Invisible object in the mass gap: neither neutron star nor black hole

What is the invisible object?

The authors, writing in the journal Science Advances, first suspected a neutron star. These are extremely dense remnants left when a massive star explodes as a supernova. Their light is usually too faint for standard instruments, and they typically weigh about 1.4 Suns.

This one doesn't fit. The researchers estimate its mass at between 1.8 and 2.8 solar masses, with 2.2 as the most likely value. Light black holes start at around three solar masses. So the object lands in what the study calls the mass gap between neutron stars and black holes.

That leaves two options: an exceptionally heavy neutron star or an extremely light black hole. According to the study, a very massive neutron star is the more likely answer. Such objects are rare, because they sit right at the limit where a star would normally collapse into a black hole.

How did it form?

The team doesn't know yet. Heavy neutron stars can grow in binaries by pulling matter off a partner star. Here, though, the red giant and its companion are too far apart for matter to pass between them.

One idea is that the system began with three stars. Two neutron stars, or a neutron star and an ordinary star, could have merged into a single heavy object. The researchers also don't rule out that supermassive neutron stars can come from perfectly normal supernovae. Either way, our knowledge of extremely massive objects clearly has gaps.