Microquasars Identified by LHAASO as Highest-Energy Cosmic Ray Accelerators in the Galaxy

Scientists utilizing China's Large High Altitude Air Shower Observatory (LHAASO) have advanced a hypothesis suggesting that microquasars function as the most potent particle accelerators responsible for generating the highest-energy cosmic rays within the Milky Way. This finding addresses one of the most enduring questions in modern astrophysics: the origin of cosmic rays, a mystery that has persisted for nearly seven decades.

These microquasar systems are characterized as binary pairs where a black hole accretes matter from a companion star, subsequently ejecting relativistic jets. Research conducted by scientists from institutions including the Institute of High Energy Physics of the Chinese Academy of Sciences (CAS), Nanjing University, the University of Science and Technology of China, and Sapienza University of Rome saw these results published in the journals National Science Review and Science Bulletin. The observations confirm that microquasars can accelerate particles to PeV levels, addressing the long-standing dilemma surrounding the "knee" in the cosmic ray energy spectrum.

A key piece of data involved the observation of photons with energies exceeding 1 PeV within a structure in the Cygnus star-forming region, with the maximum measured energy reaching 2.5 PeV. This observation strengthens the role of microquasars as cosmic ray sources, particularly in the energy region known as the "knee," a focus of recent research. The findings challenge the long-held assumption that supernova remnants alone account for galactic cosmic rays up to this energy threshold, as both observational data and theory suggest they cannot reach such extreme energies.

The LHAASO observatory, the site of this pioneering research, is situated in Daocheng, Garzê Tibetan Autonomous Prefecture, Sichuan, China, at an elevation of 4,410 meters above sea level. Construction of the main facility began in 2017 and was completed in 2021, covering an observation area of approximately 145 hectares. LHAASO commenced observations in April 2019 and by 2021 had identified dozens of ultra-high-energy accelerators, or PeVatrons, in the Milky Way. The observatory passed its national acceptance inspection on May 10, 2023, and is now a central hub for international cosmic ray research with China as the primary contributor.

LHAASO employs three primary arrays: the ground-based particle shower detector array (KM2A) with 5,216 electromagnetic particle detectors and 1,188 muon detectors, a 78,000 square meter Water Cherenkov Detector Array (WCDA), and an array of 18 wide-field Cherenkov telescopes. The confirmation that microquasars are the primary accelerators up to the 20 PeV threshold will significantly refine current astrophysical models of particle acceleration within the galaxy.

Chief researcher of LHAASO and CAS Academician, Cao Zhen, concluded that these studies illuminate the primary mechanism behind cosmic ray origins and advance the understanding of extreme physical processes within black hole systems. These discoveries provide critical observational evidence for understanding the role of black holes as cosmic ray sources, thereby resolving the "knee" mystery that has perplexed the academic community for nearly 70 years.