The Boundary Between Black Holes & Neutron Stars

A gravitational-wave merger detected in 2019 appears to involve a “small” black hole candidate paired with a companion mass of 2.6 times the Sun—an object that sits right on the boundary between neutron stars and black holes. The masses inferred from the waveform are about 23.2 and 2.59 solar masses, and the smaller mass is the problem: it is heavier than most neutron-star measurements and lighter than the black-hole masses astronomers typically see. With no accompanying burst of light, the system’s nature remains ambiguous, but the stakes are high because either outcome would force a rethink of physics at the most extreme densities.

The event was picked up by LIGO in Washington State and Louisiana and by Virgo in Italy as the detectors registered tiny changes in the lengths of their kilometer-scale arms. The timing across the three observatories narrowed the source to arcs on the sky, prompting rapid telescope sweeps for an electromagnetic counterpart—an approach that worked in 2017 when two neutron stars collided and produced light across the spectrum. This time, nothing was found. That absence fits two scenarios: the merger could have involved black holes that swallowed each other without a flare, or it could have included a neutron star that was simply consumed by the heavier companion before any detectable radiation escaped.

The mass of the lighter object is what makes the detection unusually informative. Neutron stars are the ultra-dense remnants of supernova cores, packed with roughly one-and-a-half solar masses into a sphere only about city-sized. Their surface gravity is so extreme that escape velocity approaches half the speed of light, and neutron stars are thought to be near a threshold where adding more mass would push them into becoming black holes. Unlike ordinary matter, neutron-star matter is governed by quantum physics: as mass increases, the star’s radius does not behave like a simple scaling law and can even shrink at the highest masses. That leads to a “phantom event horizon” concept—an effective horizon scale that grows with mass while the surface can move inward—so the overlap of these scales marks the transition to a true black hole.

The uncertainty lies in the star’s interior. Near the center, neutrons may transform into different forms of quark matter, potentially including strange quarks. Those unknown states determine how the star’s size changes with mass and therefore set the maximum stable neutron-star mass. Prior estimates place that maximum in the range of roughly 2 to 3 solar masses, with observational constraints from neutron-star mergers, pulsars, and X-ray binaries typically landing below 2.6 solar masses. The new candidate therefore sits at or beyond the theoretical edge.

On the black-hole side, astronomers have long observed a gap: black holes in X-ray binaries appear to start around five solar masses, consistent with stellar evolution models where a successful supernova leaves behind a neutron star, and only then does additional infalling material push the remnant above the black-hole threshold. A 2.6-solar-mass black hole would be hard to produce in that picture. If the lighter object is truly a neutron star, the event would probe the densest possible quantum states of matter. If it is a black hole, it would challenge assumptions about how stars collapse and how mass thresholds are crossed. Either way, the detection opens a new test of gravity and matter under conditions that are otherwise inaccessible.