A team of astronomers recently identified a red giant star with an unseen binary companion after examining data from the Large Sky Area MultiObject Fiber Spectroscopic Telescope (LAMOST) and the Apache Point Observatory Galactic Evolution Experiment (APOGEE). Although the companion object can’t be observed visually, astronomers can see the gravitational effects it has on the red giant star.
All of this is nothing new—researchers often discover binary systems with effectively invisible companions thanks to gravitational effects. However, the inferred mass of the unseen object, along with the system’s other characteristics, makes it a bit of an anomaly. The research on this pair, published in Science Advances, suggests the unseen companion is likely an unusually massive neutron star that is pushing the boundaries of how massive neutron stars can get.
The unclear boundary between neutron stars and black holes
Neutron stars are the extraordinarily dense remnants of exploded high-mass stars. Usually, neutron stars have masses around 1.4 times the mass of our sun, all of which must fit into a roughly city-sized sphere around 20 kilometers, or 12 miles, across. So far, no neutron stars above 2.5 solar masses have been found. Black holes, remnants of even larger stars, on the other hand, are typically at least five solar masses and go up from there, with supermassive black holes reaching billions of times the mass of the sun.
This leaves an odd mass gap of around 2.5 to 5 solar masses. Astronomers aren’t sure about the mass limits between neutron stars and black holes. They do know that the maximum possible mass of a neutron star is related to how matter behaves under pressures in the interior of these objects, but the equations of state that describe this are still poorly understood. Rare, unusually heavy examples can help establish where neutron stars give way to black holes.
The unseen companion defying simulations
The researchers that discovered the new binary system, called J0008+0128, estimate the red giant’s companion to have a mass somewhere between 1.8 and 2.8 solar masses, with a best estimate of 2.2 solar masses. The object’s upper range places it in the mass gap between neutron stars and black holes. This makes it, at the very least, a rare and valuable object for studying this gap.
The team’s analysis also found that the pair completes an orbit in about 3.3 years, meaning there is a relatively wide separation between the two objects. It also follows a surprisingly near-circular path despite this wide separation. No light was detected outside that of the red giant, meaning the companion could not be another regular star. The most likely possibility was a neutron star. However, this combination of high mass, near-circular orbit and wide separation is unusual for a binary system with a neutron star.
Many other heavy neutron stars occupy tighter binaries and likely gained their mass by drawing material from their companions, but this potential neutron star is likely too far away from the red giant to steal away material from it, and none of the typical signs of mass transfer are present, according to the team’s analysis.
The researchers also conducted around 300,000 POSYDON simulations of isolated binaries to gain some insight into how the system may have formed. However, none of the simulations reproduced the system’s combination of masses and orbital properties.
The study authors write, “The scarcity of J0008+0128–like systems is unlikely to be driven solely by limited POSYDON sampling, and instead points to a strong evolutionary bias within isolated binary evolution. Nonetheless, the formation of such a system is not precluded in principle and may require a combination of extreme but physically plausible parameters, such as an unusually efficient common envelope (CE) ejection during the evolution of the [neutron star] progenitor, a high [neutron star] birth mass, and minimal supernova kick.”
A (probable) neutron star
Despite its unusual characteristics, the team says the unseen companion is most likely a neutron star. The most similar example is Gaia NS1, a neutron star candidate with a mass of about 1.9 suns in a roughly two-year orbit. Although J0008+0128 appears to be a little more extreme, it does not fit well with many other possible explanations.
Still, the researchers do say there are some other possibilities. The binary system could actually be a triple system, with two very close, smaller neutron stars and the red giant or a neutron star, white dwarf and red giant. It may also previously have been a triple system in which the two smaller bodies merged. It is still even possible that the object might be a very small black hole, although none this small have ever been identified.
Further data on the system may enlighten researchers in the future. Whatever the case, the system is providing additional data for the understanding of the mass gap between neutron stars and black holes.
The study authors write, “The wide, low-eccentricity orbit is difficult to explain through standard binary evolution channels, pointing to the need for improved theoretical models to account for such systems.”
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Publication details
Xinlin Zhao et al., Discovery of a likely supermassive neutron star in a remarkably wide binary, Science Advances (2026). DOI: 10.1126/sciadv.aeg3307
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A likely supermassive neutron star might shed light on the mass gap between neutron stars and black holes (2026, November 13)
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