A Black Hole Feasted on a Neutron Star. 10 Days Later, It Happened Again.

Two years later, LIGO detected the collision of two neutron stars — the burnt-out remnants of stars 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 located in Italy, astronomers were able to pinpoint the part of the sky where the explosion occurred, and a series of telescopes were then able to detect particles of light, from radio waves to X-rays, emanating from that fireball.

Astronomers had long expected to find a neutron star orbiting a black hole, but in nearly half a century of searches of our Milky Way galaxy, they never found one. “So in effect, we’ve had this mystery question,” Dr. Brady said. “Why have we not seen a neutron star-black hole system?”

In 2019, two gravitational wave detections appeared to have finally bagged this elusive astronomical quarry. But one of them, in April 2019, did not hold up under scrutiny. It might have been what they were hoping it was — the rumblings of a black hole-neutron wave collision — or it might have just been random and meaningless jiggles in imperfect data.

“We think it’s unlikely that that was really an astrophysical signal,” Dr. Brady said. “So it sort of sits there as one of these things that might be, but right now we don’t have sufficient evidence to say it was.”

The second detection, on Aug. 14, 2019, remains puzzling. The larger object in the collision was definitely a black hole. The smaller one had a mass of 2.6 times that of the sun. That is larger than any neutron star that has ever been detected — and smaller than any black hole that has ever been detected. Astronomers remain unsure whether it was a neutron star or a black hole.

The new gravitational wave observations finally prove, without doubt, that these pairs exist, albeit far away from the Milky Way. The first detection of a neutron star merging with a black hole occurred on Jan. 5, 2020. The facility in Hanford, Wash., was temporarily offline, so the signal was detected in Livingston, La. A similar but smaller detector in Italy called VIRGO detected a faint signal, providing corroboration.

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