Seeking Solo Super-Fast Pulsars

Millisecond pulsars are some of the most extreme objects in the universe. Surveys of pulsars in globular clusters aim to find these fast-spinning stars and understand why some of them fly solo when they’re expected to be paired up.

Dynamic Duos and Solo Acts

When massive stars expire, they can leave behind their compressed cores as neutron stars, which pack about twice the mass of the Sun into a sphere that could nestle into the narrowest stretch of the English Channel. Pulsars are neutron stars that spin exceptionally quickly and have strong magnetic fields, leading them to produce beams of radio emission that sweep across the sky as the star spins.

The most extreme pulsars are those that complete each rotation in less than 10 milliseconds. Millisecond pulsars are thought to reach their record-setting speeds with the help of a friend, starting out as slower rotators that are “spun up” by accreting material from a binary companion. While that theory explains the origins of millisecond pulsars that inhabit binary systems, it can’t account for those that drift through space alone. To find out where these solo rotators come from, we’ll need to find them first — and researchers are homing in on the best places to look.

Seeking Single Pulsars

Of the thousands of known pulsars in the Milky Way, a small but significant fraction can be found in globular clusters: spherical groupings of tens of thousands to millions of stars on the outskirts of the galaxy. Recent observations suggest that globular clusters might be among the best places to find isolated millisecond pulsars.

Using the Five-hundred-meter Aperture Spherical radio Telescope — FAST, the largest filled-aperture radio dish in the world — Dejiang Yin (Guizhou University) and collaborators went pulsar hunting in the globular cluster NGC 6517. Previous searches spotted nine pulsars in NGC 6517, only one of which was in a binary system, suggesting that this cluster may be home to other isolated pulsars.

Best Place to Look

From the new observations, Yin’s team picked out the characteristic pulsed signals from eight millisecond pulsars, all of which appear to lack binary companions. With these new additions, NGC 6517 is now the most pulsar-rich globular cluster accessible to FAST and and the third most of all Milky Way globular clusters. (After this research article was submitted, the team found three more pulsars in their data, so expect to hear more about NGC 6517’s pulsar population soon!)

Why is NGC 6517 home to so many isolated millisecond pulsars? Its density may be responsible: NGC 6517 is one of the most densely packed globular clusters in the Milky Way, which could mean that binary systems containing a millisecond pulsar are more likely to be split apart by close encounters with other stars. Other proposed isolated millisecond pulsar formation mechanisms like neutron star mergers might also be common in dense clusters.

Yin’s team performed statistical modeling of NGC 6517 and other pulsar-hosting globular clusters in the Milky Way. They found that the estimated number of pulsars within a cluster scales with the cluster’s escape velocity, leading them to suggest clusters with high escape velocities — Messier 2, Messier 92, Liller 1, NGC 6388, NGC  2808, Messier 54, and Messier 75 — as the best places to search for more isolated millisecond pulsars.

Citation

“FAST Discovery of Eight Isolated Millisecond Pulsars in NGC 6517,” Dejiang Yin et al 2024 ApJL 969 L7. doi:10.3847/2041-8213/ad534e