The Milky Way likely harbors hundreds of millions of neutron stars, remnants of stellar explosions. Yet only a few thousand have been observed to date, as they are too faint and isolated.
A recent study in
Astronomy and Astrophysics indicates that the future Nancy Grace Roman Space Telescope, thanks to its ability to detect gravitational microlensing effects, could change the situation. It would allow identifying these otherwise invisible compact objects. Their exact number remains unknown to astronomers.
The Nancy Grace Roman Space Telescope, NASA's infrared observatory, is expected to transform the study of neutron stars and black holes. Credit: NASA's Goddard Space Flight Center
These neutron stars are the collapsed cores of massive stars, concentrating more matter than the Sun into a city-sized sphere. They emit little visible light, making them difficult to spot. Only those that emit regular radio waves, called pulsars, or X-rays are detectable. The majority thus remain hidden. This is where Nancy Grace Roman comes in.
Gravitational microlensing is a method used by astronomers to detect these invisible stars. When a massive object, such as a neutron star, passes in front of a distant star, its curvature of space deflects the star's light. This causes a brief brightening and a slight shift in its apparent position. The telescope will be able to measure both effects with high precision.
What makes Nancy Grace Roman so valuable is its astrometric capability. Most telescopes only detect the brightening, but it can also measure the tiny angular displacement of the star. Since neutron stars are very massive, this displacement is more pronounced than for other objects. This opens the way to a direct measurement of their mass, a rare piece of information. Scientists have so far measured the mass of only a few neutron stars, all in binary systems.
Researchers hope to detect dozens of microlensing events caused by neutron stars. These observations could also reveal the violent "kicks" received during their formation, which propel them through the Galaxy at high speed. Measuring these speeds would allow a better understanding of supernovas. It is a missing piece in our understanding of these phenomena.
Diagram of astrometric microlensing: the gravity of a neutron star bends the light of a background star, creating a positional shift. The more massive the object, the larger the shift. Credit: NASA, STScI, Joyce Kang (STScI)
Even a small number of discoveries would have a significant impact. According to Zofia Kaczmarek of Heidelberg University, a single mass measurement of an isolated neutron star would already be very valuable. It would allow testing models of stellar explosions and studying matter under extreme conditions. Currently, all known masses come only from binary systems. Isolated neutron stars might have different masses.
The Nancy Grace Roman Telescope was not designed for this purpose. Intended to search for exoplanets, it carries very sensitive instruments that open new possibilities. As Peter McGill of Lawrence Livermore National Laboratory notes, it is now accepted that it will be able to detect isolated neutron stars and black holes. Scientists are eager to receive the first data.
Gravitational microlensing
Gravitational microlensing is a phenomenon predicted by Einstein's general relativity. When a massive object, such as a star or black hole, passes in front of a more distant light source, its gravity curves spacetime. This bends the source's light, creating a distorted and often magnified image. This effect is used in astronomy to study objects that would otherwise be invisible.
In the case of so-called "photometric" microlensing, a brief brightening of the source is observed. Most telescopes focus on this increase in brightness. But there is also an astrometric effect: the apparent position of the source shifts slightly. This shift is tiny, on the order of a few millionths of a degree, but it contains valuable information about the object's mass.
The Roman telescope is designed to measure both effects. Thanks to its high astrometric precision, it will be able to detect the small positional shifts caused by neutron stars or black holes. The more massive the object, the larger the shift. This allows not only detection of these objects but also direct determination of their mass, a rare and valuable piece of information.
Neutron stars
Neutron stars are the remnant cores of massive stars after a supernova explosion. Their mass is comparable to the Sun's, but compressed into a sphere about 12 miles (20 km) in diameter. This extreme density makes them natural laboratories for studying matter under conditions impossible to reproduce on Earth. They also possess intense magnetic fields and rotate very rapidly.
Most neutron stars are difficult to observe because they emit little visible light. Some are detectable as pulsars, emitting regular radio pulses. Others are visible in X-rays if they accrete matter. But the vast majority remain invisible. Models predict that there would be between 100 million and 1 billion in the Milky Way.