China's Five hundred metre Aperture Spherical Telescope (FAST) has mapped a pulsar whose almost flawless circular orbit gives astronomers a rare, quiet laboratory for studying how stellar corpses get spun back up by their companions.
Twelve thousand light-years from Earth, a city-sized ball of collapsed star stuff, denser than anything on this planet, is spinning 220 times every second. Its lighthouse beam of radio waves crosses the galaxy on a clock so steady it rivals the best atomic timekeepers humans have built. What makes PSR J1810−0623 newly interesting is not the spin. It is the shape of the path the pulsar traces around its companion star: a circle so clean that, as a curated science-news write-up of the result puts it, you could almost draw it with a compass.
A pulsar is what is left when a star much more massive than the Sun runs out of fuel and collapses. The core compresses into a neutron star, an object roughly 20 kilometers across but heavier than the Sun, and its magnetic field whips charged particles around the poles. If the neutron star's spin axis does not line up with that magnetic axis, the resulting beam sweeps past Earth like a lighthouse, and astronomers see a pulse. Millisecond pulsars are a special, fast-spinning subset: their periods are measured in thousandths of a second.
The China-led FAST radio telescope, the Five-hundred-metre Aperture Spherical Telescope, sits in a natural bowl in Guizhou Province and is the largest single-dish radio telescope on Earth. That collecting area matters for a system like PSR J1810−0623. Faint, short pulses from a fast-spinning pulsar are easy to lose in the noise on a smaller dish. FAST can resolve the timing precisely enough to model the orbit in detail, and the EurekAlert press release corroborates the result: the orbit is a near-perfect circle, with an eccentricity of roughly 0.000015.
That circularity is the actual news. Recycled pulsars, the family PSR J1810−0623 belongs to, are neutron stars that were once slow rotators. A companion star overflowed its orbit and dumped gas onto the neutron star. That infalling matter carried angular momentum, the way a falling bicycle chain pulls the rear wheel faster and faster, and over hundreds of millions of years it spun the dead core back up to hundreds of rotations per second. The magnetic field also decays in the process, which is one of the clues that recycling actually ran for that long. For PSR J1810−0623, the magnetic field has fallen to roughly 100 million Gauss, consistent with an extended accretion phase rather than a young, unrecycled pulsar.
The orbit should tell the same story. Mass transfer in a close binary usually circularizes the orbit, because tidal forces and gas drag smooth it out, but the longer a system lives after recycling ends, the more chances gravitational nudges from passing stars have to deform the path. A recycled pulsar found today with an almost perfectly circular orbit is therefore a fossil of a process that ran for a long time and was not much disturbed afterward. Most recycled millisecond pulsars do show measurable eccentricity, even small fractions of a percent. Sitting near zero, as PSR J1810−0623 appears to, is rare in the known population.
A clean orbit buys astronomers two things. First, timing noise from an oblong orbit does not contaminate the pulse train, so the pulsar can be used as a precision clock to look for tiny gravitational effects, including the possibility of a third body, or to test how the neutron star's interior responds to its own rotation. Second, the system can be compared against binary-evolution models that try to predict how often the universe should produce a circular recycled pulsar versus an eccentric one. The rarer near-circular objects become, the sharper the constraint on those models.
The underlying result comes from a team led by Jie Zhang, Zerui Wang, Lei Zhang, and colleagues, including Di Li and Ryan S. Lynch, reporting in arXiv:2603.13815 (published March 2026) and accepted in Science China Physics Mechanics and Astronomy. The precise parameters: a spin period of 4.55 milliseconds (≈220 rotations per second), an orbital period of 15.4 days, an eccentricity of about 1.5×10⁻⁵, and a companion white dwarf of roughly 0.64 solar masses — consistent with a carbon-oxygen white dwarf formed via prolonged mass transfer. The characteristic age is around 32 billion years, and the surface magnetic field has decayed to approximately 100 million Gauss. The discovery was made with FAST and followed up with FAST and the Green Bank Telescope. The researchers note that while the proximity of globular cluster Pal 7 raised the possibility of a dynamical origin, discrepancies in dispersion measure and proper motion argue against a physical association — PSR J1810−0623 is a genuine Galactic-field binary.
What to watch next is further timing. As FAST and other radio telescopes continue long-term observations, scientists hope to pin down the neutron star's true mass and potentially test gravitational theories through more precise orbital measurements. For now, the system stands as one of the roundest recycled pulsars known — rounder even than the celebrated PSR J1614−2230 — and a clean window into a billion-year engineering process that turns a dead star into one of the most stable metronomes in the galaxy.