NGC 1068: what does a neutrino excess establish?

A well-known galaxy, seen in neutrinos

NGC 1068, also known as Messier 77, is an active galaxy. Matter around its central supermassive black hole powers its radiation, and the surrounding material changes how much of that radiation an observer can see.

The neutrino question is narrower than whether the galaxy is energetic: does the event sample contain more neutrinos from its direction than the expected atmospheric and diffuse astrophysical backgrounds? A galaxy being bright cannot answer that.

The 2022 IceCube paper used data taken with the full detector from 2011 to 2020, with improved calibration and reconstruction. The result comes from a long observation and a better reading of the signals, not from a single dramatic alert.

Sources: [1] · [2]

The source is one part of a statistical fit

Each reconstructed track has a direction, an energy estimate and a directional uncertainty. A source model predicts how these should be distributed if part of the sample came from a given position; a background model predicts their distribution without that source.

A likelihood combines this information over all events. Events near the source direction, with suitable uncertainty and energy, can count for more than others. Drawing a circle and counting everything inside would throw away information the real analysis uses.

The fit estimates the size of the signal and the shape of its energy spectrum. The signal count is a parameter in a statistical mixture; it does not name which events came from the source. A single event can be consistent with several components at once.

Sources: [1] · [3]

A strong local result must be corrected for the search

The source-list analysis tested 110 objects chosen in advance, and NGC 1068 gave the strongest result. Looking at many objects gives background more chances to produce an impressive-looking excess somewhere.

The paper reported a local significance of 5.2 standard deviations at NGC 1068, and 4.2 after correcting for the source-list search. The 4.2 figure is the one to quote for that catalog test. The paper’s separate scan of the northern sky has its own correction.

The fitted excess below carries the statistical uncertainty reported in the paper. The count comes from a model fit and the significance from a test against background; neither gives the probability that any particular event came from the galaxy.

Sources: [1] · [3]

Where in the galaxy did the particles interact?

Neutrinos and gamma rays can be made by the same particle processes and still not be equally visible. In a dense radiation field, gamma rays may be absorbed and their energy passed on before they escape; neutrinos get out much more easily.

This contrast points to models in which the neutrinos come from a compact, obscured region near the active nucleus. Such models tie the neutrino result to electromagnetic observations. Being consistent with the data, though, does not pick out a single geometry or acceleration mechanism.

Linking neutrinos to the galaxy’s direction cannot resolve a small region near its black hole. Nor does the result mean that neutrinos escaped from inside the event horizon: the models describe matter and interactions outside it.

Sources: [1] · [4]

Read the maps and spectra together

A significance map summarizes a scan under a given model and background estimate. Its colors show test results, not brightness as in a photograph. A hot spot tells you where the test found an excess; to interpret it, you also need to know how much sky was searched.

A plot of angular distance compares a projected distribution with the modeled signal and background. It is handy for checking agreement, but it squeezes out the energy and uncertainty information used in the full fit, so a visible peak is only part of the evidence.

The fitted spectrum asks which energy distributions fit the source contribution, given the detector response. Reading these views together tells you more than a headline count, which should not be mistaken for an image of the galaxy’s engine.

Sources: [1] · [2]

Try it in the Lab

Primary sources & revision

1. IceCube Collaboration · Evidence for neutrino emission from NGC 1068 (2022; arXiv v2, 2024)
2. IceCube · NGC 1068 analysis data release (2022)
3. Particle Data Group · Statistics (2025)
4. Anchordoqui, Krizmanic & Stecker · High-Energy Neutrinos from NGC 1068 (2021)

First published 2026-10-10; last revised 2026-10-13. Original explanatory text and diagrams by Sandbox Physics. Illustrations are schematic; no experimental event records are reproduced here. This is an independent educational publication, not an official Nobel or experiment collaboration publication.