Black Hole Jet Engines Finally Crack The Cosmic Ray Knee Mystery

After seven decades of debate, an ultra sensitive observatory in China has finally tied a puzzling kink in the cosmic ray spectrum to ravenous black holes.

In two new studies in National Science Review and Science Bulletin, an international team working with the Large High Altitude Air Shower Observatory (LHAASO) shows that black hole powered micro quasars in the Milky Way act as powerful PeV particle accelerators and reveals, through a precise proton spectrum, a previously unseen high energy component that explains the long mysterious cosmic ray “knee.” The work, led by scientists at the Institute of High Energy Physics of the Chinese Academy of Sciences alongside partners in Nanjing, Hefei and Rome, connects a landmark feature in the cosmic ray spectrum to a specific class of astrophysical sources for the first time.

The mystery centers on a sharp bend in the all particle cosmic ray energy spectrum at around 3 PeV, where the flux suddenly drops and the spectrum steepens, giving it a knee like shape. Since its discovery nearly 70 years ago, that bend has been treated as a kind of Rosetta stone for high energy astrophysics, hinting at the limits of known accelerators such as supernova remnants or at changes in how cosmic rays propagate through the Galaxy. But without direct evidence of sources that can push particles to and beyond this energy, the picture remained incomplete.

LHAASO changes that by attacking the problem from two sides at once. On the source side, its gamma ray telescopes systematically scanned the sky for ultra high energy photons that trace where extreme accelerators are at work. On the spectrum side, its hybrid array of particle, muon and Cherenkov detectors teased out an exceptionally pure sample of cosmic ray protons, allowing the team to map their energy distribution from 0.15 to 12 PeV with a precision previously achievable only in space based experiments at much lower energies.

Black Hole Micro Quasars As Galactic PeVatrons

Black holes in binary systems can siphon gas from a companion star and fling part of that material out in narrow, relativistic jets. When those jets shine brightly at many wavelengths, the systems are known as micro quasars. Using its gamma ray capabilities, LHAASO has now systematically detected ultra high energy emission from five such black hole systems in the Milky Way: SS 433, V4641 Sgr, GRS 1915+105, MAXI J1820+070 and Cygnus X 1.

In SS 433, the team finds that the ultra high energy radiation overlaps with a giant atomic cloud, a spatial coincidence that strongly suggests protons accelerated near the black hole are slamming into surrounding matter. The inferred proton energies exceed 1 PeV, and the total power is estimated at about 10³² joules per second, likened in the press release to the energy released every second by four trillion of the most powerful hydrogen bombs. In V4641 Sgr, gamma rays reach 0.8 PeV, implying parent particles with energies above 10 PeV and marking the system as another “super PeV particle accelerator.”

These measurements do more than showcase individual cosmic engines. They directly challenge the long held assumption that supernova remnants alone can account for Galactic cosmic rays up to the knee. Observations and models have increasingly suggested that supernova explosions struggle to reach those energies, especially for protons. The new LHAASO detections point instead to micro quasars as a natural population of PeVatrons, each contributing a high energy component that rises above the more familiar contribution of supernova remnants.

These results prove that micro-quasars are significant PeV particle accelerators in the Milky Way, addressing a long-standing issue in science: While supernova remnants were historically recognized as cosmic ray sources, both observational and theoretical studies have shown that they cannot accelerate cosmic rays to the energies of the “knee” and beyond.

Taken together with the dozens of ultra high energy gamma ray sources already cataloged by LHAASO elsewhere in the Galaxy, the micro quasar detections help populate a Galactic map of extreme accelerators. They also set up a crucial test: if micro quasars really power a distinct high energy component of cosmic rays, that component should leave a visible imprint in the proton spectrum measured near Earth.

A New High Energy Component In The Proton Spectrum

Measuring that imprint is technically brutal. In the knee region, cosmic rays are rare, and satellite detectors have limited geometric acceptance, making it “like finding a needle in a haystack,” as the press release describes. Ground based arrays can collect huge numbers of air showers, but must infer the primary particle from cascades of secondaries that have already been scrambled by the atmosphere. Distinguishing protons from heavier nuclei event by event was long deemed effectively impossible.

LHAASO was purpose built to push past that barrier. Its square kilometer array (KM2A) uses a dense network of electromagnetic particle detectors on the surface and large buried muon detectors below to capture both the shower front and its muon content with unprecedented coverage. Above, the wide field of view Cherenkov telescope array (WFCTA) images the air shower as a streak of Cherenkov light, providing a precise estimate of shower energy and the depth of shower maximum in the atmosphere. By combining carefully chosen parameters that depend on muon counts and Cherenkov image geometry, the team constructs a two dimensional space where proton induced showers separate cleanly from those created by heavier nuclei.

With this hybrid technique, LHAASO selected a large, high purity sample of proton events and reconstructed their spectrum from 0.15 to 12 PeV. The result is not a simple curve that glides through the knee. Instead, the proton spectrum hardens relative to low energy extrapolations, climbs to a pronounced hump around 3 PeV, and then softens sharply. That distinctive shape matches the knee in the all particle spectrum and signals the emergence of a new high energy component at PeV energies.

The proton spectrum shows significant hardening relative to low-energy extrapolations, culminating at 3 PeV, followed by sharp softening. This distinct spectral structure closely aligned with the knee in the all-particle spectrum points to the emergence of a new CR component at PeV energies that might be linked to the dozens of PeVatrons recently discovered by LHAASO, and offers crucial clues to the origin of Galactic cosmic rays.

By combining this high precision proton spectrum with lower energy measurements from the space borne AMS 02 and DAMPE experiments, the researchers assemble a multi component picture of Galactic cosmic rays. At low energies, one population of accelerators dominates. At intermediate energies, another component becomes important. Around the knee, a new high energy component, consistent with PeVatrons like micro quasars, rises and then cuts off, producing the familiar bend in the all particle spectrum. Crucially, the knee energy for protons, about 3.3 PeV in the LHAASO fit, matches the all particle knee within uncertainties.

For the first time, the knee structure is observationally tied to a specific class of astrophysical sources, the black hole jet systems that LHAASO now sees as Galactic PeV particle engines. The work not only resolves a cornerstone problem in cosmic ray physics, it also opens a new window on how black holes and their companions sculpt the high energy environment of the Milky Way.

Science Bulletin: 10.1016/j.scib.2025.10.048