Researchers identify the youngest white dwarf companion to millisecond pulsar

A team of researchers from Italy and Germany identified a millisecond pulsar with a young white dwarf companion in the globular cluster NGC 362, about 29,300 light-years away, publishing the result in Astronomy and Astrophysics in 2026.

The pulsar, designated NGC 362D, rotates once every 5.83 milliseconds, or more than 10,000 times per minute. Astronomers first discovered it in December 2022 within NGC 362, a dense globular cluster where high stellar density makes neutron stars more likely to have companions. Astronomers consider dense globular clusters prime hunting grounds for such objects because neutron stars there are more likely to have companions. The system lies about 29,300 light-years away.

Its companion, designated COM-NGC 362D, is a proto-white dwarf, a low-mass helium white dwarf of about 0.18 solar masses still in the pre-cooling phase. Astronomers identified it as the youngest millisecond pulsar companion ever found because it entered the white-dwarf stage recently compared with other known systems. The recent entry means it has not had time to cool or age like older white dwarf companions, making it a record-setting companion.

Updated binary evolution models indicate COM-NGC 362D completed the mass-transfer phase about 0.6 gigayears ago. That timing strongly suggests it is the youngest white dwarf companion to a millisecond pulsar identified to date. Because mass transfer ended so recently, the companion has not had time to cool or age like older white dwarf companions, making it a rare snapshot of post-recycling evolution.

Greta Ettorre, a Ph.D. student in the Department of Physics and Astronomy at the Alma Mater Studiorum University of Bologna in Italy, led the research. The team found the object by cross-referencing radio data from the pulsar with 10 years of Hubble observations in ultraviolet and near-infrared light. Comparing those observations with stellar evolution models allowed the researchers to determine COM-NGC 362D’s age and other properties. The study emerged from work by an international team based in Italy and Germany and appeared in the journal Astronomy and Astrophysics in 2026.

Universe Today reports that the paper’s figure plots a reference cooling track for a young helium white dwarf against observed data points from COM-NGC 362D, with some observations left unfitted for technical reasons. Stellar evolution models showed less ultraviolet radiation than expected for a white dwarf like COM-NGC 362D, with significant wavelength-dependent variations consistent with residual circumstellar material likely expelled during the mass-transfer phase. The detected dimming at shorter wavelengths suggests a cloud of leftover material still surrounds the young white dwarf, reinforcing how recently the mass-transfer phase ended.

According to the authors, the trend displays larger deviations at shorter wavelengths and is consistent with a self-extinction effect likely produced by circumstellar material surrounding the proto-white dwarf companion. The detection represents the first of its kind for this category of objects, supporting the idea that leftover material from the recent mass-transfer phase still surrounds the young white dwarf.

“What makes this system particularly interesting is the possibility of observing it at a very early stage of its evolution,” lead author Ettorre said.

Whether that material could have survived for this long is unclear, the team says, though they still favor the explanation. Another possibility is that the material was expelled during a previous hydrogen-shell flash. The team considers these two possible origins for the obscuring material but prefers residual matter from the recent mass-transfer episode to an older shell-flash event.

“NGC 362D currently offers us a unique opportunity to study what happens in the immediate aftermath of the millisecond pulsar recycling process,” study co-author Emanuele Dalessandro of INAF said. Finding such a young core could make it easier to identify similar systems because the dense core is still in early transition to a white dwarf. According to Dalessandro, material around the white dwarf could produce radio signatures like those from systems with non-degenerate companions and help interpret other young systems.

Most stars in the Milky Way remain on the main sequence, fusing hydrogen into helium like the Sun does. That description sets up the normal stellar population against which the rare NGC 362D binary stands out. White dwarfs are supercompact and dense, roughly Earth-sized but Sun-massive, shining only with leftover energy; the Sun will become one when it exhausts its fuel in several billion years.

Around 97% of the Milky Way’s stars will become white dwarfs, and they are not difficult to detect. That figure shows white dwarfs are the usual end state for stars like the Sun, unlike the rare neutron stars in the story. Neutron stars form when massive stars explode as core-collapse supernovae, and because massive stars are rare, neutron stars are rare too.

Neutron stars can have two solar masses and be only 12 kilometers (7.5 miles) in diameter; a teaspoon of neutron star would weigh one billion tons. These objects are dense, far denser than white dwarfs, and typically small, dark, and cold.

Neutron stars are hard to detect because they are compact, dim and cool, yet some pulse and reveal their location. A pulsar is a rapidly rotating neutron star that emits beams from its magnetic poles, and when a beam sweeps past Earth it looks like a blinking light. The observable subset of neutron stars consists of pulsars, whose rotation makes them detectable as blinking beacons.

Millisecond pulsars are a specific type with rotation periods of 10 milliseconds or less; they only form when they have a binary companion. As mass spills from the evolving companion onto the neutron star, it gives the neutron star angular momentum and spins it up. The spin-up process requires a donor star, which is why millisecond pulsars are almost always found in binary systems. An evolving companion transfers mass and angular momentum to a neutron star, recycling it into a rapid millisecond pulsar.

When the neutron star pulls material from its companion, it drastically shapes the companion’s evolution, forcing the companion to become a white dwarf far sooner than it would alone. The neutron star strips the companion’s outer layers, speeding up the exposed core’s move toward white-dwarf status relative to isolated evolution.

Recycling models predict that mass transfer begins when the donor is a main sequence star, a subgiant, or a red-giant branch star. A slowly spinning neutron star gains speed from an evolving companion that transfers heavy mass and angular momentum to it. This describes how an evolving companion transfers mass and angular momentum to a neutron star, recycling it into a rapid millisecond pulsar.

Millisecond pulsars are often found in globular clusters, where high stellar density makes neutron stars more likely to have companions. NGC 362D was found in NGC 362, showing that crowded stellar environments help these binary systems form.

Ettorre says these pulsars rotate with extraordinary regularity and serve as true natural clocks for precise tests of general relativity and studies of gravity under extreme conditions. Understanding how millisecond pulsars form and reconstructing their history is key to exploiting their value as cosmic laboratories for fundamental physics.

Understanding the pulsars’ surroundings and reconstructing their evolutionary history is key to exploiting what these remarkable cosmic laboratories can tell us, Ettorre concluded. This conclusion ties the study of systems like NGC 362D to the broader scientific value of millisecond pulsars as tools for fundamental physics.

The study is titled “The youngest white dwarf companion to a millisecond pulsar: Insights from NGC 362D,” and appeared in Astronomy and Astrophysics in 2026 under DOI 10.1051/0004-6361/202662038. According to Universe Today, the paper appears in volume 713, article L14, of Astronomy & Astrophysics, in 2026. Those identifiers give the study’s specific location within the journal.