NASA’s Roman telescope could detect and weigh hidden neutron stars, study suggests
The Milky Way may be teeming with neutron stars that evade even the most powerful telescopes. A new study suggests NASA’s upcoming Nancy Grace Roman Space Telescope could finally bring some of these elusive remnants into view — not by seeing them directly, but by catching the tiny distortions they imprint on starlight.
Published in Astronomy and Astrophysics, the research uses simulations of the galaxy and Roman’s planned observations to estimate that the mission could detect and study dozens of isolated neutron stars via gravitational microlensing. “Most neutron stars are relatively dim and on their own,” said lead author Zofia Kaczmarek of Heidelberg University in Germany.
“They are incredibly hard to spot without some sort of help.” Neutron stars pack more mass than the Sun into a sphere roughly the size of a city, creating some of the most extreme conditions in nature. They generally reveal themselves only as radio pulsars or bright X-ray sources.
Roman is expected to find the quiet ones indirectly: when a compact object passes in front of a more distant star, its gravity magnifies and slightly shifts the background star’s light. Unlike most observatories, Roman will precisely measure both the change in brightness (photometry) and the minute positional shift on the sky (astrometry).
Because neutron stars are relatively massive, they produce a stronger astrometric signal than smaller objects, opening a path to direct mass measurements. “Photometry tells us that something passed in front of the star, but it’s the amount the star’s position shifts that tells us how massive that object is,” said co-author Peter McGill of Lawrence Livermore National Laboratory.
“By measuring that tiny deflection on the sky, we can directly weigh something that is otherwise unseen.” The team says Roman’s results could probe whether a true mass gap exists between neutron stars and black holes, and help reveal how fast neutron stars travel after the powerful “kicks” they receive in supernova explosions.
Even a modest number of confirmed detections could sharpen models of stellar death and the behavior of matter at extreme densities. “We don’t know the mass distribution of neutron stars, black holes, or where one ends and the other begins with any certainty,” McGill said.
“Roman will really be a breakthrough in that.” The work will lean on Roman’s planned Galactic Bulge Time Domain Survey, which will repeatedly monitor millions of stars across large swaths of sky. McGill said the team aims to begin sifting for promising microlensing events as soon as data start flowing, with candidates expected in the first months after commissioning.
To date, astronomers have identified only a few thousand neutron stars, most as pulsars; Roman could significantly broaden the sample by finding those that otherwise remain dark.