Scientists have obtained phase-connected timing solutions for two newly discovered binary pulsars, PSR J1929+4105 and PSR J2359+4239, through long-term follow-up observations. While both pulsars now reside in similar configurations—each orbiting a massive white dwarf companion—the real surprise lies in the different evolutionary pathways that led them to these nearly identical destinations.

Led by the Xinjiang Astronomical Observatory of the Chinese Academy of Sciences, the study offers new insights into the evolution of intermediate‑mass binary systems into pulsar–white dwarf binaries.

The findings were recently published in The Astrophysical Journal.

Discovered by the Commensal Radio Astronomy FAST Survey (CRAFTS), both pulsars have spin periods of 41.6 and 60.5 ms, and both show very low orbital eccentricities. From the mass functions, the minimum companion masses are 0.72 and 0.467 solar masses, respectively. The researchers suggested that their companions are likely massive white dwarfs rather than neutron stars. Such systems represent an intermediate stage in the pulsar "recycling" process.

Unlike fully recycled millisecond pulsars that form through substantial accretion, these two pulsars underwent only limited mass and angular momentum transfer, resulting in spin periods of tens of milliseconds. They are thus classified as mildly recycled pulsars.

Simulations with the COMPAS binary evolution code suggest that, although the two systems have similar final configurations, they likely took different evolutionary pathways: PSR J1929+4105 may have undergone a relatively late common‑envelope phase, while PSR J2359+4239 probably entered the common‑envelope phase earlier and subsequently underwent a short‑lived mass‑transfer episode.

The researchers also included other newly discovered FAST binary pulsars to update the Galactic height distribution for various system types. The results show that systems with more massive companions tend to lie closer to the Galactic plane, while those with lighter companions are more widely scattered.

These findings shed new light on how supernova kicks, binary mass, and evolutionary timescales collectively shape pulsar population distribution, the researchers said.

With longer timing baselines in the future, the researchers aim to detect post‑Keplerian parameters such as Shapiro delay and periastron advance, which will better constrain the true masses of the neutron stars and white dwarfs. This will help clarify binary evolutionary pathways and provide additional key samples for studies of compact‑object mass distributions, Galactic electron‑density models, and pulsar population statistics, they added.