Discovery of a Compact Binary System: A Neutron Star Orbiting Within Another Star

This binary system comprises a PSR J1928+1815 along with a rapidly spinning millisecond pulsar known as the Helium Star Companion.

The millisecond pulsar consists of rapidly rotating neutron stars that emit radio waves.

These stars attain remarkable rotational velocities by harvesting material from surrounding stellar groups.

The development of such exotic binary systems remains partially understood, as it encompasses a range of complex processes.

The theory suggests that binary systems may undergo a common envelope phase, where a star orbits within the outer layer of its companion.

If the companion in this evolutionary phase is a neutron star, the theory indicates that the outer layer will be swiftly ejected, resulting in a binary system of recycled pulsars and stripped helium stars.

In the recent study, Dr. Zonglin Yang, a national astronomer at the Chinese Academy of Sciences, along with colleagues, examined the millisecond pulsar PSR J1928+1815.

Utilizing data from a high-speed 500-meter aperture spherical radio telescope, they discovered that the pulsar has a spin period of 10.55 ms and resides in a close binary system with companion helium stars, completing an orbit every 3.6 hours.

They employed a stellar model to demonstrate that this system originated following an unstable mass transfer from companion stars to neutron stars, leading to the formation of a common envelope around both stellar objects.

The neutron star approached the core of the other star, ejected the outer envelope, and released energy, resulting in a tightly bound binary system.

“The companion star has a mass between 1.0 and 1.6 solar masses, obscuring the pulsar approximately 17% of its orbit and is undetectable at other wavelengths, suggesting it is likely a stripped helium star,” the authors noted.

“We interpret this system as having recently undergone a common envelope phase to create compact binaries.”

“Such systems are thought to be rare, yet we anticipate the existence of others,” they added.

“We estimate that there could be between 16 and 84 undiscovered examples within the Milky Way.”

The findings are documented in a paper published in the journal Science.

____

Zl Yang et al. 2025. A pulsar helium star compact binary system formed by common envelope evolution. Science 388 (6749): 859-863; doi: 10.1126/science.ado0769