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A long time ago, in a galaxy far, far away … were emitted neutrinos which are this week the subject of two studies in the review Science . Astronomers from several international collaborations write that they have identified, somewhere 4 billion light-years away, an extremely luminous galaxy, or blazar. Nothing surprising until then. Except that this blazar emits cosmic neutrinos of high energy, particles carrying with them essential information on our Universe. A first for thirty years.
Neutrinos are usually the subject of particle physics studies in which the authors examine their mbad and oscillation, in other words their ability to change their shape or flavor. These experiments are often conducted with neutrinos of terrestrial origin, created in nuclear reactors or particle accelerators.
The cosmic neutrinos we are talking about here are very different: they are "wild" neutrinos, created in the heart of the stars or in the most violent galactic phenomena
The cosmic neutrinos we are talking about here are very different: they are "wild" neutrinos, created in the heart of stars or in the most violent galactic phenomena (explosions, holes black…). Our star, the Sun aside, scientists knew so far only one source of cosmic neutrinos: the 1987A supernova, born from the explosion of a star in 1987.
Small Neutrals
A the only source in thirty years is little. But logical, both neutrinos are paradoxical particles. They may exist anywhere in the universe (200 million billion of them will have pierced time to read this article), they are still elusive. The "little neutrals", according to their name in Italian, which refers to the nature of their electric charge, do not really interact with matter. They cross it briskly. Detectors included, that goes without saying.
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Physicists provide definitive proof of the mbad of neutrinos
Far from being discouraged, physicists have built (very) large instruments to catch them, with a particular interest for so-called high-energy cosmic neutrinos. This energy is incomparably greater than that of the neutrinos created on Earth or even in the 1987A supernova. Where will these neutrinos get this crazy energy? The theorists suspect them of acquiring them in blazars, these colossal galaxies with a rotating black hole as their core, emitting two jets of light and particles along its axis of rotation.
Thus, the experimenters were left to validate or sweep this hypothesis. "The theory predicts that it is these jets that give certain neutrinos and cosmic rays their exceptional energy. It has been a hundred years since physics awaited an experimental validation! "Enthuses Fabian Schüssler, from the Department of Particle Physics of the Atomic Energy and Alternative Energy Commission (CEA) in Saclay, near Paris.
In fact, neutrinos and cosmic rays are linked. Deviated by magnetic fields, cosmic rays do not deliver information about their source. But by interacting with matter, the particles that compose them (protons, electrons …) can create neutrinos. They are not deviated because they go straight ahead without stopping. Physicists were betting on these to trace the trace of cosmic rays of high energy.
20 000 leagues under the ice
Thus was built IceCube, gigantic detector of one cubic kilometer built under 1500 meters of ice in Antarctica. The instrument puts each year the grappling on about twenty neutrinos of high energy, without being able to determine the origin of it. This time he succeeded. On September 22, 2017, one of these neutrinos under EPO crossed the IceCube instruments. The alert was given 43 seconds later to a network of astronomical devices spread around the world. Crossing a maximum of independent measurements in order to triangulate the position of the source, the scientists established that the general direction of this neutrino coincided precisely with a blazar named TXS 0506 + 56. Three decades after 1987A, physicists finally held a real source of high energy neutrinos!
The small neutral in question, poetically christened IceCube170922A, would have an estimated energy of about 290 teraelectronvolts. This is 200 times more than the energy of proton collisions in the LHC casings, CERN's particle accelerator. Cautious, the authors do not speak of detection. With a precision of three sigmas, there is a one in a thousand chance that this detection is an error.
Multimedia astronomy is a sector that is gaining importance in recent times
astronomers will not hesitate to study this neutrino from every angle. Its trajectory, flavor and energy will be dissected to learn more about the cosmic processes that led to its formation. "Since neutrinos almost do not interact with matter, they come to us almost intact by taking with them information about these extreme phenomena that govern their formation," says Teresa Montaruli, of the Department of Nuclear Physics. corpuscular of the University of Geneva and member of the collaboration IceCube. Concerning the acquisition of its energy, "it could be explained by the action of the magnetic fields badociated with the shocks in the jets of the TXS blazar", advances the physicist.
To read also: Neutrinos , always more megalos
Quality against quantity
A single neutrino, it can seem very thin. But quality matters more than quantity, says Damien Dornic of the Center for Particle Physics in Marseille: "The twenty or so neutrinos detected during the 1987A supernova provided enough data to revolutionize the knowledge of these phenomena. The standard model of the Sun has, for its part, been established thanks to the initial detection of a small number of neutrinos. "
These two studies also underline the rise of so-called multimessage astronomy, in which cosmic events are observed. in all wavelengths, through a maximum of different particles (photons, gamma-ray protons, X-rays, gravitational waves, etc.). Such badyzes are possible by cooperating a maximum of independent instruments within the same network. Once a suspicious event is isolated, the alert is given and everyone can turn their telescope in the same direction. "Four hours after the signal, we were able to observe this blazar," says Fabian Schüssler, who is also a member of the HESS collaboration, a high-energy gamma ray detector located in Namibia. Multimedia astronomy is an area that is gaining in importance in recent times. "With cosmic neutrinos as new eyes to observe galaxies, there is no doubt that this discipline has not finished being talked about.
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