A Black Gap Feasted on a Neutron Star. 10 Days Later, It Occurred Once more.

In January last year, astronomers definitely first observed a black hole swallowing a dead star, like a raven devouring a roadkill.

Then, ten days later, they saw the same act of looting repeated in another, far-off sector of the cosmos.

These triumphs, reported in an article published Tuesday in the Astrophysical Journal Letters, are the latest in the fledgling field of gravitational astronomy, which captures the literal stretching and narrowing of space-time caused by some of the most catastrophic events in the universe.

“It’s the first time we’ve actually seen a neutron star and a black hole colliding anywhere in the universe,” said Patrick Brady, professor of physics at the University of Wisconsin-Milwaukee, spokesman for the LIGO Scientific Collaboration.

Astronomers had suspected that pairings of black holes and neutron stars exist. But until they saw these collisions, these assumptions were not confirmed. The discovery helps expand knowledge of the binary star systems that populate the universe, while also raising the question of why astronomers have never seen such a pair in our Milky Way galaxy.

For more than 20 years LIGO – Laser Interferometer Gravitational-Wave Observatory – has been looking for this rumble, a prediction of Einstein’s general theory of relativity. For years the laser beams in the observatory, one in Hanford, Washington, the other in Livingston, LA, did not detect anything.

In September 2015, both LIGO locations then observed the long-awaited ringing of gravitational waves.

These waves were created by a collision of two stellar-sized black holes – punctures in the space-time structure that occur when the most massive stars explode as supernovae at the end of their lives. The two black holes orbited each other and swiveled closer and closer until they finally merged into one.

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, and a number of telescopes have detected particles of light, from radio waves to X-rays, emanating from this explosion.

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?”

The new gravitational wave observations prove 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.

By studying the changes in the frequency of gravitational waves, astrophysicists were able to determine the properties of the objects colliding in the far reaches of the universe.

The black hole was 7.4 to 10.1 times the mass of the Sun; the neutron star was smaller, but still about twice the mass of the star orbiting our world. The collision occurred about 900 million light years from Earth.

On January 15, 2020, the Hanford site was operational again, and all three instruments discovered the second collision of a black hole and a neutron star. This was a little further away. Both objects were a bit lighter. The neutron star had about 1.5 times the solar mass and the black hole between 3.6 and 7.5 times the solar mass.

In contrast to the collision of two neutron stars in 2017, telescopes could not detect any light particles from the explosions. The black holes appear to have been large enough to swallow the neutron stars quickly, reducing the likelihood of detectable emissions.

Alessandra Buonanno, director at the Max Planck Institute for Gravitational Physics in Potsdam and member of the LIGO science team, said the collisions were generally what they expected. “Nothing you would say in a strikingly unexpected way,” she said.

Astrophysicists have been unable to see any signs that the black holes are tearing the neutron stars apart before swallowing them. The tidal forces of a black hole on a neutron star would indicate the diameter of the neutron star, which in turn would indicate what it is made of.

But as more such collisions are observed, patterns will emerge and the likelihood of seeing more detail will increase.

“If you’ve found a system where the black hole was a bit smaller, the tidal effects on the neutron star will be bigger before it merges with the black hole,” said Dr. Brady. “And so it can shred the neutron star on its final orbits.”

Dr. Brady said one of the remaining questions is why no black hole neutron pairs were found in the Milky Way. It is possible that the search techniques were not entirely correct, or perhaps the pairs are quickly merging and there are none in our galaxy. “This is really an open question now,” he said.

VIRGO is undergoing upgrades that will increase its sensitivity. The next observation round by LIGO and VIRGO should start in June next year at the earliest. A third gravitational wave detector goes online in Japan, and another LIGO instrument is being planned in India.

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