NASA’s Roman Telescope could finally expose the Milky Way’s hidden population of invisible neutron stars.
Astronomers believe neutron stars should be scattered throughout the Milky Way, left behind after massive stars explode in supernova blasts. But despite their expected abundance, most of these ultra-dense objects remain invisible to telescopes. A new study published in Astronomy and Astrophysics suggests NASA’s upcoming Nancy Grace Roman Space Telescope may finally be able to uncover them.
Researchers used detailed Milky Way simulations along with projections of Roman’s future observations to estimate how many isolated neutron stars the telescope could detect. Their results indicate Roman may be able to identify and study dozens of these hidden stellar remnants using a phenomenon 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.”
How Gravitational Microlensing Reveals Invisible Objects
Neutron stars contain more mass than the Sun compressed into a sphere roughly the size of a city. Scientists study them to better understand how stars evolve, explode, and distribute heavy elements throughout the universe. They also offer a unique way to explore matter under the most extreme conditions (pressures and densities) imaginable.
Most neutron stars are difficult to detect because they emit very little visible light. Unless they appear as pulsars emitting radio waves or produce strong X-ray emissions, they can remain hidden even from advanced observatories.
Roman may detect them indirectly through gravity. When a neutron star passes in front of a distant background star, its gravity bends the star’s light and slightly alters its apparent position in the sky. This effect, known as microlensing, briefly causes the background star to brighten and shift.
Many telescopes can observe the temporary brightening caused by microlensing, but Roman is expected to measure both the brightening (photometry) and the tiny positional movement (astrometry) with exceptional accuracy.
Because neutron stars are more massive than many other objects that create microlensing events, they generate a stronger astrometric signal. That could allow Roman not only to detect isolated neutron stars, but in some cases directly measure their masses, something extremely difficult to accomplish 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.”
Searching for Missing Neutron Stars
The measurements collected by Roman could help astronomers answer major questions about neutron stars and black holes, including whether there is a real gap between their masses. The telescope may also reveal how quickly neutron stars travel through the galaxy.
Scientists are especially interested in the violent “kicks” neutron stars receive during supernova explosions. These kicks can launch them across the Milky Way at hundreds of miles per second.
To search for these events, researchers plan to use Roman’s future Galactic Bulge Time Domain Survey. The survey will repeatedly image enormous star fields containing millions of stars at a high observing 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 discoveries could greatly 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.”
Roman Telescope Could Transform Neutron Star Research
Only a few thousand neutron stars have been identified so far, most of them discovered as pulsars. Yet scientists estimate the Milky Way may contain anywhere from tens of millions to hundreds of millions of neutron stars. Researchers have also only managed 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 points to an unexpected scientific opportunity for the Roman mission. Although Roman’s microlensing survey was mainly designed to search for exoplanets through photometric microlensing, its advanced astrometric precision may allow it to detect entirely different classes of hidden objects.
“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 these predictions prove accurate, Roman could produce the first large collection of isolated neutron stars detected solely through their gravitational effects. Scientists expect the mission to dramatically expand the study of microlensing and uncover previously hidden populations of objects across the Milky Way, including rogue planets, black holes, and 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.
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