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."