Two years later, LIGO discovered the collision of two neutron stars – the burned-out remains of stars that are more massive than the Sun but not large enough to collapse into black holes. Such collisions create most of the gold and silver in the universe.
With the help of VIRGO, a similar but smaller European gravitational wave observatory in Italy, astronomers were able to locate the part of the sky where the explosion occurred, and a number of telescopes were then able to detect particles of light from radio waves X-rays emanating from this fireball.
Astronomers had long expected to find a neutron star orbiting a black hole, but in nearly half a century of searching our Milky Way, they never found one. “So we actually had this mysterious question,” said Dr. Brady. “Why didn’t we see a neutron star black hole system?”
In 2019, two gravitational wave detections seemed to have finally sacked this elusive astronomical quarry. But one of them did not stand up to scrutiny in April 2019. It could have been what they were hoping for – the rumble of a black hole colliding with a neutron wave – or it could have just been random and meaningless jiggles in imperfect data.
“We think it is unlikely that this was really an astrophysical signal,” said Dr. Brady. “So it’s kind of one of those things that it could be, but at the moment we don’t have enough evidence for it.”
The second discovery on August 14, 2019 remains a mystery. The larger object in the collision was definitely a black hole. The smaller one was 2.6 times the mass of the Sun. That’s bigger than any neutron star ever discovered – and smaller than any black hole ever discovered. Astronomers aren’t sure if it was a neutron star or a black hole.
The new gravitational wave observations finally prove beyond any doubt that these pairs exist, albeit far from the Milky Way. The first detection of a neutron star fused to a black hole was made on January 5, 2020. The Hanford, Washington facility was temporarily offline, so the signal was detected in Livingston, La. A similar but smaller detector in Italy called VIRGO detected a weak signal that provided confirmation.