China's quest to unravel the universe's secrets has led to a groundbreaking discovery, shedding light on a cosmic enigma that has puzzled scientists for decades. But what exactly is this mysterious 'knee' in the cosmic ray energy spectrum?
The Cosmic 'Knee' Mystery:
China's Large High Altitude Air Shower Observatory (LHAASO) has revealed a fascinating insight into the origin of cosmic rays. The observatory's data shows a peculiar 'knee' in the energy spectrum, a sharp drop-off at around 3 peta-electronvolts (PeV). This phenomenon has baffled researchers since its initial observation nearly 70 years ago.
Scientists have long theorized that the 'knee' is linked to the limits of cosmic ray acceleration in astrophysical sources. But here's where it gets controversial—the exact cause has remained a mystery, until now.
Unveiling the Cosmic Accelerator:
Two groundbreaking studies published in prestigious journals have identified the culprits behind the 'knee'. Micro-quasars, driven by black hole systems, are the likely source of this cosmic conundrum. These micro-quasars, formed when black holes pull material from companion stars, act as powerful particle accelerators in our Milky Way galaxy.
The studies, conducted by a team of Chinese and international scientists, detected ultra-high-energy gamma rays from five micro-quasars, a first-time achievement. This suggests that the cosmic rays generating these gamma rays have energies exceeding 1 PeV, surpassing the 'knee' energy threshold. And this is the part most people miss—previous cosmic ray sources, like supernova remnants, couldn't reach these energy levels.
A New Cosmic Structure:
To fully comprehend this phenomenon, scientists needed to measure the energy spectra of different cosmic ray species and their respective 'knee' regions. However, this task was incredibly challenging due to the rarity of cosmic rays in this energy range and atmospheric interference. Distinguishing protons from other nuclei was once deemed impossible.
LHAASO's advanced multiparameter measurement techniques came to the rescue. The team successfully obtained a large sample of high-purity protons, comparable in precision to satellite experiments. This led to the discovery of a new, unexpected structure in the energy spectrum, revealing a 'high-energy component'.
A Galaxy of Accelerators:
LHAASO's findings, combined with space-borne experiments, paint a fascinating picture. They suggest the presence of multiple accelerator populations within the Milky Way, each with distinct acceleration capabilities and energy ranges. The 'knee' represents the limit of these accelerators' power, specifically those generating the high-energy component.
These discoveries provide a comprehensive understanding of the cosmic 'knee' mystery and highlight the role of black holes in cosmic ray production. LHAASO's unique design enables the detection of cosmic ray sources and precise measurement of cosmic ray particles near our solar system.
Implications and Future Prospects:
For the first time, the 'knee' has been linked to a specific astrophysical source—black hole jet systems. Cao Zhen, a leading scientist, estimates there are around a dozen such sources in our galaxy. Moreover, the principles of these cosmic accelerators could inspire the design of next-generation particle accelerators on Earth.
LHAASO's location at a high altitude in China's Sichuan Province has made it a leader in high-energy cosmic ray research. Its contributions are advancing our understanding of the universe's extreme physical processes, leaving us with a profound appreciation of the cosmos and its mysteries.
