with a number density of ~10^3 Gpc^{-3}.
• Neutrino production occurs within 10–100 Schwarzschild radii of supermassive black holes, primarily via pγ interactions in dense, X-ray-emitting coronas.
• The observed neutrino flux implies that the accompanying gamma rays are suppressed by over an order of magnitude due to X-ray opacity, consistent with MAGIC's upper limits.
• The total energy in cosmic rays and neutrinos in the universe is comparable, supporting the hypothesis that active galaxies are the dominant sources of cosmic rays.

Figure 2: The flux of cosmic muon neutrinos [ 6 ] inferred from the $9.5$ -year upgoing-muon track analysis (solid line) with $1\sigma$ uncertainty range (shaded) is compared with the flux of showers initiated by electron and tau neutrinos [ 9 ] . The measurements are consistent assuming that each n