NASA’s Roman Space Telescope could reveal millions of invisible neutron stars

Using advanced simulations of the Milky Way and predictions of Roman's future observations, researchers found the space telescope could detect and study dozens of isolated neutron stars through a phenomenon known as 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."

How Roman Could Detect Invisible Neutron Stars

Neutron stars contain more mass than the Sun packed into an object roughly the size of a city. Scientists study them to better understand how stars evolve, explode, and distribute heavy elements throughout the cosmos. They also offer a rare opportunity to investigate matter under the most extreme conditions (pressures and densities) imaginable.

Most neutron stars remain hidden unless they appear as pulsars that emit radio waves or shine brightly in X-rays. Even the most powerful telescopes can miss isolated neutron stars that produce little or no detectable light.

The Roman Space Telescope could find them indirectly. When a massive object such as a neutron star passes in front of a more distant star, its gravity bends and magnifies the background star's light. This effect, called microlensing, temporarily makes the distant star appear brighter and slightly shifted in the sky.

Many telescopes can detect the brief brightening caused by microlensing, but Roman is expected to do much more. The observatory will precisely measure both the increase in brightness (photometry) and the tiny positional movement (astrometry) of the background star.

Because neutron stars are relatively heavy, they create a stronger astrometric signal than smaller objects. That means Roman may not only detect hidden neutron stars, but also measure their masses, something extremely difficult to achieve using 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."

Solving Neutron Star Mysteries

Roman's observations could help scientists answer major questions about neutron stars and black holes, including whether there is a true gap between their masses. The mission may also reveal how quickly neutron stars travel through the galaxy.

Researchers are especially interested in the powerful "kicks" neutron stars receive during supernova explosions. These violent events can launch them through space at hundreds of miles per second.

The team plans to use Roman's future Galactic Bulge Time Domain Survey, which will repeatedly observe millions of stars in enormous sections of the sky.

"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 modest number of confirmed discoveries could significantly improve models of stellar explosions and the behavior of matter under extreme conditions.

"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."

A Hidden Population Waiting To Be Found

So far, astronomers have identified only a few thousand neutron stars, most of them detected as pulsars. However, scientists estimate the Milky Way could contain anywhere from tens of millions to hundreds of millions of neutron stars. Researchers have also only been able to measure neutron star masses in binary systems where two objects orbit each other.

"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."

The study also highlights an unexpected scientific advantage of the Roman mission. Although the telescope's survey was originally designed mainly to discover exoplanets through photometric microlensing, its advanced astrometric precision may open the door to entirely new kinds of discoveries.

"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 are correct, Roman could deliver the first large collection of isolated neutron stars detected purely through their gravitational effects. The mission is expected to dramatically expand the study of microlensing and uncover hidden populations of objects throughout the Milky Way, including rogue planets and stellar remnants such as 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.