Preprint suggests two supernova remnants may accelerate cosmic rays to PeV energies

A new paper from the LHAASO Collaboration says two supernova remnants are emitting gamma rays up to hundreds of teraelectronvolts, a signal the authors interpret as evidence that the remnants accelerated hadronic cosmic rays to petaelectronvolt energies.

If that interpretation holds up, it would mark an important step in a long-running astrophysics question: whether supernova remnants are the main source of Galactic cosmic rays up to the so-called knee, a bend in the cosmic-ray spectrum near about 3 petaelectronvolts.

But the result comes with an immediate caveat. The study, titled “Ultra-high-energy γ-ray imprints from PeV particles accelerated by supernova remnants,” was posted to arXiv on April 24 and had not been peer-reviewed as of Sunday. The arXiv entry lists Zhen Cao and the LHAASO Collaboration as authors, and no separate LHAASO press release tied to the paper had been found as of April 27.

The observations were made with the Large High Altitude Air Shower Observatory, or LHAASO, a facility in China designed to detect cascades of particles produced when very high-energy gamma rays and cosmic rays strike Earth’s atmosphere.

In the paper’s abstract, the authors write: “Here we report observations of very-high-energy γ-ray emission up to hundreds of TeV from two middle age shell-type SNRs, G150.3+4.5 and γ-Cygni, with the Large High Altitude Air Shower Observatory (LHAASO).”

The two objects are middle-aged shell-type supernova remnants, expanding debris fields left by exploded stars. One is G150.3+4.5. The other is γ-Cygni, also known as G78.2+2.1, a well-studied remnant in the Cygnus region.

According to the abstract, both sources show two, and possibly three, distinct morphological and spectral components, with convex spectral shapes. The lower-energy component is also more extended in space than the higher-energy component.

That structure matters because it feeds into the paper’s preferred explanation for the highest-energy signal. The authors argue the top-end gamma-ray emission is best explained by hadronic cosmic rays — meaning proton-like particles rather than high-energy electrons — that were accelerated to PeV energies and then collided with nearby molecular clouds, dense concentrations of gas in interstellar space.

The paper says that interpretation is supported by an apparent association between the high-energy gamma-ray component and molecular clouds at similar distances to the remnants, along with the weakness or absence of pulsar wind nebulae inside the remnants. Pulsar wind nebulae can also produce high-energy gamma rays, so their lack strengthens the case, in the authors’ view, for a hadronic origin.

The abstract frames the finding as fitting a longstanding theoretical picture: particles accelerated near the end of a supernova remnant’s free-expansion phase later escape and illuminate nearby clouds. “These results are compatible with the classic model prediction that PeV particles accelerated near the end of the free expansion phase of SNR evolution can illuminate nearby molecular clouds (MCs) to produce strong γ-ray emission,” the authors wrote.

Neither remnant is new to gamma-ray astronomy. G150.3+4.5 had already been the subject of earlier gamma-ray studies, including a 2024 paper in Astronomy & Astrophysics. γ-Cygni has previously been observed by VERITAS, HAWC and NASA’s Fermi-LAT space telescope. What is new in the LHAASO preprint is not the existence of the sources, but the claimed extension of their emission to hundreds of TeV and the argument, based on morphology and cloud associations, that the highest-energy component traces PeV hadrons.

The work also fits into a broader run of LHAASO results on ultrahigh-energy Galactic gamma rays, including reports of PeV photons in 2021 and a Cygnus bubble result in 2024. This paper is more specific: It ties the signal to shell-type supernova remnants and nearby molecular clouds.

For astrophysicists, that is the key point. Supernova-remnant shocks have long been leading candidates to explain most Galactic cosmic rays below the knee, but direct evidence that they can accelerate hadrons to PeV energies has been hard to pin down.

This new result does not settle that question on its own. But if the analysis survives peer review and is confirmed independently, it would strengthen the case that at least some supernova remnants can act as Galactic PeVatrons — accelerators capable of pushing proton-like particles to the energies needed to help explain the knee.