Astronomers have long believed that the Milky Way is filled with neutron stars, the ultra-dense remnants left behind when massive stars explode. The problem is that most of these objects are invisible. New research published in astronomy and astrophysics It suggests that NASA’s upcoming Nancy Grace Roman Space Telescope may eventually be able to discover some of them.
Using advanced simulations of the Milky Way and Roman’s predictions of future observations, researchers discovered that the space telescope can detect and study dozens of isolated neutron stars through a phenomenon known as gravitational microlensing.
“Most neutron stars are relatively faint and exist alone,” said study leader Zofia Kaczmarek from the University of Heidelberg in Germany. “It’s incredibly difficult to find them without some help.”
How Roman was able to detect invisible neutron stars
Neutron stars contain more mass than the Sun packed into an object the size of a city. Scientists study them to better understand how stars evolve and explode and heavy elements are distributed throughout the universe. It also provides a valuable opportunity to study materials under the most extreme conditions possible (pressure and density).
Most neutron stars remain hidden unless they appear as radio-emitting pulsars 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 may be able to detect them indirectly. When a massive object, such as a neutron star, passes in front of a more distant star, its gravity bends and the light of the background star is magnified. This effect, called microlensing, temporarily brightens distant stars, making them appear slightly offset in the sky.
Many telescopes can detect the short-term brightness caused by microlenses, but Roman is expected to do more than that. Observatories accurately measure both the increase in brightness of background stars (photometry) and their minute positional movements (astrometry).
Because neutron stars are relatively massive, they produce stronger astronomical signals than smaller objects. This means that Roman may not only be able to detect hidden neutron stars, but also measure their mass, which is extremely difficult to achieve using photometry alone.
“What’s really cool about using microlenses is that you can measure mass directly,” said study co-author Peter McGill of Lawrence Livermore National Laboratory. “Photometry tells us that something has passed in front of a star, but the amount the star’s position has moved tells us how much that object has mass. By measuring small deflections in the sky, we can directly measure the weight of things that we can’t see otherwise.”
Solving the mystery of neutron stars
Roman’s observations could help scientists answer key questions about neutron stars and black holes, including whether there is a true gap between their masses. The mission may also reveal how fast neutron stars move through galaxies.
Researchers are particularly interested in the powerful “kick” that neutron stars receive during supernova explosions. These violent events can travel through space at hundreds of miles per second.
The research team plans to use Roman’s future Galactic Bulge Time-Domain Survey, which will repeatedly observe millions of stars across vast swaths of the sky.
“We’ll get to work as soon as the data starts coming in,” McGill said. “We expect that even in the first few months after commissioning, promising events will begin to be identified.”
Even a small number of confirmed discoveries could significantly improve models of stellar explosions and the behavior of matter under extreme conditions.
“We don’t know for sure the mass distribution of neutron stars or black holes, or where one ends and the other begins,” McGill said. “Roman will be a breakthrough in just that regard.”
A hidden group waiting to be discovered
Astronomers have so far seen only a few thousand neutron stars, most of them detected as pulsars. However, scientists estimate that the Milky Way may contain tens to hundreds of millions of neutron stars. The researchers were also able to measure the mass of neutron stars only in binary systems where two objects orbit each other.
“What we are seeing is a small sample that is not representative of the whole picture,” Kaczmarek said. “Even a single mass measurement would be very powerful. If we could find even one isolated neutron star, it would already be an incredible stimulus for our research.”
The study also highlights the unexpected scientific benefits of the Roman mission. Although this telescope survey was originally designed to primarily discover exoplanets through photometric microlenses, its advanced astronomical precision could open the door to entirely new kinds of discoveries.
“This was not part of the original plan,” McGill said. “But it turns out that Roman’s astronomical measurement capabilities are very good at detecting neutron stars and black holes, so we can add a whole new kind of science to Roman’s investigations.”
If the predictions are correct, Roman could provide the first large collection of isolated neutron stars detected purely through gravitational effects. The mission is expected to dramatically expand microlensing research and reveal populations of hidden objects across the Milky Way, including stellar remnants such as rogue planets and neutron stars.
The Nancy Grace Roman Space Telescope is managed at NASA’s Goddard Space Flight Center in Greenbelt, Maryland, with participation from NASA’s Jet Propulsion Laboratory in Southern California. Caltech/IPAC, Pasadena, California. Space Telescope Science Institute in Baltimore. A scientific team consisting of scientists from various research institutions. A key industrial partner is BAE Systems Inc., Boulder, Colorado. L3Harris Technologies (Rochester, NY); Teledyne Scientific & Imaging, Thousand Oaks, CA;
