The object has 2.6 times the mass of our sun, making it too light for a black hole and again too heavy for a neutron star.

Mergers between two black holes or two neutron stars are now known. This results in gravitational waves that researchers can detect with powerful instruments such as LIGO or Virgo. However, a recently observed merger reveals a mysterious object. An object with a rather peculiar mass.

Pulp
When the heaviest stars die, they fall apart under their own gravity, leaving black holes. When slightly less massive stars become defunct, they explode in a supernova. What remains are the dense and dead remains of stars called neutron stars. The heaviest known neutron star has no more than 2.5 times the mass of our sun. The lightest known black hole is about five solar masses. And that poses a pressing question to astronomers: are there also objects that are exactly in between in terms of mass?

Gap
Yes, researchers now say. On August 14, 2019, astronomers picked up the gravitational waves from an amalgamation of an unknown object and a black hole from about 23 solar masses. The mysterious object appears to have 2.6 times the mass of our sun. And that is interesting. It means that the object is too light for a black hole and too heavy for a neutron star. “This compact object in the mass gap is a complete surprise,” said researcher Vicky Kalogera.

Mysterious object
The exact nature of the object remains shrouded in mystery to date. “Although we cannot classify the object with conviction, it must be the most heavily known neutron star or the lightest known black hole,” Kalogera continues. “It could be a neutron star merging with a black hole. This is an exciting possibility that has been predicted theoretically, but has not yet been observed in real life. But with a mass 2.6 times that of our Sun, the object is actually too heavy for a neutron star. So it could also be the lightest black hole ever detected. Either way, it is record breaking. ”

Factor 9
But that is not even the only special thing. Before the two objects merged, their masses differed by a factor of 9. And that is unprecedented. This therefore includes the most extreme mass ratio ever. “These exciting and unprecedented findings challenge all astrophysical models attempting to shed light on the origins of events of this kind,” said researcher Mario Spera. “We are pretty sure that the universe is telling us for the umpteenth time that our ideas about how compact objects arise, evolve and merge are still very vague.”

The curious cosmic fusion – dubbed GW190814 – eventually resulted in a black hole about 25 times the mass of our sun. Part of the merging mass was converted into an explosion of energy in the form of gravitational waves. This newly formed black hole is about 800 million light-years away from Earth.

Whether the researchers will ever find out whether the mysterious object is a neutron star or black hole? Probably. Future observations with LIGO and possibly other powerful telescopes could catch similar events. And that would help find out if there may be more objects in the perceived mass gap. “The mass gap has been an enigmatic puzzle for decades,” said researcher Pedro Marronetti. “And we have now discovered for the first time an object that falls exactly in it.” So whether the object is a heavy neutron star or a light black hole; Either way, the discovery is the first in a new class of binary mergers.