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A dozen ultra-high-energy particle accelerators discovered in the Milky Way

Crab Nebula
The Crab Nebula is one source of ultra-high-energy cosmic rays. (Image credit: NASA, ESA, J. Hester and A. Loll (Arizona State University))

A century-old celestial mystery is one step closer to being solved as researchers discover a dozen ultra-powerful natural particle accelerators in our galaxy. 

The findings help astronomers understand the origin of cosmic rays — charged particles and atomic nuclei flying through space at near light speed that have been imbued with mind-boggling amounts of energy. 

Discovered in 1912, cosmic rays arrive from almost every direction in the Milky Way, though scientists have yet to determine exactly how they reach their ultra-fast speeds, according to NASA.

Many researchers suspected that cosmic rays are flung away from massive stars as they die in supernova explosions, Siming Liu, an astrophysicist with the Southwest Jiaotong University in Chengdu, told Live Science. During such events, "stars release the same amount of energy in two months as over their whole life," he added. 

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But even a powerful blast like this is only capable of imparting less than a peta-electron-volt (PeV), or a quadrillion electron-volts, to cosmic rays, Liu said. Observatories have captured ultra-high-energy cosmic rays with energies that exceed that and, so far, nobody has been able to figure out where in the universe they come from.

Discovering the sources of cosmic rays has been difficult, because as charged entities they are deflected by magnetic fields, which are abundant in the Milky Way, Liu said. That means a cosmic ray captured on Earth won't point directly back to its origin point, he added. 

But as they jet away from their sources, cosmic rays can interact with surrounding gases and generate gamma rays with a 10th of the cosmic ray's energy. These rays aren't charged and so travel in straight lines, offering a means of discovering where they came from.

Along with his colleagues, Liu used China's Large High Altitude Air Shower Observatory (LHAASO), a facility under construction atop Haizi Mountain at the edge of the Tibetan Plateau in Sichuan Province, to indirectly look at gamma ray light. As gamma rays impinge on Earth's atmosphere, they generate a shower of particles that can be captured in LHAASO's thousands of detectors, which will eventually spread over an area of 0.4 square miles (1 square kilometer), according to a press release

Though the data was taken with only half the array operational, it was able to reveal a dozen sources — dubbed PeVatrons for their ability to imbue subatomic particles with peta-electron-volts’ worth of energy — all over the Milky Way. These entities are at least 100 times more powerful than the largest particle accelerator on Earth, the Large Hadron Collider

The team also detected the most powerful gamma-ray photon, or light particle ever seen — an object with 1.4 PeV. They reported their findings on May 17 in the journal Nature

Among the PeVatrons are familiar objects, such as the Crab Nebula, which is known to contain a dead star known as a pulsar that is a potential suspect as the cosmic rays' accelerator. But the list also includes an active star-forming region in the constellation Cygnus, leaving researchers scratching their heads over what is shooting out such powerful particles there.

LHAASO is only capable of pinpointing the PeVatron sources to within a few tens or hundreds of light-years, Liu said, so it's difficult to know exactly what objects in each region are causing the acceleration. 

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Nevertheless, "this is a major step," Razmik Mirzoyan, an astroparticle physicist at the Max Planck Institute for Physics in Germany, told Live Science. LHAASO will soon be four times larger than any previous telescope of this type, allowing it to unlock a new era of ultra-high-energy observations, Mirzoyan added.

Mirzoyan is part of a collaboration that is building a similar facility in the Southern Hemisphere to hone in on ultra-high-energy cosmic ray sources. By combining information from this facility with data from telescopes that observe in the electromagnetic spectrum and those looking at neutrinos, it's possible the field will finally know where these mysterious entities originate from within about 10 years, he said. 

Liu agreed that future observations with LHASSO and other instruments should one day help pinpoint how cosmic rays reach such prodigious speeds and energies. "We hope we can address this issue," he said. "These observations open the possibility to answer this question."

Originally published on Live Science.

Editor's Note: This story was updated to correct Liu's affiliation; he is with the Southwest Jiaotong University in Chengdu, not the Purple Mountain Observatory in Nanjing.