NASA’s Roman Poised to Transform Hunt for Elusive Neutron Stars
Astronomers have long known that neutron stars, the crushed cores left after massive stars explode, should be scattered throughout the Milky Way. However, most of them are effectively invisible. A new study published in Astronomy and Astrophysics suggests that NASA’s upcoming Nancy Grace Roman Space Telescope might spot them anyway.
Using detailed simulations of the Milky Way and future Roman observations, the researchers showed that 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 faint and self-contained,” said Zofia Kaczmarek of the University of Heidelberg in Germany, who led the study. “They are incredibly difficult to spot without help.”
Find what is invisible
Neutron stars pack more mass than the Sun into a sphere the size of a city. Their study helps us understand how stars live, die and spread heavy elements throughout the universe. They also provide the opportunity to study what happens under the most extreme conditions (pressures and densities) imaginable.
Yet unless they are pulsars emitting radio wavelengths or 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 distorts space-time and bends light from the background star. This microlensing effect briefly makes the background star brighter and appears offset from its true position in the sky.
While many telescopes can detect temporary brightening, Roman can measure both the brightening (photometry) and the tiny change in position (astrometry) of the lensing 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, which is almost impossible with photometry alone.
“What’s really interesting about using microlensing is that you can get direct mass measurements,” said Peter McGill, co-author of the paper from Lawrence Livermore National Laboratory. “Photometry tells us that something passed in front of the star, but it’s the shift in the star’s position that tells us the mass of that object. By measuring this small deviation in the sky, we can directly weigh something that would otherwise be invisible.”
Roman’s measurements could help astronomers determine whether there is a real discrepancy between the masses of neutron stars and black holes and how fast the neutron stars are moving.
Scientists are particularly interested in understanding the powerful “kicks” that neutron stars receive when they are born in supernova explosions. These kicks can send them running across the galaxy at hundreds of kilometers per second.
Huge surveys, big chances of winning
The research team will use Roman’s upcoming Galactic Bulge Time Domain Survey, which will monitor millions of stars at once in vast images of the sky, taken at high frequency.
“We’re going to get to work as soon as the data starts coming in,” McGill said. “Even in the first few months after go-live, we hope 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 for sure the mass distribution of neutron stars, black holes, or where one ends and the other begins,” McGill said. “Roman will truly be a breakthrough in this area.”
Although only a few thousand neutron stars have been detected so far, mostly in the form of pulsars, scientists estimate there could be tens to hundreds of millions in the Milky Way. Additionally, to date, researchers have only been able to measure the mass of neutron stars in binary pairs.
“We are looking at a small sample that is not representative of the bigger picture,” Kaczmarek said. “Even a single mass measurement would be very powerful. If we found a single isolated neutron star, that would already be incredibly stimulating for our research.”
Looking to the future
The study also highlights creative use of mission capabilities. While Roman’s survey is primarily designed to find exoplanets using photometric microlensing, its powerful astrometric capabilities open the door to entirely new discoveries using astrometric microlensing.
“This was not part of the original plan,” McGill said. “But it turns out that Roman’s astrometric abilities are really good at detecting neutron stars and black holes, so we can add a whole new type of science to Roman’s investigations.”
If the predictions prove 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. Novel is expected to transform the study of microlensing and hidden populations of objects in our galaxy, from rogue exoplanets to stellar remnants like neutron stars.
The Nancy Grace Roman Space Telescope is operated at NASA’s Goddard Space Flight Center in Greenbelt, Maryland, with participation from NASA’s Jet Propulsion Laboratory in Southern California; Caltech/IPAC in Pasadena, California; the Baltimore Space Telescope Science Institute; and a scientific team composed of scientists from various research institutions. Key industry partners include BAE Systems Inc. in Boulder, Colorado; L3Harris Technologies in Rochester, New York; and Teledyne Scientific & Imaging in Thousand Oaks, California.
To find out more about the Roman visit:
https://nasa.gov/roman
By Hannah Braun
Space Telescope Science Institute, Baltimore, Maryland.
hbraun@stsci.edu
Media contacts:
Claire Andreoli
NASA Goddard Space Flight CenterGreenbelt, Maryland.
301-286-1940
Christine Pulliam
Space Telescope Science Institute, Baltimore, Maryland.
cpulliam@stsci.edu
