Astronomers Find Youngest White Dwarf Yet Circling a Fast-Spinning Pulsar

Astronomers have identified the youngest white dwarf ever found orbiting a millisecond pulsar, catching the pair at an early and rare stage of their shared evolution, according to Universe Today. The pulsar, called NGC 362D, sits in the globular cluster NGC 362, about 29,300 light years away.

The finding comes from a team of researchers in Italy and Germany. Their paper appears in the journal Astronomy and Astrophysics, with Greta Ettorre, a PhD student at the University of Bologna, as first author.

A pulsar that spins more than 10,000 times a minute

A pulsar is a rapidly rotating neutron star that sends beams of light from its poles. If a beam points toward Earth, astronomers see it blink, much like a lighthouse. Millisecond pulsars are a specific type with rotation periods of 10 milliseconds or less, and they only form when the neutron star has a binary companion.

In that process, the companion swells as it evolves, and mass spills onto the pulsar. That gives the pulsar extra rotational energy and makes it spin faster. Astronomers call this recycling.

NGC 362D rotates once every 5.83 milliseconds, which works out to more than 10,000 rotations per minute. It was first discovered in December 2022. Globular clusters are good places to look for such pulsars, Universe Today explained, because stars there are packed so tightly that neutron stars are more likely to have companions.

What makes the companion special

The pulsar's partner, named COM-NGC 362D, is a proto-white dwarf, meaning it is only just entering the earliest stages of becoming a white dwarf. White dwarfs are the compact remnants of stars that have run out of fuel. The team's analysis shows it is a very low-mass helium white dwarf of about 0.18 solar masses that is still in the pre-cooling phase.

Comparisons with updated binary evolution models indicate the system finished its mass-transfer phase only recently, about 0.6 Gyr (billion years) ago. On that basis, the authors say this is the youngest white dwarf companion to a millisecond pulsar identified to date.

To reach those results, the researchers combined radio data from the pulsar with ten years of Hubble observations in ultraviolet and near-infrared light. They then compared the measurements with stellar evolution models to estimate the companion's age and other properties.

An unexpected dimming in ultraviolet light

The same models also showed something odd: the white dwarf gave off less ultraviolet radiation than expected. Something appeared to be blocking or weakening part of that light.

According to the paper, this is the first time that brightness changes depending on wavelength have been seen in this class of objects. The authors say the changes are consistent with leftover material around the star, likely thrown off during the mass-transfer phase. Ettorre said the weakened ultraviolet signal could point to material still surrounding the white dwarf.

The team acknowledges that it is unclear whether such material could have survived for this long. It is, however, their preferred explanation, so the circumstellar material should be treated as a leading interpretation rather than a confirmed detection.

Why it matters

For readers following how stars live and die, the system offers a rare look at a stage that astronomers normally only infer. Finding millisecond pulsars and their companions is of great interest to astronomers, and this pair was caught soon after the mass transfer ended.

It also shows how strongly a pulsar can shape its partner. By drawing material away, the pulsar forces the companion to become a white dwarf far sooner than it would on its own.

The result is a scientific one with no direct effect on daily life, and it rests on a single study of one system. Further observations would be needed to settle whether the dimming truly comes from surrounding material.

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Around Prague, based on reporting by Universe Today