University of Wisconsin-Madison

Neutrino Blazar FAQ

Why neutrinos to explore the universe?

Neutrinos are among the tiniest elementary particles in the universe. They are about a million times smaller than an electron. But, unlike electrons, neutrinos are uncharged particles.

You can think of a neutrino as a particle very similar to a photon, the particle of light. But photons interact with everything––they can’t even go through a sheet of paper––while neutrinos sail through large amounts of matter. That’s why we sometimes call neutrinos ghost particles. They are not affected by magnetic fields and travel for millions, even billions, of years through the universe in a straight line, almost undisturbed.

These properties make the neutrino an idea cosmic messenger for exploring the extreme universe. Scientists think that about 20% of the extreme universe can’t be explored with photons, because in those energetic and dense environments photons can be absorbed.

And that’s one of the reasons why we need neutrino telescopes such as IceCube. Neutrinos can escape the densest environments in the universe, carrying unique information about the places where they have been created.

Are neutrinos only produced in the distant universe?

No. Neutrinos are produced everywhere. There are neutrinos, trillions of them, going through us all the time, most of them produced by the Sun. All stars produce neutrinos. But you don’t need a star to produce a neutrino. Every time there’s a nuclear reaction, there’s a neutrino involved. Think about all the places where that takes place! But the neutrinos IceCube is looking for come from the distant, extreme universe. We want to explore regions of the universe that cannot be studied with light.

Learn more about this in this TED-Ed video

How is this new result important for the field of astrophysics?

This search, for the first time, points us to the superenergetic particle accelerators that we know exist in the universe. These accelerators can emit particles with energies a billion times higher than CERN’s Large Hadron Collider, the most powerful human-made particle accelerator.

We would like to know how nature builds such accelerators, but first we have to find them. The neutrinos that IceCube saw in this new result are our first direct sign that these accelerators are located in active galactic nuclei, which are galaxies with supermassive black holes at their center. The black holes draw in matter, emitting some of it as relativistic twin jets along the axis of the galaxy. This jet had been considered a likely site of acceleration, and now we have evidence for that. Although there’s still much more to learn, this is a huge step forward.

Learn more about this in a short video featuring IceCube PI Francis Halzen.

What’s next for IceCube in neutrino astronomy?

Everything is next! These papers mark the beginning of real neutrino astronomy. IceCube was built to find and study the sources of high-energy neutrinos and cosmic rays. Finding the first one is a big accomplishment, but now we want to find many more, and detect many neutrinos from each one of them. This is the only way to really know how these powerful cosmic accelerators work. Given the number of neutrinos IceCube has been detecting, we know that to do precise neutrino astronomy we will need a larger detector. We are already working on the extension of our detector, which we call IceCube-Gen2.

Can we count on more discoveries?

We think so, although we don’t know when the next one will come. This multimessenger effort has not only taught us about blazars, but also given us hints of how to search more efficiently in our data. We are already working with our partners to move our collaboration to the next step, and also improving IceCube searches. IceCube sends a few high-energy alerts to the international community every year. The alert that led to these results was the tenth one of this type. We don’t have enough data to know how often this will happen again. But the huge number of cosmic rays we see tell us that there are many more sources to discover.

Are all sources of cosmic rays blazars?

We don’t know yet. This is the first time we have been able to identify a likely source of neutrinos and cosmic rays. Scientists think there are other cosmic objects that might also accelerate cosmic rays, many even think that most of them will only be revealed with a larger neutrino detector, i.e., with larger samples of neutrino data, since the highest energy gamma rays might be absorbed in the environments around these sources or on their way to Earth.

Learn more about this in a short video featuring IceCuber Chad Finley, former convenor of the IceCube point-source working group.

Why IceCube had not seen the 2014-15 flare before?

The reason for this is what scientists call the look-elsewhere effect. We see neutrinos from everywhere in the sky, most of them of atmospheric origin. Just by chance, these non-cosmic neutrinos, which can also reach fairly high energies, can cluster and produce a signature similar to an astrophysical source. We needed the joint observation with gamma rays and other low energy photons to have strong evidence that blazar TXS 0506+056 was the source. And once we knew where to look, we could refine our analysis in that location, and then we did find a previous neutrino flare.

Learn more about this in a short video featuring IceCuber Chad Finley, former convenor of the IceCube point-source working group.

Why are the 2014-15 and the 2017 neutrino flares so different? Do we see gamma rays from both?

Thanks to the follow-ups of the IceCube alert, we have learned more about this blazar than any other blazar before. Blazars, as many other cosmic objects, are dynamic environments. We still don’t know all the details about how blazars work, but the two neutrino flares seem to point to different types of activity around the central black hole. There are several follow-up papers released today that give more details about them. And, yes, there are hints that there was also gamma-ray activity during the 2014-15 flare.

https://doi.org/10.1093/mnras/sty1852
https://doi.org/10.3847/2041-8213/aad083
https://doi.org/10.3847/2041-8213/aad053

Why is IceCube in Antarctica?

The detection of high-energy neutrinos requires huge detectors because for every neutrino we see, there are about a million more that go through our detector without leaving a trace. The idea of building a cubic-kilometer detector in Antarctica—the only place with so much ice—goes back to the late 1980s. The first neutrino detectors were envisioned in water, but the ice has many good properties: very clean, very dark, and very stable. But building such a detector at the South Pole came with many challenges. Yet IceCube construction was a success: it was built on time, on budget, and exceeding design specifications. Before that, a smaller detector called AMANDA proved that we could detect neutrinos in the ice. And only three years after completion, IceCube announced the detection of extragalactic neutrinos. It’s been 30 years of Antarctic research, and we are looking forward to many more to come.