Cosmic ray puzzle resolved as scientists link ‘knee’ formation to black holes

Milestone results released by the Large High Altitude Air Shower Observatory (LHAASO) on November 16 have solved a decades-old mystery about the cosmic ray energy spectrum—which shows a sharp decrease in cosmic rays above 3 PeV, giving it an unusual knee-like shape.

The cause of the “knee” has remained unclear since its discovery nearly 70 years ago. Scientists have speculated that it is linked to the acceleration limit of the astrophysical sources of cosmic rays and reflects the transition of the cosmic ray energy spectrum from one power-law distribution to another.

Now, however, two recent studies—published in National Science Review and Science Bulletin, respectively—demonstrate that micro-quasars driven by black hole system accretion are powerful particle accelerators in the Milky Way and are the likely source of the “knee.” The studies also advance our understanding of the extreme physical processes of black hole systems.

The research was conducted by researchers from the Institute of High Energy Physics of the Chinese Academy of Sciences (CAS), Nanjing University, the University of Science and Technology of China of CAS, La Sapienza University of Rome, and other institutions.

Micro-quasars identified as cosmic accelerators

Black holes, one of the most enigmatic objects in the universe, generate relativistic jets when accreting material from companion stars in binary systems, forming “micro-quasars.” In this study, LHAASO systematically detected for the first time ultra-high-energy gamma rays from five micro-quasars: SS 433, V4641 Sgr, GRS 1915+105, MAXI J1820+070, and Cygnus X-1.

In particular, the ultra-high-energy radiation from SS 433 was found to overlap with a giant atomic cloud, strongly suggesting that the high-energy protons are accelerated by the black hole and collide with surrounding matter.

The proton energy in this system exceeded 1 PeV, with a total power output of approximately 10³² joules per second, equivalent to the energy released per second by four trillion of the most powerful hydrogen bombs. The gamma-ray energy from V4641 Sgr was found to reach 0.8 PeV, making it another “super PeV particle accelerator,” while the parent particles generating these gamma rays had energies exceeding 10 PeV.

These results prove that micro-quasars are significant PeV particle accelerators in the Milky Way, addressing a long-standing issue in science: While supernova remnants were historically recognized as cosmic ray sources, both observational and theoretical studies have shown that they cannot accelerate cosmic rays to the energies of the “knee” and beyond.

Challenges in measuring cosmic ray spectra

To fully understand this phenomenon, precise measurements of the energy spectra of the various cosmic ray species including their respective “knees” are essential. The first step is to measure the energy spectrum of the lightest nuclei—protons. However, cosmic rays in the “knee” region are sparse and satellite detectors have limited acceptance, making detection akin to finding a needle in a haystack.

In ground-based indirect measurements of cosmic ray particles, it is impossible to avoid atmospheric interference. This makes it difficult to distinguish protons from other nuclei. For a long time, this measurement was considered impossible.

In this study, leveraging its world-leading ground-based cosmic ray observational equipment, LHAASO developed multi-parameter measurement techniques and selected a large statistical sample of high-purity protons, allowing for precise measurement of their energy spectrum, with precision comparable to that of satellite experiments. This measurement revealed an energy spectrum structure that was entirely unexpected, clearly displaying a new “high-energy component” instead of a simple transition between power-law spectra.

Multiple accelerators shape cosmic ray origins

LHAASO’s new findings, together with the low-energy component measured by the space-borne AMS-02 experiment and the intermediate-energy component measured by the space-borne DArk Matter Particle Explorer (DAMPE) experiment, revealed the existence of multiple accelerators within the Milky Way, with each possessing its own unique acceleration capability and energy range. The “knee” represents the acceleration limit of the sources responsible for generating the high-energy component.

The complex structure of the proton energy spectrum indicates that cosmic ray protons in the PeV energy range primarily originate from “new sources” such as micro-quasars, which have an acceleration limit significantly higher than that of supernova remnants. This enables them to generate high-energy cosmic rays that exceed the “knee.”

Breakthroughs connect black holes to cosmic rays

These two discoveries support each other, presenting a comprehensive scientific picture. This not only marks a significant advancement in resolving the long-standing mystery of the “knee” origin, but also offers crucial observational evidence for understanding the role of black holes in the origin of cosmic rays.

LHAASO’s hybrid detector array design allows for the detection of cosmic ray sources through ultra-high-energy gamma rays, while enabling precise measurement of cosmic ray particles in the vicinity of the solar system. This approach offers insights into the acceleration capabilities of sources at PeV energies and the spectral characteristics they contribute to cosmic rays. For the first time, the “knee” structure has been observationally connected to a specific type of astrophysical source—the black hole jet system.

LHAASO, which was designed, constructed, and is operated by Chinese scientists, has taken the lead in high-energy cosmic-ray research due to its sensitivity in both gamma ray astronomical exploration and cosmic ray precision measurement. It has achieved a series of discoveries that have a global impact, thereby contributing to our knowledge of the extreme physical processes in the universe.