The Large High Altitude Air Shower Observatory (LHAASO) reveals that the peculiar binary system Cygnus X-3 in the constellation Cygnus is the highest energy gamma-ray source known to date. The temporal variability and spectral characteristics measured by LHAASO collectively confirm this object as the source of cosmic rays, with particle energies reaching at least 30 PeV (1PeV=1015eV), far exceeding current theoretical expectations for Galactic accelerators. The study was completed through collaboration among scientists from the Institute of High Energy Physics of the Chinese Academy of Sciences (IHEP, CAS), the Tsung-Dao Lee Institute at Shanghai Jiao Tong University, the Shanghai Astronomical Observatory of CAS, and other research institutions. The results were published in National Science Review under the title "Cygnus X-3: A variable petaelectronvolt γ-ray source," with corresponding authors including Professor CAO Zhen, Associate Researcher LI Cong, Associate Professor WANG Jieshuang, Associate Researcher ZHOU Jianeng, and Professor Felix Aharonian.

LHAASO detected flaring activity in ultra-high-energy (>1014eV) gamma rays, which showed a clear temporal correlation with observations at the GeV (1GeV=109eV) band, one million times lower in energy. During the GeV flaring periods, both the Fermi satellite detector and the LHAASO simultaneously detected highly significant signals, while no signals were observed during GeV quiescent periods for LHAASO. Furthermore, the signal exhibited a 4.8-hour periodicity in both GeV and ultra-high-energy gamma rays, reflecting the orbital modulation of the binary system. This allowed the accelerator to be localized within a region roughly three times the size of the Sun—the highest-precision localization ever achieved for an ultra-high-energy particle accelerator in the Universe.

By confirming Cygnus X-3 as the first ultra-high-energy gamma-ray source with temporal variability, this work opens a new frontier in ultra-high-energy time-domain astronomy and provides a novel avenue for studying the physical processes near compact objects such as black holes. As an extreme cosmic ray accelerator, Cygnus X-3 also has important implication for future multi-messenger astronomy research. The results have already attracted significant attention in the astrophysics community and have become one of the most widely discussed topics at international conferences.

Figure 1. Schematic diagram of high-energy photons and neutrinos produced by interactions between protons from the base of the Cygnus X-3 jet and ultraviolet photons from the companion star. (Credit: LHAASO Colllaboration)

Paper link: https://doi.org/10.48550/arXiv.2512.16638