In August 2019, astronomers using gravitational wave detectors in the US and Italy detected ripples in space and time, a gravitational wave event they dubbed GW190814. Given the fact that this occurred so far from Earth, the event occurred 780 million years ago, but the gravitational waves are just now reaching us.
The merger occurred between an object that was 2.6 times the mass of our sun with a black hole that was 23 times the mass of our sun. This large difference in the sizes of both objects, differing by a factor of nine, makes it the most extreme mass ratio for a gravitational wave event known to date.
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.
The Laser Interferometer Gravitational-Wave Observatory, or LIGO, and Virgo were used to detect the event. The National Science Foundation's LIGO includes two detectors — one in Livingston, Louisiana, and another in Hanford, Washington. The Virgo detector is located in Cascina, Italy.
Black holes are created when massive stars die and collapse. Stars that are less massive explode in a supernova. The remnant of this outburst is a neutron star, which is small but very dense.
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.
An international team of astronomers were involved in the study, which published Tuesday in The Astrophysical Journal Letters.
"We've been waiting decades to solve this mystery," said Vicky Kalogera, study coauthor and Daniel I. Linzer distinguished university professor of physics and astronomy at Northwestern University, in a statement.
"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.
"Even though we can't classify the object with conviction, we have seen either the heaviest known neutron star or the lightest known black hole. Either way, it breaks a record," said Kalogera, who is also director of Northwestern's Center for Interdisciplinary Exploration and Research in Astrophysics (CIERA).
A different kind of detection
When LIGO and Virgo scientists detected this event, they sent out an immediate alert across the astronomy community to allow for follow-up by Earth and space-based telescopes. The hope is to find light waves also caused by the event.
This has only been seen once during a gravitational wave event, known as GW170817, which occurred in August 2017. It was the result of two neutron stars colliding and releasing light, gravitational waves and even creating elements like gold. Neutron star collisions are fiery, energetic and messy, releasing matter in all directions, and light is a by-product.
Mergers between two black holes, however, aren't believed to create light.
When telescopes followed up on the August 2019 event, they didn't pick up any signals of light waves. Scientists believed this is due to the distance of the event, which was six times farther away than the 2017 merger. If it was in fact a merger between two black holes, no light would be produced. And if it was a neutron star, the black hole was so much larger that it may have simply swallowed it.
"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."
The detection challenges current theoretical models of how stars die as well as how they pair up in binary systems. Binary systems, like two stars orbiting each other, occur when the two objects are close enough for gravity to create a central orbit.
"The mass gap has been an interesting puzzle for decades, and now we've detected an object that fits just inside it," said Pedro Marronetti, program director for gravitational physics at the National Science Foundation, in a statement.
"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."
An asymmetric binary system
The researchers said they didn't expect to find a binary system including two objects with such different masses, but now they know these are actually being created somewhere in the universe. Next, they have the challenge of trying to figure out what exactly they are and how they work, according to Alberto Vecchio, study coauthor and director of the Institute for Gravitational Wave Astronomy.
Though the opportunity to study this event in detail has passed, this discovery will change the way astronomers understand and study neutron stars and black holes going forward.
Future detections of similar events could help astronomers determine if there are more objects that exist in the mass gap.
"This is the first glimpse of what could be a whole new population of compact binary objects," said study coauthor Charlie Hoy, a member of the LIGO Scientific Collaboration and a graduate student at Cardiff University, in a statement.
"What is really exciting is that this is just the start," Hoy said. "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."
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