That makes the lighter object more massive than the heaviest type of dead star, or neutron star, previously observed - of just over two solar masses.
They said the object formed when another unknown body merged with a large black hole around 800 million years ago, in doing so emitting a huge gravitational wave detectable on Earth.
Astronomers have never detected black holes lighter than five times the mass of the sun.
The data come from physicists working with the Laser Interferometer Gravitational-Wave Observatory (LIGO), a pair of detectors in Louisiana and Washington state, and Virgo, a similar detector in Italy. By sensing only the gravitational waves from the collision, LIGO and Virgo can not tell for sure what the object is, she says. The European Gravitational Observatory (EGO) hosts the Virgo detector near Pisa in Italy, and is funded by Centre National de la Recherche Scientifique (CNRS) in France, the Istituto Nazionale di Fisica Nucleare (INFN) in Italy, and Nikhef in the Netherlands.
If, however, this new class of object is a heavy neutron star then theories for how they form may also need to be revised, according to Prof Bernard Schutz of the Max Planck Institute for Gravitational Physics in Potsdam, Germany.
Portsmouth PhD student Connor McIsaac ran one of the analyses that computed the significance of this event. In August of 2019, they observed gravitational waves from the merger of a 23 solar mass black hole with an object 2.6 times the mass of the Sun.
Still image from a numerical simulation of an unequal mass binary black hole merger. "On the other hand, if the collision involved two black holes, it's not likely that it would have shone with any light".
Since its gravitational wave is proof of its formation, the researchers said the object's discovery questions the existence of the mass gap. Earlier this year, scientists announced a black hole merger with a mass ratio of roughly 3:1, but GW190814 is way more extreme. "This discovery implies these events occur much more often than we predicted, making this a really intriguing low-mass object", explained astrophysicist Vicky Kalogera of Northwestern University in IL.
Artist's rendition of a mystery object in'Mass Gap found by LIGO and Virgo
This object exceeds that, making it too large to fit neatly with other neutron stars that astronomers know about.
The mysterious object is said to be 2.6 times the mass of our sun, while the black hole was that was 23 times the mass of our sun.
When the LIGO and Virgo scientists spotted this merger, they immediately sent out an alert to the astronomical community.
Dozens of ground- and space-based telescopes followed up in search of potential optical or infrared radiation generated in the event, but none picked up any such radiation.
Or it might have been too far away for observatories looking for light to see. That event, discovered by the LIGO-Virgo network in August of 2017, involved a fiery collision between two neutron stars that was subsequently witnessed by dozens of telescopes on Earth and in space. Neutron star collisions are messy affairs with matter flung outward in all directions and are thus expected to shine with light.
In theory, black holes could be lighter than five suns. But Prof Fairhurst's colleague, Prof Fabio Antonioni, has proposed that a solar system with three stars could lead to the formation of light black holes. In the latter case, this would mean it is the lightest black hole ever detected.
"I think of Pac-Man eating a little dot", says Kalogera.
"Mergers of a mixed nature - black holes and neutron stars - have been predicted for decades, but this compact object in the mass gap is a complete surprise".