Most supernovae, when they explode, either leave behind a black hole or produce a neutron star. This duality depends on the original mass of the star and is seen in the population of objects they produce. The heaviest neutron star is no more than 2.5 times the mass of the Sun. The lightest black hole observed is 5 times our star. In the middle of this range is the so-called "mass gap", and scientists have been puzzled by it for decades. Now, researchers from the gravitational wave observatories LIGO and Virgo have announced they have found an object with an in-between mass.

The object was estimated to be 2.6 solar masses and was part of a collision detected on August 14, 2019 (GW190814) with a black hole 23 times the mass of the Sun. As reported in The Astrophysical Journal Letters, the event is record-breaking for two reasons: It is the gravitational wave emission with the most extreme mass ratio (9:1), and the object itself is either the heaviest known neutron star or the lightest black hole.

"It's a challenge for current theoretical models to form merging pairs of compact objects with such a large mass ratio in which the low-mass partner resides in the mass gap. This discovery implies these events occur much more often than we predicted, making this a really intriguing low-mass object," co-author Vicky Kalogera, a professor at Northwestern University, said in a statement.

"The mystery object may be a neutron star merging with a black hole, an exciting possibility expected theoretically but not yet confirmed observationally. However, at 2.6 times the mass of our Sun, it exceeds modern predictions for the maximum mass of neutron stars, and may instead be the lightest black hole ever detected."

Once the detection from LIGO and Virgo occurred, an alert was sent out to the astronomical community. Dozens of telescopes on the ground and in space looked for the event but unfortunately, no transient event was spotted. So far, only one event has been confirmed with optical telescopes, the first neutron star collision GW170817, which created an object that sat on the limit of the mass gap. This new event was six times as far as GW170817 making it much more difficult to actually find. 

The true nature of this object will remain ambiguous but more discoveries of similar events will hopefully provide some retroactive knowledge about this one.

"This is the first glimpse of what could be a whole new population of compact binary objects," said Charlie Hoy, a member of the LIGO Scientific Collaboration and a graduate student at Cardiff University. "What is really exciting is that this is just the start. As the detectors get more and more sensitive, we will observe even more of these signals, and we will be able to pinpoint the populations of neutron stars and black holes in the universe."