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

The results, published by the Large High Altitude Airstream Observatory (LHAASO) on November 16, resolved a long-standing mystery about the energy spectrum of cosmic rays, which shows a sharp decrease in cosmic rays with energies above 3 PeV, giving them an unusual knee-shaped shape.

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

However, now two recent studies published in National Science Review And Scientific BulletinAccordingly, they demonstrate that microquasars, driven by the accretion of black hole systems, are powerful particle accelerators in the Milky Way and are the likely source of the knee. The research also expands our understanding of extreme physical processes in black hole systems.

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

Microquasars identified as space accelerators

Black holes, one of the most mysterious objects in the Universe, generate relativistic jets when they accrete material from companion stars in binary systems, forming “microquasars.” In this study, LHAASO systematically detected ultrahigh energies for the first time. gamma rays from five microquasars: SS 433, V4641 Sgr, GRS 1915+105, MAXI J1820+070 and Cygnus X-1.

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

The proton energy in this system exceeded 1 PeV, and the total output power was about 10³² joules per second, which is equivalent to the energy released per second by four trillion of the most powerful hydrogen bombs. The energy of the gamma rays from V4641 Sgr was found to be as high as 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 microquasars are important PeV particle accelerators in the Milky Way, solving a long-standing problem in science: although supernova remnants have historically been considered sources of cosmic rays, both observational and observational. theoretical research showed that they cannot accelerate cosmic rays to “knee” energies and higher.

Problems of measuring cosmic ray spectra

To fully understand this phenomenon, precise measurements of the energy spectra of different types of cosmic rays, including their corresponding “knees,” are needed. The first step is to measure the energy spectrum of the lightest nuclei – protons. However, cosmic rays in the knee region are rare and reception from satellite detectors is limited, making detection akin to searching for a needle in a haystack.

In ground-based indirect measurements of cosmic ray particles, atmospheric interference cannot be avoided. This makes it difficult to distinguish protons from other nuclei. For a long time this measurement was considered impossible.

In this study, using its world-leading ground-based cosmic ray observation equipment, LHAASO developed multi-parameter measurement techniques and selected a large statistical sample of high-purity protons, allowing precise measurements of their energy spectrum with an accuracy comparable to that of satellite experiments. This measurement revealed a completely unexpected structure in the energy spectrum, clearly demonstrating a new “high-energy component” instead of a simple transition between power-law spectra.

Multiple accelerators determine the origin of cosmic rays

The new LHAASO results, along with the low-energy component measured by the AMS-02 space experiment and the intermediate-energy component measured by the DArk Matter Particle Explorer (DAMPE) space experiment, reveal the existence of multiple accelerators in the Milky Way, each with 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 mainly originate from “new sources” such as microquasars, which have an acceleration limit much higher than supernova remnants. This allows them to generate high-energy cosmic rays beyond the knee.

Breakthroughs link black holes to cosmic rays

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

LHAASO's hybrid detector array design enables the detection of cosmic ray sources using ultra-high energy gamma rays and also provides precise measurements of cosmic ray particles in the solar system's vicinity. This approach provides insight into the accelerated capabilities of sources at energies in pVah and the spectral characteristics to which they contribute. cosmic rays. This is the first time that a knee structure has been experimentally linked to a specific type of astrophysical source—a black hole jet system.

LHAASO, which was designed, built and operated by Chinese scientists, has taken a leading role in high-energy cosmic ray research due to its sensitivity in both astronomical gamma-ray studies and precision cosmic ray measurements. He made a number of discoveries that are of global significance, thereby contributing to our knowledge of extreme physical processes in the Universe.