The Mystery of Cosmic Rays: Unraveling the Knee Phenomenon
Since the discovery of a sharp drop in the cosmic ray spectrum about 70 years ago, scientists have been intrigued by the reasons behind this mysterious shift. This drop, known as the “knee,” was long thought to represent the highest energy levels that cosmic ray sources could achieve, indicating a transition in the spectrum from one power-law behavior to another.
Microquasars: The New Generation of Particle Accelerators
Recent studies suggest that microquasars, powered by black hole accretion, may be responsible for this shift in the cosmic ray spectrum. These compact systems act as extremely powerful particle accelerators within our galaxy, the Milky Way, and can reasonably generate the energies associated with the knee. These discoveries deepen scientific understanding of how black hole systems drive extreme physical processes.
The Role of LHAASO in Recent Discoveries
The Chinese space observatories LHAASO have played a crucial role in these discoveries. They have detected high-energy gamma emissions from five microquasars, including SS 433 and Cygnus X-1. This marks the first time such objects have been observed at these high energy levels. Observations from SS 433 were particularly astounding, showing gamma rays interacting with a massive atomic cloud, indicating the acceleration of high-energy protons by the black hole.
New Scientific Insights
These studies have led to a new understanding that microquasars can accelerate particles to energy levels exceeding a peta-electronvolt, surpassing the known capabilities of supernova remnants. This new insight offers a solution to the long-standing challenge in cosmic ray research regarding high-energy sources beyond the knee.
Challenges in Measuring the Proton Spectrum
Understanding the full spectrum of cosmic rays requires precise measurement of the energy spectra of various particle types. Measuring the proton spectrum, the lightest nuclei, is the first and most crucial step. Despite the rarity of protons in the knee energy range, LHAASO has developed multi-criteria analysis techniques to isolate a large and statistically pure sample of protons, enabling highly accurate energy spectrum measurements.
Conclusion
These discoveries represent a significant step forward in our understanding of cosmic rays and their sources. By directly linking the knee to black hole systems, a long-standing mystery about the origins of this phenomenon has been resolved. The research highlights the role of microquasars as new and powerful sources of high-energy particles in our galaxy, opening new avenues for studying the universe and understanding the extreme physical processes occurring within it.