Chinese and Japanese scientists announced on Friday that they have found "unequivocal evidence" of the existence of PeVatrons, the most powerful cosmic accelerator known to date, in our Milky Way galaxy, but exactly what PeVatron is and how it works still remains to be discovered.
PeVatron is capable of accelerating particles to a staggering energy level of 1 PeV, or 1 million billion electron volts, which is 100 times more energetic than the highest level achieved by man-made accelerators.
The discovery marks a key milestone in revealing the origin of high-energy cosmic rays—extremely energetic and charged particles including protons and atomic nuclei that travel close to the speed of light, experts said. The findings will be published in the journal Physical Review Letters on Monday.
Since its discovery in 1912, scientists have been struggling to find the origins and acceleration mechanisms of these mysterious space travelers, said Huang Jing, a co-author of the study and a researcher from the Institute of High Energy Physics of the Chinese Academy of Sciences.
As a result, how do cosmic accelerators work was listed by the United States National Research Council as one of the 11 science questions for the 21st century.
"Although we have yet to find the origin of cosmic rays, the discoveries and efforts we made along the way have greatly accelerated the development of particle physics," said Huang.
Scientists hypothesized that supernova explosions, star-forming regions in the universe or supermassive black holes at the galactic centers are PeVatron candidates, but none have been substantiated by observational evidence.
This is mainly because most cosmic rays carry an electric charge, therefore magnetic fields from celestial objects and events can alter the particles' paths as they fly through space, making it extremely difficult to trace their origins, Huang said.
However, cosmic rays can interact with the thin gas of interstellar medium, said Wang Xiangyu, a professor of astrophysics at Nanjing University.
This interaction would produce gamma rays, the most energetic type of wave in the electromagnetic spectrum, with roughly 10 percent of the energy of the original cosmic rays.
Since gamma rays are electrically neutral, they are unaffected by magnetic fields. Coupled with its tremendous penetrating power, scientists believe they can tell where PeVatrons are in the universe by studying ultrahigh energy gamma rays, Wang said.
But this approach also has limitations. First, the higher the energy of the gamma rays, the rarer they are. Second, gamma rays require extremely sensitive instrument to detect them, Huang said.
Since 1990, dozens of researchers from China and Japan have been hunting for the elusive high-energy gamma rays at 4,300 meters above sea level in Yangbajing, Tibet autonomous region, in the Tibet ASgamma experiment.
Over the years, the observatory for the experiment, which consists of over 500 detectors that cover about 65,000 square meters and an underground particle detector, has discovered 23 ultrahigh energy cosmic gamma rays with energy levels above 398 trillion electron volts from the galactic disk region of the Milky Way.
The highest energy recording was 957 trillion electron volts, which is close to the 1 PeV mark and it is the new world record for gamma ray photons.
But what surprised scientists was that these gamma rays did not point back to their sources, but were somehow spread throughout the Milky Way. This prompted scientists to theorize that multiple PeVatrons may be responsible for producing different gamma rays with extreme levels of energy in our home galaxy.
"These cosmic rays produced by PeVatrons may have been trapped by our galaxy's magnetic field and bounced around in it for millions of years," Huang said.
"The gamma rays are like cosmic footprints for the PeVatrons. We now know for a fact that they exist, but exactly what celestial object is responsible for PeVatrons and how many there are in the Milky Way remain a mystery."
