A Pulsar in a Rare Triple System

Follow-up observations show that a recently discovered pulsar has two stellar companions, making it one of just a handful of millisecond pulsars known to be in triple-star systems.

An Unusual New Pulsar

Earlier this year, researchers reported the discovery of a pulsar with the Five-hundred-meter Aperture Spherical radio Telescope (FAST). The newfound pulsar, PSR J0435+3233, is a millisecond pulsar, rotating once every 3.2 milliseconds. What’s unique about this pulsar is how quickly it’s spinning down: its spin rate is decreasing roughly 100 times faster than other millisecond pulsars in our galaxy, suggesting an unusually strong magnetic field and the possibility of an exotic formation pathway.

The FAST observations also showed that the pulsar is not alone: it appears to be on an 8-day orbit with a white dwarf. Now, follow-up observations have revealed the pulsar’s membership in an intriguing stellar system.

From Radio to Gamma Rays

Zonglin Yang (National Astronomical Observatories, Chinese Academy of Sciences) and coauthors followed up on the discovery of PSR J0435+3233 using multiwavelength data from FAST, multiple optical and infrared observatories, and the Fermi Gamma-Ray Space Telescope.

With this broad dataset, the team confirmed the identity of the pulsar’s white-dwarf companion, detected gamma-ray emission from the pulsar, and identified a second previously unknown companion, a subgiant star roughly as massive as the Sun. These three stars are arranged in a hierarchical triple: the pulsar and white dwarf orbit one another tightly, and the subgiant is a more distant companion, orbiting in decades rather than days. Current observations show that the orbits of the inner binary and the outer star are tilted relative to one another, either nearly perpendicularly or at about 54 degrees. While a triple system of three closely intertwined stars invites chaos, this configuration is stable long term.

Insights into Stellar Evolution

While it’s common to find millisecond pulsars in binary systems — in fact, many millisecond pulsars are thought to spin up to their incredible speeds via accretion from a stellar companion — it’s rare to discover them in triple systems. Because this system is isolated out in the field rather than crowded together with other stars in a dense cluster, Yang and collaborators suspect that the three stars have been together since birth, as opposed to being assembled dynamically later on.

The most massive star in the trio would have hurried through its main-sequence lifetime in less than 10 million years, culminating in a supernova. The remaining two members of the system would have had to withstand this explosion, which the team found is possible but not especially likely; the likelihood of the system coming unglued due to a star going supernova depends on how much of a “kick” the explosion gives the newborn pulsar, and even in a zero-kick scenario, the likelihood of the system remaining bound is only 35%. The future evolution of the system will be less dramatic. The outer star will eventually evolve into a white dwarf, undergoing a gentle transition that poses no threat to the stability of the system.

Triple-star systems are common among massive stars, but how this configuration affects stellar evolution is still being explored. This discovery offers a valuable opportunity to study dynamics and stellar evolution in a rare triple system.

Citation

“The PSR J0435+3233 Triple System,” Z. L. Yang et al 2026 ApJL 1010 L9. doi:10.3847/2041-8213/aeaa29