Chinese astronomers discover rare compact binary system of pulsar, helium star

Chinese astronomers have discovered a rare compact binary system consisting of a millisecond pulsar and a companion helium star formed by common envelope evolution, which sheds light on future research on stellar evolution theory.

The research, conducted by a team led by professor Han Jinlin from the National Astronomical Observatories of the Chinese Academy of Sciences using China's Five-hundred-meter Aperture Spherical Radio Telescope (FAST), was published on Science online on Thursday.

Pulsars are spinning neutron stars that originate from the imploded cores of massive dying stars through supernova explosions. They emit twin beams of radio waves from their magnetic poles. These beams appear to pulse because astronomers see them only when a pulsar pole is pointed at Earth.

Thanks to the high sensitivity and detection abilities of FAST, the Chinese team identified a pulsar with a spin period of 10.55 milliseconds in a compact binary system, which has an orbit radius of 500,000 kilometers and an orbital period of 3.60 hours.

The companion star has 1.0 to 1.6 solar masses, eclipses the pulsar for about 17 percent of the orbit, and is undetected at other wavelengths, so it is most likely a stripped helium star.

The scientists interpret this system as having recently undergone a common envelope phase, thus producing a compact binary.

Such binary systems are very rare and hard to detect, because they only exist for about 10 million years. The team estimates only dozens of other examples of this system may exist in the entire galaxy.

This discovery is of great significance to the study of stellar evolution, physics of accretion onto compact stars, and gravitational-wave sources from mergers of binary stars.

FAST, the world's largest filled-aperture and most sensitive radio telescope, is located in a naturally deep and round karst depression in southwest China's Guizhou Province. It started formal operations in January 2020 and was officially opened to the world in March 2021.

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