Scientists find particles of ultrahigh energy in Milky Way

The Milky Way glows with iridescent beauty above the Gobi grassland in Urad Rear Banner, North China's Inner Mongolia autonomous region.
[Photo by Bao Changqing/For]

A Chinese cosmic ray observatory has found 12 sources of ultrahigh-energy photons and detected a photon with a record-breaking energy level of 1.4 peta-electron volt, or 1.4 million billion electron volts, according to a study published in the journal Nature on Monday.

Experts called these findings exciting, describing them as opening a new chapter in mankind's astronomical study of the most energetic particles in the universe, whose origins and acceleration mechanisms have fascinated and baffled scientists for over a century.

In April last year, Wang Lingyu, an associate professor at the Chinese Academy of Sciences' Institute of High Energy Physics, was checking data collected by the Large High Altitude Air Shower Observatory, or LHAASO, when she discovered something surprising, according to China Science Daily, the official newspaper of the academy.

LHAASO is an extensive assembly of detectors and telescopes covering more than 1.36 square kilometers and located at an altitude of 4,410 meters in Daocheng county, Sichuan province. It had picked up what Wang believed to be an ultrahigh-energy gamma-ray photon.

After catching her breath, Wang relayed the anomaly calmly to colleague Chen Songzhan.

Chen, in turn, shouted: "What!"

After rechecking the data, they emailed the report to Cao Zhen, LHASSO's chief scientist, and said they needed "to further verify its authenticity"-following the statement with six exclamation points.

Three months later, they confirmed that the mysterious space traveler was from the Milky Way Galaxy and was packing a staggering energy level of 1.4 PeV. That is 140 times higher than what can be achieved by the Large Hadron Collider, which is near Geneva and is the most powerful particle accelerator on Earth. It is the highest recorded energy photon to reach Earth to date.

This proved that PeVatrons, or PeV accelerators, exist in our home galaxy, but exactly where and what they are remains a mystery, according to the study. Within 11 months last year, LHAASO found a total of 12 potential PeVatron candidates in the Milky Way.

Cao said LHAASO started operation in 2019 and was able to make the discoveries with only half of its equipment installed. The full instrument, which is set to be completed next month, will host over 5,195 electromagnetic particle detectors, 1,188 muon detectors, 18 telescopes and three giant water ponds with a total surface area of 78,000 square meters.

"A photon in the PeV energy range is an extremely rare find-typically a detector the size of one square kilometer can only pick up one or two every year from a source like the Crab Nebula-and they are mixed in over hundreds of thousands of other cosmic ray signals," Cao said, adding that LHAASO's colossal size and unmatched sensitivity are what made the discovery possible.

Karl Ziemelis, the chief physical sciences editor at Nature, said cosmic rays are charged particles, such as protons and atomic nuclei, flying through space with velocities close to the speed of light.

Since their discovery in 1912, scientists have been struggling to understand how these speedy space bullets are created. The question of how cosmic accelerators work was listed by the United States National Research Council as one of the top 11 science questions of the 21st century.

Scientists have long hypothesized that supernova remnants, star-forming regions in the universe or supermassive black holes at the galactic center are PeVatron candidates. With the new discovery, clusters of young but very massive stars should also be considered.

"Where and what they are remains an open question, but this new paper provides some important clues," Ziemelis said. "Tracing the origin of these high-energy cosmic rays is not a straightforward task, as magnetic fields in space can cause their paths to twist and bend when they travel."

"So when we detect such a cosmic ray arriving at Earth, we may be able to determine the direction it arrived from, but this is not necessarily the same direction it started from."

One way to tackle this problem is by observing gamma rays, the most energetic type of wave in the electromagnetic spectrum, produced when ultrahigh cosmic rays from PeVatrons interact with the thin gas of the interstellar medium around them. These gamma rays can carry roughly 10 percent of the energy of the original cosmic rays.

Gamma rays are electrically neutral and thus are unaffected by magnetic fields. Scientists believe they can tell where PeVatrons are in the universe by studying these high energy gamma rays, which act as footprints for the cosmic rays produced by the accelerator.

Felix Aharonian, a professor of astrophysics at the Dublin Institute for Advanced Studies, said LHAASO will dominate in the field of ultrahigh-energy gamma-ray astronomy and play a major role in its development, and "the results reported in the Nature article reveal only the tip of the iceberg".

"In the coming years, we anticipate breakthrough discoveries by LHAASO that could dramatically change the current concepts about the most energetic and extreme phenomena in the nonthermal universe," he said.

Benedetto Piazzoli, a professor of astrophysics at the University of Naples Federico II, said LHAASO will explore in depth a wide energy range where the transition from galactic to extragalactic cosmic ray origins is expected.

"Many of LHAASO's discoveries will no doubt bring surprises. Thus there is a good chance that the cosmic-ray mystery could be unveiled," he said. "There are exciting times ahead."