China's LHAASO Observatory Confirms Cygnus X-3 as First Ultra-High-Energy Gamma-Ray Variable Source

China's Large High Altitude Air Shower Observatory (LHAASO) has confirmed that Cygnus X-3 — a binary system in which a black hole or neutron star feeds on a massive companion star — is the first ultra-high-energy gamma-ray source with time-variable characteristics, researchers reported in the National Science Review on July 31. The cosmic-ray particles are accelerated to at least 30 PeV (3 billion billion electron volts), far exceeding the mainstream theoretical limit of roughly 1 PeV, making Cygnus X-3 the most energetic particle accelerator observed to date.

A 4.8-Hour Heartbeat

The breakthrough comes from the signal's variability: gamma-ray emission from the system pulses with a 4.8-hour periodicity that reveals the binary's orbital motion, pinpointing the particle accelerator to a region only about three times the size of the Sun. The time-variable signature is what lets astronomers confirm the source — and it opens the door to a new field of ultra-high-energy time-domain astronomy.

"For the first time, we can watch a PeV-scale accelerator turn on and off as its two stars orbit each other," said a member of the LHAASO collaboration.

Why It Matters

The finding challenges models of where cosmic rays are born: if a binary system — not just supernova remnants and active galactic nuclei — can accelerate particles past 30 PeV, the Milky Way's cosmic-ray inventory has a new candidate engine. The region is also a natural laboratory for extreme physics near black holes and neutron stars, where magnetic fields and accretion flows reach conditions impossible to reproduce on Earth.