A huge telescope built under the ice of the South Pole has detected neutrinos coming from our galaxy, the Milky Way, for the first time. The finding is confirmation of a phenomenon expected for years, and implies that in our own cosmic neighborhood there are unknown bodies capable of producing the most energetic particles in the universe.
Neutrinos are the most abundant particles in the cosmos. Every second about 100 trillion pass through our body without us noticing. These ghostly particles have no electric charge and hardly any mass. The vast majority of neutrinos pass through the Earth from side to side without a trace. But a few interact with an atom and produce a flash of blue light that allows to determine its origin.
The IceCube telescope is a mass of one cubic kilometer of Antarctic ice in which more than 5,000 spherical detectors capable of capturing the flashes left by neutrinos in their wake have been set. In 2013, the observatory discovered the first two neutrinos from outside our galaxy. They were baptized Epi and Blas and inaugurated a new era of astronomy, although it could not be determined where exactly they came from.
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In 2017 and 2022, a few dozen more neutrinos were trapped that made it possible to identify the first two sources outside our galaxy: two black holes that could fit millions of stars like the Sun. In the first case, neutrinos had traveled at nearly the speed of light for more than 4 billion years from the distant galaxy TXS 000+0506. In the second, they came from Messier 065, a galaxy just 77 million light-years from our solar system.
Neutrinos are associated with cosmic rays — beams of charged particles that are the most energetic in the universe. In 1993, a telescope in Utah (United States) captured the most powerful particle of this type known. It traveled at the speed of light and its energy was millions of times higher than that which can reach the most powerful particle accelerator in the world, the LHC. At first it was called WTF signal (initials of what the cojones!, in English), although it was finally baptized as OMG (Oh, my God). Astrophysicists use large neutrino detectors to try to figure out where cosmic rays are coming from.
This time IceCube has discovered hundreds of neutrinos that arrive from the center of our galaxy, about 25,000 light years, and with an energy 10,000 times greater than that of a particle accelerator, explains Ignacio Taboada, spokesman for the IceCube telescope. "Discovering these galactic neutrinos should be the easiest, but it turns out the Milky Way doesn't produce many. We have finally managed to catch them and so we know that in our cosmic environment there are also objects capable of producing them," says Taboada, a researcher of Venezuelan origin who has worked on IceCube since its construction in 2010. The discovery is published today in the journal Science, a benchmark of the best science in the world.
Representation of the Milky Way with galactic neutrino signals. IceCube
The mass of IceCube is located below the American base Amundsen-Scott of the South Pole, where the average temperature is 50 degrees below zero. During the Antarctic winter, which lasts six months in which it is always night, only two people remain at the base to keep IceCube running. Meanwhile, a team of more than 300 scientists from more than 12 countries can access the data in real time.
For today's finding, an artificial intelligence has been developed that has analyzed a trillion neutrino signals captured between 2011 and 2022, and has selected the few hundred that come from the Milky Way. Juanan Aguilar, an astroparticle physicist from Albacete who is part of the IceCube team, details that "before less accurate statistical models were used to analyze the signals captured by the telescope." The new tools based on neural networks allow to clean the noise produced by other particles and stay only with "the signals that arrive from inside the galaxy".
The Milky Way is shaped like a flattened spiral, like a cookie, and the signals seem to come right from the edge. The IceCube data shows that there is a kind of blurred neutrino cloud that extends throughout the galactic center. In addition, there may be one or more point sources of neutrinos. It is possible that one of them is Sagittarius A*, a black hole with a mass equivalent to four million stars like the Sun that lurks right in the center of the galaxy. It's also possible that there are other unknown objects that produce cosmic rays and neutrinos, such as a black hole that is swallowing a nearby star.
Astroparticle physicist Ignacio Taboada, at the Amundsen-Scott base at the South Pole. T.
"The finding of IceCube reveals that in our own galaxy there must be huge particle accelerators" comparable to those previously detected in other more or less distant galaxies, summarizes Francisco Salesa, scientist at the Institute of Corpuscular Physics, in Valencia. Earlier this month, the Antares neutrino detector, located under the Mediterranean Sea off the coast of Toulon, France, picked up a signal of galactic neutrinos. "The reliability of that signal was two sigma, that is, there was a one in 100 chance that the signal was an error," explains Salesa. "The IceCube observation has 4.5 sigma, one chance of error in 10 million." It is much more reliable, but it remains on the verge of being able to claim a discovery, which requires five sigma – a possibility in 3.5 million.
Now begins a worldwide race to identify the origin of galactic neutrinos. IceCube, driven mainly by the United States, will continue to operate for several years and could achieve it. But being in the southern hemisphere means that the center of the galaxy is just above it, which introduces a lot of noise caused by other elementary particles produced in the atmosphere, the Sun and other objects. KM3Net, a new underwater telescope in the Mediterranean with two sites, Arca and Orca, located near Toulon and Sicily (Italy), is currently under construction. Being in the northern hemisphere the Earth will serve as a filter and theoretically it will be able to refine much better the origin of the elusive galactic neutrinos.
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