A new study has confirmed that some of the brightest and most energetic objects in the universe are a mysterious source of high-energy cosmic neutrinos.
Comprehensive analyzes have quite convincingly linked galaxies containing blazing cores known as blazars to these enigmatic particles.
This is a result that provides a truly unexpected solution to a problem that astrophysicists have puzzled over for years.
“The results provide, for the first time, incontrovertible observational evidence that a subsample of PeVatron blazars are extragalactic neutrino sources and therefore cosmic ray accelerators,” said astrophysicist Sarah Buson of the Julius Maximilian University of Würzburg in Germany.
Neutrinos were strange 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 neutrinos, ordinary matter, which makes up most of the universe, could be a shadow; that’s why they are known as ghost particles.
We know pretty well where neutrinos come from – ordinary neutrinos.
They are formed as a result of radioactive decay, which is quite common. Most of the neutrinos we detect on Earth are by-products of nuclear reactions in the Sun, but they can also be produced, for example, by supernovae, artificial nuclear reactions, or interactions between cosmic rays and atoms.
But a special observatory in Antarctica has revealed some really bizarre ones.
Although neutrinos interact little with ordinary matter, they interact from time to time. When they interact with water atomic molecules, they can produce a very small flash of light.
The IceCube neutrino observatory has detectors embedded deep in the Antarctic ice at the south pole that can detect these flares. These detections could reveal the energy of the neutrino.
In 2012, IceCube discovered two neutrinos unlike anything we’ve ever seen. Their energies were measured in petaelectronvolts (PeV), 100 million times the energy of supernova neutrinos. And these high-energy neutrinos came from intergalactic space, the source is unknown.
We got a hint at this source in 2018. Because neutrinos don’t interact, they pretty much travel in a straight line through space, which is why a huge international collaboration of scientists has been able to trace the origin of high-energy neutrinos to a blazar.
This is the core of a massive galaxy powered by an active supermassive black hole, angled so that jets of ionized matter, accelerated to almost the speed of light, are directed straight at Earth.
“Interestingly, there was a general consensus in the astrophysicist community that blazars were unlikely to be sources 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 a group of scientists led by Buson did what scientists do: they started digging.
They took 7 years of all-sky neutrino data from IceCube and carefully compared it to a catalog of 3,561 objects that are either confirmed blazars or very likely to be.
They performed positional cross-matching of these catalogs in an attempt to determine whether high-energy neutrinos could be definitively associated with blazar locations in the sky.
“With these data, we had to prove that the blazars, whose position in the direction coincided with the position of the neutrino, did not appear by chance,” explained astrophysicist Andrea Tramacere from the University of Geneva in Switzerland.
“After several rolls of the dice, we found that random association can only outperform real data once in a million trials! This is strong evidence that our associations are correct.”
According to the team’s analysis, the probability of a random occurrence is 0.0000006. This suggests that at least some blazars are capable of producing high-energy neutrinos, which in turn helps solve another problem. The origin of high-energy cosmic rays – protons and atomic nuclei that fly through space at close to the speed of light – is also a huge mystery.
According to Bewson, high-energy neutrinos are produced exclusively in processes associated with the acceleration of cosmic rays. This means we can now relate blazars to the acceleration of cosmic rays, the team says.
“The process of accretion and the rotation of a black hole lead to the formation of relativistic jets in which particles are accelerated and emit radiation with an energy of a thousand billion energies of visible light!” Tramaser said.
“The discovery of a connection between these objects and cosmic rays could be the Rosetta Stone of high-energy astrophysics.”
From here, there are several paths that require further study. One is to try to figure out why some blazars are efficient particle accelerators and others are not. This will help the team figure out what the characteristics of the neutrino factory are and where else in space we can find them.
In addition, further more detailed analysis of the neutrino data may yield more insights into the birthplaces of these unusual ghost particles.
The study was published in Letters from an astrophysical journal.
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