1LHAASO J1852+0050u: GeV-100TeV Gamma-ray emission Powered by Star-forming Region?

High above the Earth, in the thin air of the Tibetan plateau, a massive array of sensors waits for something extraordinary. These sensors are part of the Large High Altitude Air Shower Observatory, known as LHAASO, and they are designed to catch the faint, fleeting signatures of the most energetic particles in the universe. These particles, called cosmic rays, are atomic nuclei that travel through space at nearly the speed of light. For nearly a century, scientists have wondered where these particles get their incredible speed. While some are known to come from the violent explosions of dying stars, the most powerful ones, reaching energies a million times higher than what our most powerful particle accelerators can create, have remained a mystery. Finding the source of these "PeVatrons"—objects capable of accelerating particles to such extreme energies—is one of the great quests in modern astronomy, as it would finally reveal the engines that power the high-energy universe.

In a recent study, researchers turned their attention to a specific patch of sky in the constellation Aquila, where a mysterious source of high-energy gamma rays had been spotted. This source, cataloged as 1LHAASO J1852+0050u, is a vast, diffuse glow of radiation that had been detected at energies exceeding one hundred trillion electron volts. The puzzle was identifying what was creating this glow. The region is crowded with known astronomical objects, including the remnants of two exploded stars and a rapidly spinning neutron star, known as a pulsar. The team, led by researchers from the LHAASO collaboration, set out to determine if one of these familiar objects was the culprit, or if something more exotic was at work.

To solve the mystery, the scientists combined data from multiple telescopes. They re-examined four years of observations from LHAASO, which provided a detailed map of the high-energy gamma rays, and paired this with sixteen years of data from the Fermi-LAT satellite, which observes lower-energy gamma rays. They also looked at the invisible molecular gas in the region using radio telescopes, which can reveal the presence of dense clouds of gas and dust where stars are born. By stitching these different views together, the researchers were able to separate the faint signal of the main source from the glare of its neighbors. They found that the gamma-ray glow is not a single point but an extended region that overlaps with a bustling nursery of new stars.

The first suspect the team considered was a middle-aged pulsar, a dense, spinning star that had been spinning for about two hundred thousand years. Pulsars are known to create clouds of high-energy particles that can emit gamma rays. However, when the researchers ran simulations to see if this specific pulsar could generate the amount of energy observed, the numbers did not add up. The pulsar simply did not have enough power to explain the bright, high-energy glow seen by LHAASO. The team also looked at the nearby remnants of exploded stars, but the distances and the specific types of gas in the region ruled them out as the primary source. The evidence pointed away from these violent, explosive origins.

Instead, the data pointed toward a more continuous, yet equally violent, process: the birth of massive stars. The gamma-ray source sits directly on top of a dense cloud of molecular gas, which is the raw material for star formation. Within this cloud lies a specific region known as G34.26+0.15, a site where massive stars are currently being born. The researchers found that the lower-energy gamma rays detected by the Fermi satellite match the location of young, forming stars, while the highest-energy gamma rays detected by LHAASO align with the presence of massive, fully formed stars in the same cluster.

The scientists propose a two-part explanation for the radiation. They suggest that the lower-energy gamma rays are produced by protons accelerated by the powerful jets and outflows from young, forming stars. These young stars are still gathering mass, and their energetic activity stirs up the surrounding gas, accelerating particles to high speeds. The highest-energy gamma rays, reaching up to one hundred trillion electron volts, are likely produced by even more powerful winds from the massive, adult stars in the same cluster. These massive stars blow material into space at tremendous speeds, creating shock waves that can accelerate particles to the extreme energies required to produce the observed radiation.

The study concludes that this star-forming region is likely a "PeVatron," a natural particle accelerator capable of boosting cosmic rays to energies previously thought to be the domain of only the most violent explosions. The researchers estimate that the cluster contains between twenty and two hundred massive stars, whose combined winds provide the necessary energy to sustain this high-energy emission. While they cannot completely rule out the presence of other hidden, unseen pulsars in the region, the evidence strongly favors the star-forming region as the dominant engine. This discovery adds a new chapter to our understanding of cosmic rays, suggesting that the chaotic, energetic nurseries where stars are born are just as capable of creating the universe's most energetic particles as the catastrophic deaths of stars.