A Neutron Star's Missing "Birth Certificate" Comes to Light

An international research team led by scientists from the Chinese Academy of Sciences (CAS) has discovered an isolated pulsar located within the tidal tail of NGC 6791, one of the oldest and most massive open clusters in the Milky Way.

By combining observations from the Five-hundred-meter Aperture Spherical radio Telescope (FAST), the MeerKAT radio telescope and the Canada-France-Hawaii Telescope (CFHT) with direct N-body simulations, the researchers have obtained the first direct observational evidence of a neutron star in the tidal tail of an open cluster—revealing a key stage in the journey of neutron stars from their birth clusters into the Galactic field.

In 1934, astronomers Walter Baade and Fritz Zwicky proposed that supernova explosions create neutron stars. A decade later, Baade introduced the concept of stellar populations, characterizing the distinct groups of stars that make up the Milky Way. However, a critical link between these two foundational ideas was missing: observational proof of how a neutron star born in a dense star cluster eventually becomes one of the thousands scattered across the galactic field.

The researchers targeted NGC 6791, an 8-billion-year-old open cluster, to search for this missing evolutionary stage. The discovery of the pulsar, designated PSR J1921+3745, effectively provides a "birth certificate" that traces the object's journey from the cluster to the Galactic tide.

Identifying this unique object required a global scientific collaboration combining multi-telescope observations and advanced simulations. The researchers first detected PSR J1921+3745 with FAST, identifying it as an isolated pulsar with a 1.9-second spin period. MeerKAT then determined its position with arcsecond-level precision, and deep CFHT observations mapped the surrounding stellar field.

These combined observations place PSR J1921+3745 squarely within a prominent tidal tail of NGC 6791, a stream of stars gradually being stripped from the cluster by the Milky Way's gravity. The researchers confirmed this physical association based on the pulsar's estimated distance, the low probability of a chance alignment, and the absence of other young stellar populations in the vicinity.

To understand how the pulsar arrived at this location, the researchers utilized direct N-body simulations tailored to the specific dynamics of NGC 6791. Simulations show that more than 95% of neutron stars are rapidly expelled from clusters by powerful supernova kicks immediately after formation. They found that a neutron star formed via a relatively gentle "electron-capture supernova," characterized by a small kick velocity, could remain bound to the cluster for billions of years. Over time, stellar encounters and galactic tides gradually carried the neutron star into the cluster's tidal tails.

This discovery provides the first direct observational evidence of a neutron star in the tidal tail of an open cluster, revealing a previously missing stage between a neutron star's birth in a stellar group and its later life in the galactic field. It also opens a new avenue at the intersection of stellar evolution, supernova explosion, pulsar astronomy, star-cluster dynamics and the structure of the Milky Way.

Nearly a century after Baade connected supernovae, neutron stars, and stellar populations, this discovery now provides a traceable route from a neutron star's birth cluster to its future in the wider Milky Way.

The study was led by ZHANG Lei of the National Astronomical Observatories of CAS, FU Xiaoting of the Purple Mountain Observatory of CAS; and LI Di of Tsinghua University, with collaborators from Australia, Canada, China, Germany, Italy, South Africa and the United States.

Artist's impression of the pulsar PSR J1921+3745 within the tidal tail of the 8-billion-year-old open cluster NGC 6791. Three telescopes contributed to the study: FAST detected the pulsar, MeerKAT pinpointed its position, and CFHT mapped the surrounding stellar structure. (Image by FU Xiaoting)