A research team led by Han Jinlin from the National Astronomical Observatories of the Chinese Academy of Sciences has discovered a rare millisecond pulsar using China’s Five-hundred-meter Aperture Spherical radio Telescope (FAST), also known as the "China Sky Eye." The newly identified pulsar orbits a companion star every 3.6 hours and is eclipsed by its companion for approximately one-sixth of each orbit — a phenomenon similar to solar or lunar eclipses, the CCTV News reported on Friday.
This discovery provides valuable insights into stellar evolution theories, the physics of accretion onto compact objects, and gravitational wave sources from binary mergers. The findings were published on Friday in the international journal Science online.
In the vast Milky Way galaxy, most stars are not solitary — they evolve in pairs as binary systems. While the life cycles of single stars are relatively well understood, how binary stars interact and evolve together remains one of the most challenging frontiers in modern astrophysics.
China's FAST, with its exceptional sensitivity, is a powerful instrument for detecting pulsars —particularly those in ultra-short orbital periods. During an in-depth pulsar survey of the Milky Way, the research team discovered a millisecond pulsar with a spin period of just 10.55 milliseconds. After multiple follow-up observations, the team confirmed that the pulsar resides in an extremely compact orbit with a radius of only 500,000 kilometers. The pulsar and its companion revolve around each other every 3.6 hours, with the pulsar signal being eclipsed by its companion for roughly one-sixth of each orbit.
Estimates suggest that the companion has a mass at least equivalent to that of the sun —significantly more massive than typical companions in eclipsing pulsar systems. However, such a compact orbit cannot accommodate a sun-like star. Based on multiple lines of evidence, researchers inferred that the companion is neither a normal star nor a typical evolved compact object. Instead, it is likely the exposed core of a star — a hot helium star — that has undergone a common-envelope phase and lost its outer layers. The pulsar’s signal is eclipsed by stellar wind material expelled from this helium star.
Such exotic binary systems are extremely rare and difficult to observe, as they survive for only about 10 million years — an eye-blink in the 13.8-billion-year history of the universe, much like a fleeting meteor across the night sky. According to simulations conducted by the research team, only a few dozen such systems are expected to exist among the hundreds of billions of stars in the Galaxy.
The discovery of this rare celestial object holds the potential to advance astronomical research on multiple fronts. It offers valuable insights into the detailed processes of stellar — particularly binary — evolution, including how binary stars spiral inward and shrink their orbits, how they exchange material, how a neutron star accelerates its spin to just a few milliseconds after plunging into its companion, and how the shared hydrogen envelope is blown away by the compact star. The system also serves as a crucial observational case supporting the theory of neutrino cooling during heavy mass accretion onto compact stars.
This finding may also help improve predictions of gravitational wave sources, drive research into deep optical and infrared observations of helium stars and significantly enhance our understanding of celestial bodies and their evolution in the cosmos.
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