Cosmic rays, the charged particles traveling at close to the speed of light from deep space, constantly bombard the Earth. Since their very first observation in 1912 by the physicist Victor Franz Hess, carried out from a balloon, our knowledge of these extremely energetic particles has increased. We know, for example, that the vast majority of them exist in the form of hydrogen nuclei, the lightest and most common element in the atmosphere. But also, less often, in the form of helium nuclei and, even more rarely, of nuclei of heavier elements such as uranium.
Energy monsters of mysterious origin
We also know that the energy of cosmic rays ranges from about 1 GeV – equivalent to the energy produced by a relatively small particle accelerator – to 108 TeV, in other words a million times the energy generated by the rays of the Large Hadron Collider ( LHC) at CERN, the most powerful accelerator in the world. The source of the less energetic rays, also called primary cosmic rays, are more than familiar to us: they are produced by the Sun and brought to Earth in the form of the solar wind. Low-energy cosmic rays are essentially associated with supernovae and gas bubbles. But the origin of the most powerful rays remains a mystery to this day.
Read also about “The Research”: Neutrinos reveal the mystery of cosmic rays
These so-called ultra-high-energy cosmic rays actually raise a number of questions for astrophysicists: how do they reach such energies? What could be the natural accelerators that generate them? And above all, how can we go back to their source? Unfortunately, it is impossible to simply “trace their course”: made of charged particles, the turbulent magnetic fields of our galaxy have constantly deflected them on their way to us.
However, a team of researchers led by Sara Buson from Julius-Maximilians-Universität (JMU) Würzburg in Bavaria, Germany, has uncovered the most serious clue yet in this arduous hunt. This group’s work will be published in The Astrophysical Journal Letters, already available on arXiv.
Indeed, the astrophysicist and her collaborators, associate professor of physics and astronomy at the University of Clemson (South Carolina) Marco Ajello and Andrea Tramacere, from the University of Geneva, provide solid evidence that cosmic neutrinos, neutral particles with almost zero mass, come directly from extreme accelerators such as quasars, the most luminous active galactic nuclei. More specifically, they demonstrate that they must be associated with blazars, which are none other than quasars, and therefore supermassive black holes whose matter jet is oriented toward the Solar System (in other words, toward their terrestrial observers).
Unlike the charged particles of cosmic rays, neutrinos have the enormous advantage of passing through galaxies, planets and the human body almost without leaving a trace. Since the electromagnetic forces do not affect them, it is therefore possible to go back to their astrophysical sources. All very well, but what does this have to do with our famous ultra-high energy cosmic rays?
Here it is, crystal clear: Scientists now believe that the most active particles of these cosmic rays and astrophysical neutrinos are accelerated in the same place. “Astrophysical Neutrinos (also called “cosmic”, editor’s note) produced exclusively by processes involving the acceleration of cosmic rays”assures Sara Buson in a press release. “This is precisely what makes neutrinos unique messengers that pave the way for locating sources of cosmic rays.” In short, neutrinos could help locate sources of ultrahigh-energy cosmic rays.
This is not the first demonstration of the link between cosmic neutrinos and blazars, which would behave like natural particle accelerators capable of producing the energies observed for cosmic rays. In 2017, the IceCube neutrino observatory, buried deep under the South Pole’s ice and considered the most sensitive detector of its kind to date, detected a neutrino for which Sara Buson and her colleagues were able to establish a direct link with the blazar TXS 0506. +056, 4 billion light years away in the direction of the constellation Orion. Using data obtained by IceCube and a catalog of astrophysical objects identified with certainty as blazars, this time the team was able to determine that a subset of blazars were indeed the origin of neutrinos at observed high energy.
“We have a clue. [du lien entre neutrinos astrophysiques et blazars] then, now we have the proof”, said Marco Ajello. And Andrea Tramacere to continue: “With this data, we had to prove that the blazars whose directional positions coincided with the positions of the neutrinos were not there by chance. After rolling the dice several times, we discovered that the random association could not exceed that of the actual data only once in a million trials! This is solid proof of the correctness of our associations.”
The “Rosetta Stone” of high-energy astrophysics?
It is now up to scientists to determine the main difference between objects that emit neutrinos and those that do not. “This will help us understand to what extent the environment and the accelerator ‘dialogue’. We can then rule out certain models, improve the predictive power of other models and finally add new pieces to the eternal puzzle of the acceleration of cosmic rays. “continues Andrea Tramacere.
“The process of accretion and rotation of the black hole leads to the formation of relativistic jets, in which the particles are accelerated and emit radiation of up to a trillion times the energies of visible light! The discovery of the connection between these objects and cosmic rays could be the high-energy astrophysics ‘Rosetta Stone’!”