Astrophysicists Think They've Found a Mysterious Source of High-Energy Neutrinos

Some of the brightest and most energetic objects in the Universe are the mystery source of high-energy cosmic neutrinos, new research has confirmed.

A comprehensive analysis has been convincing enough to link the galaxies that host the fiery cores known as blazars with these mysterious particles.

It’s a result that provides a completely unexpected solution to a problem that has kept astrophysicists scratching their heads for years.

“The results provide, for the first time, irrefutable observational evidence that the PeVatron blazar sub-sample is a source of extragalactic neutrinos and thus an accelerator of cosmic rays,” said astrophysicist Sara Buson of the Julius Maximilian University of Würzburg in Germany.

Neutrinos are the odd little things at the best of times. These subatomic particles are ubiquitous and are among the most abundant in the Universe.

However, their mass is almost zero, they are electrically neutral, and they interact very little with anything else in the universe. For the neutrino, the normal matter that consists mostly of the Universe might as well be a shadow; this is why they are known as ghost particles.

We know very well where neutrinos – normal neutrinos – come from.

They are produced by radioactive decay, which is quite common. Most of the neutrinos we detect on Earth are byproducts of nuclear reactions on the Sun, but they can also be generated by supernovae, artificial nuclear reactions, or interactions between cosmic rays and atoms, for example.

But a special observatory in Antarctica revealed some downright strange ones.

Although neutrinos don’t interact much with normal matter, they do occasionally do. When they interact with the molecules in the water atom, they can produce very small flashes of light.

The IceCube Neutrino Observatory has detectors embedded deep in the Antarctic ice at the south pole that can detect these flashes. This detection can reveal the energy of the neutrino.

In 2012, IceCube detected two neutrinos we had never seen before. Their energy is on the petaelectronvolt (PeV) scale – 100 million times more energetic than supernova neutrinos. And these high-energy neutrinos come from intergalactic space, the source of which is unknown.

We got a lead on that source in 2018. Because neutrinos don’t interact, they pretty much travel in straight lines through space – so a large international collaboration of scientists was able to track high-energy neutrinos back to the blazar.

It is the core of a massive galaxy powered by an active supermassive black hole, tilted so that jets of ionized matter are accelerated to near the speed of light directly on Earth.

“It’s interesting that there is a general consensus within the astrophysics community that blazars cannot possibly be the source of cosmic rays, and here we are,” University of Wisconsin-Madison physicist Francis Halzen said at the time.

However, some questions remain about the relationship between blazars and high-energy neutrinos. So the team of scientists led by Buson did what scientists do: they dug.

They took 7 years of celestial neutrino data from IceCube, and painstakingly compared it to a catalog of 3,561 objects confirmed as blazars, or very likely.

They cross-matched the positions of this catalog, trying to determine whether high-energy neutrinos could be conclusively linked to the blazar’s location in the sky.

“With these data, we had to prove that the blazar whose direction coincided with the neutrino was not there by chance,” explains astrophysicist Andrea Tramacere of the University of Geneva in Switzerland.

“After rolling the dice several times, we found that random associations can only exceed real data once in a million trials! This is solid evidence that our association is correct.”

According to the team’s analysis, the probability of a random event is 0.0000006. This suggests that at least some blazars are capable of producing high-energy neutrinos, which, in turn, help solve other problems. The origin of high-energy cosmic rays – protons and atomic nuclei flowing through space at near the speed of light – is also a great mystery.

According to Buson, high-energy neutrinos are produced exclusively in processes involving the acceleration of cosmic rays. This means, by conclusion, that we can now relate the blazar to the acceleration of cosmic rays, the team said.

“The process of accretion and rotation of black holes leads to the formation of relativistic radiance, in which particles are accelerated and emit radiation of up to a thousand billion energies of visible light!” Tramacere said.

“The discovery of a connection between these objects and cosmic rays may be the ‘Rosetta stone’ of high-energy astrophysics.”

From here, there are several paths that require further exploration. One is to try to discover why some blazars are efficient particle accelerators while others are not. This will help the team figure out what the characteristics of the neutrino factory are, and where else we can find them in the universe.

In addition, further analysis of the neutrino data could lead to more discoveries about the birthplace of these strange and ghostly particles.

This research has been published in Astrophysics Journal Letter.

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