Caption: This image is a still from a video visualization of the coalescence of two black holes that inspiral and merge, emitting gravitational waves. A mysterious astronomical object merged with a black hole 780 million light-years away and created gravitational waves that could be detected on Earth. The object exists inside what scientists call the "mass gap," a range between the heaviest known neutron star and the lightest known black hole.
The merger led to a black hole about 25 times the mass of the sun. Some of the mass was blasted out as gravitational waves. Currently, the heaviest known neutron star is 2.5 times the mass of our sun and the lightest black hole is five times the mass of our sun. In between is the "mass gap" into which this object fits.
"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. We are really pushing our knowledge of low-mass compact objects," Kalogera said. "I think of Pac-Man eating a little dot," Kalogera said. "When the masses are highly asymmetric, the smaller compact object can be eaten by the black hole in one bite."
"That cannot be explained without defying our understanding of extremely dense matter or what we know about the evolution of stars," Marronetti said. "This observation is yet another example of the transformative potential of the field of gravitational-wave astronomy, which brings novel insights to light with every new detection.