For years, scientists have searched for the sources of the most energetic particles in our galaxy, cosmic rays that carry energies far beyond what human-made accelerators can achieve.

Now, observations from the Large High Altitude Air Shower Observatory (LHAASO) have revealed a binary star system pushing particles past a critical energy barrier. The system, LS I +61° 303, has been found emitting gamma rays above 100 tera–electron volts (TeV)—firmly placing it in the category of ultra-high-energy sources. 

For comparison, this is more than 15 times the energy carried by a single proton in the most powerful human-made accelerator, the Large Hadron Collider, which tops out at around 6.5 TeV per proton.

This is the first time such extreme emission has been confirmed from a gamma-ray binary, suggesting that these systems can act as PeVatrons, capable of accelerating particles to peta–electron volt (PeV) energies. 

“In this study, we report the first definite detection of gamma-ray emission up to the UHE range from the gamma ray binary LS I +61◦ 303 using LHAASO observations,” the study authors note.

In simple terms, a pair of stars has demonstrated a level of particle acceleration that theories did not clearly predict for such systems.

Reading particle footprints instead of light

Catching something this extreme is not as simple as pointing a telescope at the sky. Gamma rays at these energies don’t reach detectors directly—they collide with Earth’s atmosphere and trigger particle cascades, known as air showers. 

LHAASO is built to capture these cascades. By tracking how these secondary particles spread out and arrive at the ground, scientists can work backward to estimate the energy and origin of the incoming gamma ray.

This approach allowed researchers to push far beyond earlier measurements of LS I +61° 303, which had only been tracked up to about 10 TeV. With LHAASO’s sensitivity, the team extended the observed spectrum to nearly 200 TeV, clearly identifying signals above the 100 TeV threshold. 

This jump changes how the system is classified, effectively upgrading it into an ultra-high-energy emitter. “These results provide compelling evidence of extreme particle acceleration in LS I +61° 303,” the study authors note.

An orbit that rewires the accelerator

The system, LS I +61° 303, is far from stable. A massive star and a compact object—likely a neutron star or black hole—circle each other every 26.5 days, constantly reshaping their surroundings. What the researchers found is that the gamma-ray output doesn’t just rise and fall—it changes differently at different energies as the orbit progresses.

This energy-linked variation points to a shifting acceleration environment. Conditions such as magnetic field strength, particle density, and collision zones evolve as the stars move, meaning the engine powering the gamma rays is never in a steady state.

That variability also helps identify the particles involved. In such intense magnetic fields, electrons lose energy quickly and struggle to reach ultra-high energies. So when gamma rays above 100 TeV show up, they strongly hint at protons or heavier particles doing the work. 

“We identify 16 photon-like events above 100 TeV against an estimated 5.1 background events,” the study authors said.

These particles can travel farther and collide with dense stellar winds, producing gamma rays through high-energy interactions. This sets gamma-ray binaries apart from more familiar candidates like supernova remnants, where the acceleration process is comparatively steadier. 

Here, the energy output appears tied to orbital motion, making the system more dynamic and less predictable.

Expanding the shortlist of cosmic heavyweights

The origin of the highest-energy cosmic rays has remained unresolved for over a century, partly because no single type of source has fully explained the observations. This result adds a new contender. It shows that gamma-ray binaries are not just energetic—they can reach the extreme conditions needed to act as PeVatrons. 

Our “results provide unprecedented access to the UHE regime and offer new constraints on the nature of particle acceleration in gamma-ray binaries,” the study authors said.

At the same time, the discovery complicates existing models. The strong dependence on orbital phase suggests that particle acceleration can switch modes or efficiency over short timescales. This is harder to model than steady, one-time events like supernova explosions.

There are still uncertainties, though. The exact mechanism driving the acceleration is not pinned down. Also, while the gamma rays point to hadronic processes, direct confirmation will require additional signals, such as neutrinos.

Next steps will likely focus on combining observations across different messengers, including gamma rays, cosmic rays, and neutrinos, to build a clearer picture of what’s happening inside these systems. 

The study is published in the journal Physical Review Letters.