Unveiling the Most Elusive Feature in Cosmic Radiation

For nearly seven decades, astrophysicists have been trying to decode the strange, abrupt feature in the cosmic ray energy spectrum known as the knee. This dip—an unexpected decline in cosmic ray abundance at energies above roughly 3 petaelectronvolts (PeV)—has long resisted explanation. Since the 1950s, the prevailing assumption was that supernova remnants were the primary drivers of galactic cosmic rays across all energies. Their massive shock waves were believed powerful enough to accelerate charged particles to extreme velocities. But as observations grew more precise, it became increasingly clear that supernovae alone could not account for the highest-energy cosmic rays detected near Earth.

Now, a watershed discovery has redefined the landscape of high-energy astrophysics. Using the Large High Altitude Air Shower Observatory (LHAASO) in China, scientists have uncovered definitive evidence that the cosmic ray knee originates not from supernova explosions but from micro-quasars powered by black holes within our own Milky Way. These compact, violent systems—stellar-mass black holes feeding on companion stars—generate relativistic jets capable of accelerating protons to energies far beyond the reach of supernova shock waves. This breakthrough not only rewrites decades of assumptions but also reveals the Milky Way as a far more dynamic particle accelerator than ever imagined.

Understanding the Cosmic Ray Knee

The cosmic ray energy spectrum normally follows a smooth power-law curve: higher-energy particles become progressively rarer. But around 3 PeV, the curve suddenly steepens—this is the knee. For decades, astrophysicists proposed explanations ranging from supernova remnant limitations to leakage of high-energy protons out of the galaxy. However, none of these models fully matched the observed sharpness or energy threshold of the knee.

The new findings show that this feature is not due to propagation effects or supernova limitations but instead reflects the upper acceleration limit of one specific population of cosmic ray sources—black hole micro-quasars.

This is the first time researchers have tied the knee to a concrete astrophysical mechanism rather than speculative modeling.

Micro-Quasars: The Milky Way’s Hidden Particle Accelerators

Micro-quasars are scaled-down versions of quasars—cosmic powerhouses found in distant galaxies. While quasars harbor supermassive black holes millions of times the mass of the Sun, micro-quasars contain stellar-sized black holes, usually between 5 and 20 solar masses. Despite their smaller size, their jet energies are extraordinarily intense due to their rapid accretion processes.

These systems consist of:

• A black hole actively consuming matter from a companion star
• An accretion disk heating up to millions of degrees
• Relativistic jets blasting from the poles at near-light speed

Until recently, micro-quasars were known mainly for their bright X-ray and gamma-ray emissions. But LHAASO has now shown they are also major proton accelerators, capable of pushing particles to energies exceeding 1 PeV—precisely the domain of the cosmic ray knee.

How Black Hole Jets Produce Ultra-High-Energy Cosmic Rays

The newly established mechanism begins in the relativistic jets launched by the black hole. As matter spirals inward, magnetic fields twist around the black hole’s spin axis, generating tightly collimated jets that fire outward at relativistic speeds. Within these jets, protons undergo repeated acceleration through shock waves and magnetic turbulence, gaining energy with each scattering event.

When these ultra-energized protons collide with surrounding gas clouds—such as giant molecular clouds near micro-quasar systems—they produce neutral pions that decay almost instantly into high-energy gamma rays.

These gamma rays are the key: they reveal the maximum energy reached by the protons. LHAASO’s ability to detect gamma rays above 1 PeV finally exposed the true nature of these cosmic engines.

SS 433: The First Confirmed PeV Accelerator

One of the most significant detections came from SS 433, a famous micro-quasar located within the W50 nebula. SS 433 has long been known for its powerful jets and unusual precession motion, but LHAASO’s observations transformed it from an exotic object into a cosmic accelerator of unprecedented scale.

Its collision with a surrounding atomic cloud produced gamma rays with energies exceeding 1 PeV, confirming that the jets host protons at energies many times higher. The energy output is staggering—equivalent to trillions of hydrogen bombs per second, making SS 433 one of the most energetic objects in the Milky Way.

This single detection alone hinted strongly that micro-quasars could account for the knee. But one observation is never enough. That is where additional detections sealed the case.

V4641 Sgr: An Even More Powerful Accelerator

Another key source is V4641 Sgr, a micro-quasar known for its extreme variability and powerful outbursts. LHAASO detected gamma rays at super-PeV energies—far beyond the knee threshold. These energies surpass the acceleration capabilities of all known supernova remnants combined.

The existence of multiple micro-quasars producing PeV and super-PeV particles confirms that the phenomenon is widespread, not an anomaly.

Other Contributing Sources Strengthening the Evidence

Several additional black hole systems also exhibit similar acceleration behavior:

• GRS 1915+105, known for its steady jet activity
• MAXI J1820+070, a rapidly evolving and highly luminous X-ray transient
• Cygnus X-1, one of the first confirmed black hole candidates

Each of these sources shows evidence of producing high-energy gamma rays or of having jet characteristics compatible with PeV acceleration. Collectively, they form a population of galactic sources capable of shaping the high-energy tail of the cosmic ray spectrum.

Why Supernova Remnants Are No Longer the Prime Candidates

Supernova remnants (SNRs) have long been considered the dominant sources of galactic cosmic rays. While SNRs can accelerate particles to hundreds of teraelectronvolts (TeV), they fail to reach the PeV range consistently. Their maximum energies fall short of the precise energy range associated with the knee.

Despite decades of theoretical modeling, no SNR ever demonstrated observational evidence of accelerating protons to PeV energies. Conversely, micro-quasars now show direct evidence of producing these energies.

Thus, the new model of cosmic ray origins divides the sources into categories:

• Supernova remnants: dominate at lower energies (up to a few hundred TeV)
• Micro-quasars: dominate near and beyond the knee (PeV to super-PeV)
• Extragalactic sources: dominate well above the ankle (EeV range)

This layered structure resolves inconsistencies that plagued previous single-source theories.

LHAASO’s Role: The Technology That Solved the Mystery

The breakthrough would not have been possible without the unique design of the Large High Altitude Air Shower Observatory. Located on the Tibetan Plateau at 4,410 meters elevation, LHAASO captures extensive air showers—cascades of particles initiated when gamma rays or cosmic rays strike the atmosphere.

Its hybrid structure includes:

• KM2A (kilometer-square array) for high-energy gamma detection
• WCDA (water Cherenkov detector array)
• WFCTA (wide-field Cherenkov telescopes)

This combination allowed simultaneous detection of:

• The gamma rays emitted by distant micro-quasars
• The cosmic ray particles arriving at Earth
• The energy and direction of the incoming showers

For the first time, astrophysicists could connect the dots between distant black hole jets and the energy cutoff observed in cosmic rays near Earth.

A New Era in High-Energy Astrophysics

With this discovery, the cosmic ray knee is no longer a mystery—its origin is firmly tied to black hole-powered micro-quasars. This revelation reshapes our understanding of the Milky Way by showing that stellar-mass black holes play a far more dramatic role in galactic energetics than previously assumed.

More importantly, it opens the door to new questions:

• How many micro-quasars exist undetected in the Milky Way?
• What determines the maximum energy each system can achieve?
• Do newly formed black holes produce the most powerful accelerators?
• How does this affect the distribution and propagation of cosmic rays across the galaxy?

As LHAASO continues observing and new observatories come online, the next decade may reveal even more dramatic surprises hidden within our galaxy’s black hole population.

What was once a decades-old cosmic mystery has now become a gateway to understanding the true power of compact objects shaping the universe at the highest energies.