Chinese scientists discover rare eclipsing pulsar

BEIJING -- A team of Chinese scientists has discovered an extremely rare pulsar that gets partially blocked by its companion star every few hours, like a cosmic game of hide-and-seek.

The findings, published in the latest issue of the journal Science, could help solve long-standing mysteries about how stars evolve in pairs.

The discovery was made using China's Five-hundred-meter Aperture Spherical Radio Telescope (FAST), the world's largest single-dish and most sensitive radio telescope, in the southwestern province of Guizhou.

Led by Han Jinlin, a researcher from the National Astronomical Observatories of China, the team found a fast-spinning millisecond pulsar tightly orbiting a mysterious companion star. The two stars circle each other every 3.6 hours, and for about one-sixth of that time, the pulsar's radio signals are blocked.

Most stars in the Milky Way galaxy exist in pairs, but scientists still don't fully understand how these binary systems evolve. According to current stellar evolution theory, when two stars orbit each other, the more massive one evolves faster, eventually collapsing into a dense neutron star or black hole. The smaller star continues evolving, but it is enlarged due to its lost matter that is accreted by the compact dense companion, and someday, the dense star has to be within the outer layers of the smaller star. These two stars share a common envelope of hydrogen gas. Over about 1,000 years, the neutron star blows away this envelope, leaving behind a hot, helium-burning star, which orbits the dense neutron star.

According to the scientists, the newly discovered system, named PSR J1928+1815, is a rare example of what happens after this dramatic phase. The pulsar has a rotation period of 10.55 milliseconds. It must have accreted considerable material from its companion, and has spun up. Its companion is likely the helium core of the smaller star after the out layers being ejected. The team estimates there may be only a few dozen such systems in the entire Milky Way.

This discovery is a smoking gun for theories about binary star evolution that have been discussed for decades, including how stars exchange masses and shrink their orbits, how the neutron star is spun up by accreting matter from its companion, and how the shared hydrogen envelope gets ejected, Han said.

Additionally, the system could help scientists study how a neutron star accretes matter and then cools down. Such a binary will evolve to be a system of two compact stars, which finally merge and become a future source of gravitational waves (GW), he added.

According to one of the paper's reviewers, Scott Ransom, a binary pulsar expert at the U.S. National Radio Astronomy Observatory, the paper describes the discovery of an interesting new binary pulsar of a type that is so far unique. Both the system and paper should generate several interesting paths of future investigation, in a variety of different areas including population synthesis, predictions for GW sources, binary and stellar evolution calculations, deep optical/infrared follow-up, long-term timing, etc.

With FAST's unmatched sensitivity, astronomers hope to find more of such cosmic rarities, shedding light on the mysteries of the universe.