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This discovery establishes the unusual binary system Cygnus X-3, located in the constellation Cygnus, as the highest-energy gamma-ray source known to date. LHAASO detected gamma-ray emission above 1 PeV with a statistical significance of 6.4 sigma. The highest-energy photon ever detected at Earth, with an energy of 3.7 PeV, originated from this source. The temporal variability and spectral properties measured by LHAASO provide strong evidence that Cygnus X-3 accelerates protons to energies of at least 30 PeV. These energies substantially exceed current theoretical expectations for Galactic particle accelerators.
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.
The APC team contributed to the analysis of Cygnus X-3, in particular, to the validation of the event-list systematics at the highest energies, a crucial step in establishing the robustness of the PeV detection.
Journal publication: https://doi.org/10.1093/nsr/nwag435
Publication on ArXiv: https://arxiv.org/pdf/2512.16638