The exotic black hole-neutron star binaries in our Galaxy

Deep in the quiet corners of our Milky Way galaxy, astronomers have long suspected a hidden population of cosmic couples: a black hole and a neutron star locked in a tight orbit. These pairs are considered the holy grail of astrophysics because they offer a unique laboratory to test the laws of gravity and understand how the densest objects in the universe behave. Yet, despite decades of searching with powerful radio telescopes, not a single one of these binary systems has been found within our own galaxy. While we have detected the final, violent crash of such pairs in distant galaxies, the ones that should be lurking nearby remain invisible. This absence has left scientists with a puzzle: are these systems rarer than models predict, or are they simply hiding in plain sight, invisible to our current methods?

The missing link may lie in a substance that permeates the universe but remains undetected: dark matter. For years, researchers have known that massive objects like stars and planets can act as traps for dark matter particles. As these invisible particles drift through space, the intense gravity of a dense star can pull them in, where they collide with ordinary matter, lose their speed, and become trapped inside. This process transfers energy to the star, warming it up. In standard scenarios, this heating is too faint to see. However, a new study by Man Ho Chan proposes that in specific binary systems, this heating could be amplified to extreme levels, turning a cold, invisible neutron star into a blazing beacon that our telescopes can finally spot.

Chan's work suggests that some of the black holes in these missing pairs might be "primordial," meaning they formed in the very early universe rather than from the collapse of a dying star. If this is true, these black holes would have been surrounded by a dense, concentrated spike of dark matter from the moment of their birth. Unlike the diffuse dark matter found in the rest of the galaxy, this spike would be incredibly dense. When a neutron star orbits such a black hole, it moves through this dense cloud of dark matter. The neutron star acts like a net, capturing a massive amount of dark matter particles as it travels. The energy released by this capture is so intense that it keeps the surface of the neutron star at a searing temperature of about one million degrees Kelvin.

This extreme heat changes the game for detection. A normal neutron star, even one warmed by standard dark matter capture, would be too cool and small to be seen with current technology. But a neutron star heated to a million degrees would glow brightly in ultraviolet and X-ray light. Chan calculates that such a star would be visible to powerful instruments like the XMM-Newton X-ray telescope and to the James Webb Space Telescope. If these exotic binaries exist, they would appear as incredibly hot, tiny points of light that do not fit the profile of any other known object.

The study does not claim to have found these objects yet, but it provides a clear roadmap for how to find them. By looking for these specific, ultra-hot neutron stars, astronomers could finally locate the missing black hole-neutron star pairs in our galaxy. Furthermore, the brightness of these stars would tell us more than just their location; it would reveal the density of the dark matter surrounding them. If a hot neutron star is found, its temperature could be used to place incredibly strict limits on how dark matter particles interact with ordinary matter, potentially improving our understanding of this mysterious substance far beyond what current experiments on Earth can achieve.

The researchers acknowledge that other processes can heat neutron stars, so finding a hot star alone is not enough to confirm the theory. To be certain, astronomers would need to verify that the hot star is indeed orbiting a black hole and that the system shows signs of the gravitational drag caused by the dark matter spike. However, the proposal offers a compelling new strategy: instead of listening for radio signals that may not exist, we should look for the heat signature of a star that has been supercharged by the invisible matter of the universe. If successful, this approach could solve the mystery of the missing binaries and open a new window into the physics of dark matter and gravity.