An artistic illustration of a blazar accelerating cosmic rays, neutrinos and photons up to high energies. I© Benjamin Amend
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Scientists have discovered that a special type of “ghost particle” is likely forged by gargantuan black holes known as blazars, a finding that sheds light on some of the most tantalizing mysteries about our universe, such as the origin of particles called cosmic rays.
Cosmic rays are extremely energetic bits of atomic matter that shoot across space at near light speed, suggesting they hail from powerful objects that act as natural particle accelerators. One way to pinpoint sources of cosmic rays is to look for astrophysical neutrinos, another type of high-energy particle that is probably forged by the same intense mechanisms as cosmic rays.
Neutrinos are sometimes called ghost particles because they are so lightweight that they barely react to matter, and pass effortlessly through our bodies and planets such as Earth. While this makes them extraordinarily difficult to capture, it also means that neutrinos usually travel in a straight line through space, allowing astronomers to sometimes trace them back to a particular region of the sky. Discovering the source of astrophysical neutrinos can, therefore, indirectly point to the natural space factories that spew out cosmic rays.
Now, scientists led by Sara Buson, an astronomer at the Julius Maximilian University of Würzburg have presented new evidence that blazars “are astrophysical neutrino factories and hence, extragalactic cosmic-ray accelerators,” according to a recent study published in the Astrophysical Journal Letters.
Blazars have already been floated as a potential source of cosmic rays, but the new study is the first to show “a firm indirect detection of extragalactic cosmic-ray factories,” Buson and her colleagues said in the study.
“Neutrinos are the most elusive particles in the universe, capable of traveling nearly unimpeded across it,” the researchers said. “Despite the vast amount of data collected, a long-standing and unsolved issue is still the association of high-energy neutrinos with the astrophysical sources that originate them.”
“Among the candidate sources of neutrinos, there are blazars, a class of extragalactic sources powered by supermassive black holes that feed highly relativistic jets, pointed toward Earth,” they continued. “Previous studies appear controversial, with several efforts claiming a tentative link between high-energy neutrino events and individual blazars, and others putting into question such relation. In this work, we show that blazars are unambiguously associated with high-energy astrophysical neutrinos at an unprecedented level of confidence.”
Buson and her colleagues reached this conclusion after cross-referencing data from the IceCube Neutrino Observatory in Antarctica, which is the most sensitive neutrino detector on Earth, with BZCat, a catalog of more than 3,500 objects that are likely blazars. The team performed a statistical analysis to see if astrophysical neutrinos captured in “hotspots” at IceCube might point back to specific blazers in BZCat.
The results revealed that 10 of the 19 IceCube hotspots in the southern sky are probably linked to blazars, suggesting that astrophysical neutrinos, and therefore cosmic rays, originate in these explosive environments. That’s not to say that all blazars produce these high-energy particles, or that high-energy neutrinos and cosmic rays only come from blazars. But the clear connection between the particles and at least one source is still a big step forward in terms of understanding the high-energy universe.
“Cosmic rays are charged particles of energies up to 1020 [electronvolts], far higher than the most powerful human-attained particle accelerator, i.e., the Large Hadron Collider (LHC),” the researchers said in the study. “The nature and origin of these particles arriving from deep outer space remain elusive and represent a foremost challenge for the astroparticle and astrophysics fields.”
Given that astrophysical neutrinos are “unique smoking-gun signatures of a cosmic-ray source,” the team added, it’s important to keep looking for these energetic ghost particles with next-generation neutrino detectors.