Chinese scientists identify Milky Way origin for ultra-high energy cosmic rays

Scientists at an observatory in southwestern China say they have identified a super cosmic ray accelerator – a discovery that could fundamentally change understanding of the origin and source of cosmic rays in our galaxy.

The existence of a bubblelike structure – around 10 million times larger than our solar system – could explain how ultra-high energy gamma rays may have originated within the Milky Way, in contrast to previous theories about their origins, they said.

According to a paper published on Monday by the peer-reviewed Science Bulletin, the structure was observed in the Cygnus constellation with the Large High Altitude Air Shower Observatory (Lhaaso) in Sichuan province.

Corresponding author Cao Zhen, a professor at the Chinese Academy of Sciences’ Institute of High Energy Physics, told state news agency Xinhua that “this is the first super cosmic ray accelerator identified, as of now”.

Cosmic rays are charged particles that move through space at nearly the speed of light. Although they were discovered more than 100 years ago, scientists are still unsure of how the rays that reach Earth were formed and where they originated.

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Previous energy spectra of cosmic rays showed an inflection point at around the energy level of 1 petaelectronvolt (PeV) – called the “knee” because of its shape in graphs.

Cosmic rays with an energy lower than 1 PeV were thought to originate from within the Milky Way, while rays with an energy above this point were thought to originate from outside our galaxy, Cao told the Sichuan News Network.

However in 2020, Lhaaso discovered ultra-high energy photons generated by cosmic rays with over 1 PeV energy originating from Cygnus. The latest paper details how these rays could have formed.

The researchers found that the bubblelike structure in the constellation’s star-forming region – called Cygnus X – contained multiple photons exceeding 1 PeV, including one as high as 2.5 PeV.

At the centre of the bubble, the team identified a massive star cluster called Cygnus OB2 which contains many young, hot stars, some with surface temperatures of more than 35,000 degrees Celsius – which may be the super cosmic ray accelerator.

The structure and energy spectrum of the bubble “are reasonably reproduced by the assumption of a particle accelerator in the core,” according to the paper.

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The cosmic ray accelerator is capable of injecting cosmic rays into interstellar space at an energy of 10 to 25 PeV, which collide with interstellar gas to produce the observed photons with energies of 1 PeV and higher.

Cao told the Sichuan News Network that the stars in the cluster have a radiation intensity “a hundred to a million times that of the sun”. Materials blown from the stars’ surfaces by radiation pressure create high speed winds that collide with interstellar media – forming a particle accelerator.

“After the accelerator is formed, microscopic particles dominated by protons are accelerated to extremely high energies, which are the cosmic rays we observe on Earth,” Cao said.

The researchers found that the gases surrounding the bubble structure correlated to the intensity of gamma rays produced by the acceleration of high-energy particles into interstellar space.

“A robust assumption is that every cosmic ray source in our galaxy has such a bubble or halo of cosmic rays,” they wrote in the paper.


‘Supermassive’ black hole at the centre of our galaxy viewed for the first time

‘Supermassive’ black hole at the centre of our galaxy viewed for the first time

The diffusion speed of the cosmic rays was much slower than expected – 100 times less than previously believed – which made it possible for the researchers to observe the bubble, Cao told the Sichuan News Network.

“With increasing observation time, Lhaaso is expected to detect more super cosmic ray accelerators,” he said, in the Xinhua report. Cao added that he hopes further discoveries can “solve the century-long enigma of the origin of cosmic rays”.

Elena Amato, an astrophysicist from the Italian National Institute for Astrophysics, said the results have “very relevant consequences on our description of cosmic ray transport in the galaxy”.


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