The Large High Altitude Air Shower Observatory (LHAASO) has detected PeV (10¹⁵ eV) gamma-ray emission from a pulsar wind nebula powered by PSR J1849-0001 in the constellation Aquila, marking the discovery of a new PeVatron and posing a challenge to the classical theory of particle acceleration in pulsar wind nebulae.

This discovery is important because the calculated particle acceleration efficiency of this celestial structure approaches or even exceeds the theoretical limits allowed under ideal magnetohydrodynamic conditions.

This study, published in Nature Astronomy, was conducted by Prof. LIU Ruoyu, Dr. WANG Kai, and doctoral student TONG Chaonan from Nanjing University, Prof. CHEN Songzhan and Assoc. Prof. WANG Lingyu from the Institute of High Energy Physics of the Chinese Academy of Sciences, and their collaborators.

A pulsar wind nebula is a spectacular, high-energy celestial structure formed when a pulsar—a rapidly rotating, magnetized neutron star—expels a wind of charged particles moving at nearly the speed of light into the surrounding space, where it violently collides with the ambient medium.

One of the most famous examples of a pulsar wind nebula is the Crab Nebula, which has long been regarded as a “standard candle” in the field of high-energy astrophysics. It is driven by the Milky Way’s most luminous spin-down-powered pulsar—a pulsar whose energy comes from its rotation slowing down.

Based on previous observations of PeV-scale gamma rays from the Crab Nebula, LHAASO determined that its particle acceleration efficiency is at least 16% of the theoretical limit. This finding firmly established the Crab Nebula’s status as an extreme PeVatron—a PeV-scale particle accelerator—and pushed the limits of existing acceleration models.

The current study focuses on another pulsar wind nebula system, powered by PSR J1849-0001. Located in the constellation Aquila, PSR J1849-0001 has a spin-down luminosity approximately 50 times lower than that of the Crab Nebula pulsar. In conventional models of pulsar wind nebula evolution and emission, a lower injection luminosity typically corresponds to a weaker high-energy radiation luminosity.

However, LHAASO’s spectral measurements reveal that the gamma-ray spectrum of this system not only extends as a power-law up to 2 PeV, but its gamma-ray luminosity in the PeV energy range is actually several times higher than that of the Crab Nebula. This indicates that the system is astonishingly efficient at converting the energy of the pulsar wind into ultra-high-energy particles.

Through multi-wavelength observations including X-ray data, the researchers constrained the internal physical parameters of this pulsar wind nebula. They found that the particle acceleration efficiency in the nebula reached at least 27% of the theoretical limit, exceeding that inferred for the Crab Nebula and thus earning this pulsar wind nebula the nickname “Aquila Booster.”

These striking results challenge current theories of particle acceleration in pulsar wind nebulae. Under conventional models, particle acceleration is assumed to occur at the termination shock—the region where the pulsar wind abruptly slows as it collides with the surrounding nebula. If the observed particle energies were produced at this location, however, the required acceleration efficiency would exceed 100%, which is physically impossible. As a result, this discrepancy suggests that the standard termination-shock model cannot fully explain the observed emission.

This discovery by LHAASO not only adds a valuable new PeVatron candidate for the Milky Way but also reveals how the cosmos has ingeniously built an extreme particle accelerator operating at extraordinary levels of efficiency within a seemingly unremarkable pulsar system. If confirmed, this suggests that the high efficiency observed in the Crab Nebula may be a common characteristic of pulsar wind nebulae as a class of astrophysical objects.

In summary, this study provides crucial clues for refining the theoretical framework of pulsar wind nebulae and will prompt theoretical astrophysicists to re-examine the mechanisms of particle acceleration and fundamental physical processes within relativistic plasmas.

Artist’s impression of the Aquila Booster and LHAASO. (Image by LHAASO Collaboration)