La. scientists the first to see black hole swallow dead star

JAMES FINN, The Advocate
July 10, 2021


LIVINGSTON, La. (AP) — More than a billion years ago and hundreds of millions of light-years away, a ravenous mass of gravitational power known to scientists as a black hole swallowed a smaller, dead star whole, like an alligator eating its fill of nutria. Then it happened again.

The ripples created by the two collisions finally reached Earth in January 2020, marking a revelation in the burgeoning field of gravitational wave astrophysics. Scientists now regularly study the gravitational waves emitted by black holes meeting other black holes, but they had long-anticipated examining the more subtle ripples they hoped would emanate from a black hole consuming a smaller neutron star.

Last January, a group of researchers working on an isolated patch of wooded land in the middle of Livingston Parish were among the first humans to detect them.

“We’ve gotten quite used to the discovery process at this stage,” said Dr. Brian O’Reilly, a senior scientist at the LIGO center in Livingston, “but this was a good one. It’s kind of a landmark achievement to detect this new type of system.”

LIGO Livingston — the acronym stands for Laser Interferometer Gravitational-wave Observatory — is one of two such sites in the United States dedicated to mind-bending study of ruptures in the fabric of space-time.

Once impossible to detect, scientists at LIGO Livingston and its sister site in Hanford, WA, along with a third observatory in Italy, now regularly observe gravitational waves that emerge from those ruptures. To date, the waves have almost always emitted from ancient collisions of one black hole with another.

LIGO scientists describe the observatories behind the discoveries, like the one in Livingston Parish, as “remarkable physics experiments.” Each center consists of two four-kilometer-long, vacuum-sealed tubes in which are suspended some of the world’s finest mirrors — the only way to offer a reading on the infinitesimally small gravitational waves.

Studying the waves, they say, offers new windows into the very fabric of how light, time and space coalesce to form the universe. Each new discovery bolsters Albert Einstein’s famous theory of gravity.

For many years, scientists thought the gravitational waves offering evidence of those theories might be a myth, or were simply undetectable. The invention of LIGO, O’Reilly said, was like “watching a silent movie, and then somebody turned on the sound.”

Run by scientists from both LSU and the California Institute of Technology, the Livingston observatory has regularly played a role in groundbreaking gravitational wave discoveries — around 50 of them over the past several years, according to O’Reilly. Between 2015 and 2017, the observatory logged waves from a series of black hole collisions that helped form a base of knowledge in the young field.



Those collisions generally involved black holes of similar mass and size — until an event the Livingston scientists saw on April 12, 2019, when one black hole with a mass 30 times greater than the sun swallowed up another one about one-fourth its own size.

Working alongside colleagues at the Virgo observatory in Pisa, Italy, the Livingston scientists reported in a new paper this week of observing two even- rarer-still instances of black holes colliding with neutron stars.

The first merger, detected on Jan. 5, 2020, involved a 9-solar-mass black hole and a 1.9-solar-mass neutron star, according to an LSU spokesperson. The second merger was detected 10 days later and involved a 6-solar-mass black hole and a 1.5-solar-mass neutron star.

The gravitational-wave signals that emerged from the two collisions offered evidence of “a new kind of astrophysical system not previously observed,” said Guillermo Valdes, who until December of 2020 was a postdoctoral researcher at LSU and contributed to the paper announcing the discovery.

Rather than being a cataclysmic event, the collisions were likely fairly subdued.

“We looked for evidence that before the black hole and the neutron star merged that the neutron star was basically ripped apart by the black hole,” O’Reilly said. “But in this case, the neutron star basically disappeared into the black hole without a lot of being shredded apart, because the size differences were big enough between the two.”

Now, O’Reilly and the other LIGO scientists are upgrading their detectors as they keep their eyes peeled for the arrival of a supernova in the earth’s galaxy. Such an event would offer the next big data point for gravitational astrophysicists to work with, O’Reilly said.

Supernovas appear about as often as a 100-year flood.

“It goes to show that even after discovering dozens of events, there’s still a lot of new science to be explored,” O’Reilly said.