The Universe is bathed in cosmic rays, particle beams accelerated at high speeds. They come to Earth constantly, although the atmosphere protects us from them. But the origin of the most energetic cosmic rays, known for more than a hundred years, was a mystery. Until now: from the track of a single particle, a neutrino, an international collaboration of scientists led by the IceCube observatory in Antarctica, has managed to trace for the first time one of the possible origins of high-energy cosmic rays. The milestone opens new possibilities in the so-called multimensajero astronomy, which began last year with the simultaneous detection of a clash of neutron stars by gravitational and electromagnetic waves.
The source of this neutrino, and the cosmic rays that originated it, is an object known as blazar, 4 billion light years from Earth, according to the researchers published today in an article in the journal Science. It is a galaxy that at its center houses a supermassive black hole that engulfs all the matter that approaches it, an extremely violent phenomenon and in a precarious balance. When the balance breaks, it emits flashes of electromagnetic radiation and high-energy cosmic rays, just in the direction of Earth.
That is why researchers have been able to trace it, through a combined effort of twenty observatories located on Earth and space.
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“This campaign has covered all the wavelengths of the electromagnetic spectrum, from radio to gamma rays and, most importantly, the IceCube neutrino that launched the alert,” states Francis Halzen , physicist at the University of Wisconsin , by email. -Madison (United States) and principal investigator of the IceCube observatory. That is why it is multimensajero astronomy, since it has used two types of messengers: electromagnetic waves and neutrinos. “The neutrinos open a new window to observe the Universe,” says Darren Grant , a researcher at the University of Alberta (Canada) and the study’s first signatory.
IceCube, powered by the National Science Foundation of the United States, is an observatory specialized in the detection of neutrinos, practically undetectable particles. They are the ghosts of the subatomic world: since they interact very little with matter, they can pass through it without problems, it is in the form of dust, planets or stars, and they travel through the universe for billions of light-years in a straight line, without stop or deviate, and almost at the speed of light. “Neutrinos are the ideal astronomical messengers,” says Darren Grant. But at the same time they are extremely difficult to hunt: “only one in 10,000 neutrinos that reach IceCube collides with the nucleus of an atom of ice and becomes a signal that we can detect,” says Francis Halzen. To maximize the detection possibilities.
One of the phenomena that elusive neutrinos can generate are precisely the sources of high-energy cosmic rays. But, unlike what happens with neutrinos, cosmic rays, which are formed by protons and nuclei of electrically charged atoms, deviate as soon as they meet a magnetic field. Therefore, when they reach Earth, it is impossible to know where they originated. In contrast, neutrinos generated in the source of high-energy cosmic rays can help to track it.
The neutrinos open a new window to observe the Universe”
IceCube detected a single high-energy neutrino on September 22, 2017, which had to come from a source of high-energy cosmic rays as well, and traced its direction of origin in the celestial vault. In less than a minute, he alerted telescopes around the world to observe the possible source of the particle. The first to detect a candidate, on September 28, was the NASA Fermi space telescope . On October 4, the MAGIC telescope pair followed at the Roque de los Muchachos Observatory in Gran Canaria, according to Òscar Blanch , researcher at the Institute of Physics of Altes Energies of Barcelona (IFAE) in the MAGIC project. After adding observations of another 18 telescopes, all the signs pointed to the blazar known as TXS 0506 + 056, near the left shoulder of the constellation of Orion, although invisible to the naked eye. “The probability that it is a coincidence, and that the neutrino and the electromagnetic observations do not come from the same source, is less than 1%,” Blanch explains in a telephone interview.
By analyzing data collected previously by IceCube, researchers have also discovered that between 2014 and 2015 a burst of 19 high-energy neutrinos arrived from the direction of the same blazar, reinforcing the hypothesis that it is a source of high-energy cosmic rays . The results of this research are published today in another article in the journal Science.
Cosmic rays are the highest energy radiation that reaches us from the Universe. We will not be able to understand the morphology of the extreme Universe without understanding the enormous flow of neutrinos and cosmic rays
“Sources of high-energy cosmic rays have been a mystery for more than a hundred years. This is the first conclusive evidence of a source of these cosmic rays, “says Darren Grant. The test has come more than a hundred years after Austro-Hungarian Victor Franz Hess discovered them in 1912, adds Francis Halzen. The researchers intend to find new sources of high-energy cosmic rays, since there could be more besides the blazars: collisions between neutron stars could also be, according to Òscar Blanch.
“Cosmic rays are the highest energy radiation that reaches us from the Universe. We will not be able to understand the morphology of the extreme Universe without understanding the enormous flow of neutrinos and cosmic rays “, concludes Francis Halzen.
Approximately half of the known galaxies have supermassive black holes in their center that absorb matter; They are known as galaxies with active nuclei, explains Òscar Blanch. “They are very violent phenomena and their balance is sometimes broken, which causes them to emit beams of electromagnetic radiation in a direction perpendicular to the plane of the galaxy.” By colliding with the matter surrounding the black hole, the radiation accelerates the particles and converts them into high-energy cosmic rays. “They are accelerators of particles much more powerful than those we can build on Earth,” said Francis Halzen in the press conference in which the results have been presented, organized by the National Science Foundation of the United States. When it happens that the radiation beams point towards the Earth, these galaxies are called blazares; Some thousands are known, say Halzen and Grant. The black hole at the center of the Milky Way, on the other hand, is not large enough to absorb matter from its surroundings, so the galaxy’s nucleus is not active.