GW190814: The merger of a 23-solar-mass black-hole and an enigmatic object
Another unprecedented discovery has just been unveiled by LIGO–Virgo scientists. Data from the third observation period (O3) of the Advanced LIGO and Advanced Virgo detectors reveal that, at 21:10 (UTC) on the 14th of August, 2019, the three instruments in the network detected a gravitational-wave signal, called GW190814. The signal originated from the merger of an enigmatic couple: a binary system composed of a black hole, 23 times heavier than our sun, and a much lighter object, about 2.6 times the mass of the Sun. The merger resulted in a final black hole about 25 times the mass of the sun.
It is this lighter object that makes GW190814 so special. It may just be either the lightest black hole or the heaviest neutron star ever discovered in a binary system. Another peculiar feature of GW190814 is the mass ratio of the objects in the binary system. The factor 9 ratio is even more extreme than was the case with the first detected merger of a binary with unequal masses, GW190412.
"Once again, gravitational-wave observations are shedding light on the unknown. The lightest object in this system has a mass that has never before been observed", says Giovanni Losurdo, of Istituto Nazionale di Fisica Nucleare (Italy) and the spokesperson of the Virgo Collaboration. "A new discovery, which raises new questions. What is its nature? How did such a binary system form? Virgo, LIGO and, soon, Kagra in Japan, will continue to search for the answers and push forward the frontier of what we know about the cosmos in which we live."
The mass asymmetry causes the presence of higher multipoles in the gravitational radiation, a fact that allows stringent tests of General Relativity. Once again, all our tests confirm the prediction of Einstein’s theory. Moreover, the higher multiples allow us to disentangle the determination of the source distance from the inclination angle of the plane of the orbiting binary. We have found the source of the gravitational wave to be about 800 million light years away!
The signal was clearly detected by the three instruments, with an overall signal-to-noise ratio as high as 25. Thanks mainly to the delay between the signal arrival times at the three, well separated detectors, the network was able to localise the origin of GW190814 to within about 19deg2 in the sky. This is similar to the localisation achieved for the famous GW170817, which gave birth to multi-messenger astronomy with gravitational waves. In the case of GW190814, however, an electromagnetic counterpart has yet to be observed.
"We are very satisfied with the performance of Advanced Virgo during O3," says Maddalena Mantovani, scientist at the European Gravitational Observatory (EGO). "We reached the target sensitivity with a very good duty cycle. This is the result of the hard work of the scientists and technicians that have fine-tuned the machine to provide its best performance. Scientific discoveries such as GW190814 are the best rewards for all those days and nights spent on improving the detector."
Animation: Artistic rendering of the GW190814 event, in which a smaller compact object is swallowed by a 9-times-more-massive black-hole. The matter stream between the two objects and the look of the massive black hole are an artistic invention. To the best of our knowledge, the GW190814 fusion is not thought to have emitted any light.
Animation credit: Alex Andrix
📖 Research article: Astrophysical Journal Letters