The Large High Altitude Air Shower Observatory (LHAASO) has identified microquasars as key accelerators behind the cosmic ray 'knee,' a decades-old mystery in astrophysics. Released on November 16, the findings reveal these black hole systems produce ultra-high-energy particles exceeding previous expectations. Two studies confirm microquasars' role in shaping the galaxy's particle environment.
For nearly 70 years, scientists have puzzled over the 'knee' in the cosmic ray energy spectrum—a sharp drop in particle counts above 3 PeV, signaling a shift from one power-law behavior to another. Milestone results from LHAASO, released on November 16, trace this feature to microquasars: compact binary systems where black holes accrete matter from companion stars, launching relativistic jets that act as extreme particle accelerators.
Two studies, published in National Science Review and Science Bulletin, highlight microquasars as the likely origin. LHAASO detected ultra-high-energy gamma rays from five such objects: SS 433, V4641 Sgr, GRS 1915+105, MAXI J1820+070, and Cygnus X-1. These mark the first systematic observations at PeV energies.
In SS 433, gamma rays overlap with a massive atomic cloud, indicating black hole-accelerated protons colliding with surrounding material. Proton energies there surpass 1 PeV, with total power reaching about 10^32 joules per second—equivalent to four trillion hydrogen bombs exploding each second. V4641 Sgr produced gamma rays up to 0.8 PeV, with parent particles exceeding 10 PeV, confirming it as a super PeV accelerator.
LHAASO's advanced techniques enabled precise proton spectrum measurements in the knee region, revealing an unexpected high-energy component. Combined with data from AMS-02 and DAMPE, this shows the Milky Way hosts multiple accelerators, each with distinct energy limits. Supernova remnants explain lower energies, but microquasars account for the knee and beyond, resolving a long-standing debate.
The research involved scientists from the Institute of High Energy Physics of the Chinese Academy of Sciences, Nanjing University, University of Science and Technology of China, and La Sapienza University of Rome, among others. LHAASO's hybrid array provides dual insights into gamma rays and cosmic rays, linking the knee directly to black hole jet systems and advancing understanding of the universe's extreme processes.