Astronomers have long known that neutron stars, the crushed cores left behind after massive stars explode, should be scattered throughout the Milky Way galaxy. However, most of them are effectively invisible. A new study published in Astronomy and Astrophysics suggests NASA’s upcoming Nancy Grace Roman Space Telescope could spot them anyway.

Using detailed simulations of the Milky Way and Roman’s future observations, researchers showed the flagship observatory may be able to identify and characterize dozens of isolated neutron stars through a subtle effect called gravitational microlensing.

“Most neutron stars are relatively dim and on their own,” said Zofia Kaczmarek of Heidelberg University in Germany, who led the study. “They are incredibly hard to spot without some sort of help.”

Finding what’s invisible

Neutron stars pack more mass than the Sun into a sphere about the size of a city. Studying them helps us understand how stars live, die, and spread heavy elements throughout the universe. They also provide a chance to study what happens under the most extreme conditions (pressures and densities) imaginable.

Yet, unless they are pulsars that beam in radio wavelengths or glow in X-rays, they can remain hidden from even the most powerful telescopes.

Roman can search for them in a different way. When a massive object like a neutron star moves in front of a distant background star, its intense gravity warps spacetime and deflects the background star’s light. This microlensing effect briefly makes the background star brighter and appear offset from its true position in the sky.

While many telescopes can detect the temporary brightening, Roman can measure both the brightening (photometry) and the tiny positional shift (astrometry) of the lensed star with exceptional precision.

Because neutron stars are relatively massive, they produce a larger astrometric signal than lighter objects, allowing missions like Roman to not only detect them, but also weigh them in some cases, something that is nearly impossible with photometry alone.

“What’s really cool about using microlensing is that you can get direct mass measurements,” said paper co-author Peter McGill of Lawrence Livermore National Laboratory. “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. By measuring that tiny deflection on the sky, we can directly weigh something that is otherwise unseen.”

Roman’s measurements could help astronomers determine whether there is a true gap between the masses of neutron stars and black holes and how fast neutron stars are moving.

Scientists are particularly interested in understanding the powerful “kicks” neutron stars receive when they are born in supernova explosions. These kicks can send them racing through the galaxy at hundreds of miles per second.

Huge surveys, high chance of payoff

The research team will utilize Roman’s future Galactic Bulge Time Domain Survey, which will monitor millions of stars at a time in vast images of the sky, taken at a high frequency.

“We’re going to get to work as soon as the data start coming in,” said McGill. “Even in the first months after commissioning, we expect to start identifying promising events.”

Even a relatively small number of confirmed detections could significantly improve models of stellar explosions and extreme matter.

“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.”

Although only a few thousand neutron stars have been detected so far, mostly as pulsars, scientists estimate there could be tens of millions to hundreds of millions in the Milky Way. Additionally, to date, researchers have only been able to measure the masses of neutron stars in binary pairings.

“We’re seeing a small sample that’s not representative of the big picture,” Kaczmarek said. “Even a single mass measurement would be very powerful. If we found just one isolated neutron star, it would already be incredibly stimulating to our research.”

Looking ahead

The study also highlights a creative use of the mission’s capabilities. While Roman’s survey is designed primarily to find exoplanets using photometric microlensing, its powerful astrometric capabilities open the door to entirely new discoveries with astrometric microlensing.

“This wasn’t part of the original plan,” said McGill. “But it turns out Roman’s astrometric capability is really good at detecting neutron stars and black holes, so we can add a whole new kind of science to Roman’s surveys.”

If the predictions hold true, the mission could provide the first large sample of isolated neutron stars discovered through their gravity alone, revealing a hidden population that has remained out of reach until now. Roman is expected to transform the study of microlensing and the hidden populations of objects in our galaxy, from rogue exoplanets to stellar remnants like neutron stars.

The Nancy Grace Roman Space Telescope is managed at NASA’s Goddard Space Flight Center in Greenbelt, Maryland, with participation by NASA’s Jet Propulsion Laboratory in Southern California; Caltech/IPAC in Pasadena, California; the Space Telescope Science Institute in Baltimore; and a science team comprising scientists from various research institutions. The primary industrial partners are BAE Systems Inc. in Boulder, Colorado; L3Harris Technologies in Rochester, New York; and Teledyne Scientific & Imaging in Thousand Oaks, California.

To learn more about Roman visit:

https://nasa.gov/roman

By Hannah Braun
Space Telescope Science Institute, Baltimore, Md.
hbraun@stsci.edu

Media contacts:

Claire Andreoli
NASA’s Goddard Space Flight Center, Greenbelt, Md.
301-286-1940

Christine Pulliam
Space Telescope Science Institute, Baltimore, Md.
cpulliam@stsci.edu