IceCube Observatory issues first report on new method of analyzing neutrinos

High above the ice at the South Pole, a detector the size of a city block listens for some of the quietest messengers in the universe. In its latest analysis, the IceCube Collaboration combined 14 years of track data and 10 years of cascade data to carry out its most sensitive all-sky search for steady neutrinos. 

The work targets a question that has nagged physicists for decades. Where do the highest energy cosmic rays come from, and which cosmic engines push particles to such extreme energies? 

IceCube tracks neutrinos with two signals

IceCube records two main kinds of neutrino events within a cubic kilometer of ultra-clear Antarctic ice. These are long streaks called tracks, and compact fireballs called cascades.

Tracks, which are usually produced by muon neutrinos, point back to their sources with sharper angles, while cascades, often from electrons, provide cleaner energy measurements.

This pairing covers the whole sky well. Tracks favor the Northern Hemisphere because the Earth filters out unwanted muons from above.

In contrast, cascades perform better in the Southern Hemisphere where atmospheric backgrounds are tougher.

By analyzing both together in one simultaneous maximum likelihood fit, the team increased its reach without splitting the sky into separate games. That unified approach helps reveal patterns that either channel might miss on its own.

What the sky scan found

The researchers looked across the entire sky and checked 167 known gamma ray sources. They were interested to see if any gave off more neutrinos than expected by chance.

After adjusting for all the places they tested, they found one bright spot in the north that matched the location of the galaxy NGC 1068. This location showed a signal about 3.5 times stronger than normal background levels.

“We found that one spot in the sky, very close to the galaxy NGC 1068, was the “brightest” spot in the northern sky,” said Riya Shah, Drexel University.

The result is not yet strong enough to confirm a discovery, but it reinforces earlier evidence that NGC 1068 could indeed be a real source of high-energy neutrinos.

In the Southern Hemisphere, the hottest spot did not match any object from the tested catalog. It also did not appear as a hot spot when tracks or cascades were analyzed separately.

This suggests that the combined method can surface features that single channel searches overlook.

“While the southern sky’s brightest spot does not yet meet the threshold for discovery, it stands out as a previously unidentified feature in IceCube’s point-source searches,” said Riya.

Why NGC 1068 keeps showing up

NGC 1068 is a nearby galaxy with a bright center. It also has a massive black hole that draws in surrounding gas and dust. As this material falls inward, it releases large amounts of energy.

In 2022, the IceCube Observatory detected 79 high-energy neutrinos from this galaxy, marking one of the strongest signals yet seen beyond the Milky Way.

The new search strengthens the case by finding the same region again using a different and more sensitive strategy. It shows that the earlier signal is not a one-off fluke of a particular analysis recipe.

Steady neutrino signals over time

The team also asked whether the strongest hot spots could be caused by short bursts or if they are steady glows that hold over many years.

For NGC 1068, the analysis excluded short flares that lasted less than four years as the sole cause of the signal, pointing to emissions that persists across the full data set.

That matters because steady emission is easier to model and connect to the physics of the core, where dense gas and radiation fields can help convert cosmic ray collisions into neutrinos.

It also helps plan future observations that can build exposure without depending on rare flares.

Cascades recently helped IceCube spot neutrinos from our own galaxy.

In 2023, the collaboration reported evidence for emission from the Galactic plane, at 4.5 sigma significance, using ten years of data and machine learning-based reconstructions. 

That result showed that cascades can reveal sources, even where backgrounds are messy. It also explains why combining channels is so useful when the goal is uniform sensitivity across both hemispheres.

How the IceCube neutrino search works

The search uses a maximum likelihood framework that weighs each event by how signal-like it looks in angle and energy.

Then it asks whether a group of events clusters more tightly than backgrounds predict. For the catalog test, the method stacks known positions and lets the data decide which, if any, contribute.

Trial factors – the statistical penalty for looking in many places or testing many sources – are handled explicitly. That is why the northern hot spot remains intriguing but falls short of a formal discovery, even though its pre-trial map may look bright.

High energy neutrinos point straight back to their birthplaces because they are neutral and almost never interact across cosmic distances.

That makes them clean tracers of hadronic processes that also produce the most extreme cosmic rays.

If NGC 1068 continues to shine in future data, it would support the idea that dense, obscured cores in active galaxies are efficient neutrino factories. It would also explain why some of those sites are dim in gamma rays, since gamma rays can be absorbed in the same thick environments where neutrinos escape.

What to watch next

More exposure will tighten the statistics for the northern and southern hot spots.

Improved reconstructions and catalogs that include more dust-hidden active galactic nucleus candidates should also help separate steady emitters from backgrounds.

IceCube will keep operating at high uptime, and next generation arrays will expand the reach to both lower and higher energies. Each added year improves sensitivity to steady sources that add up over time.

The study is published in The Astrophysical Journal.

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