LIGO and Virgo recap smashups, including hints of a black hole eating a neutron star


Scientists work in the LIGO Hanford control room. (Caltech / MIT / LIGO Lab Photo / C. Gray)

The science teams for the Laser Interferometer Gravitational-Wave Observatory, or LIGO, and Europe’s Virgo detector today laid out the details of their recent detections, including a crash between neutron stars, three black hole mergers and what may be the first observed collision of a neutron star and a black hole.

Astronomers and their fans have been talking about the detections for days, thanks to the fact that LIGO and Virgo are quickly sharing the raw results from their current observing run. But today’s statements provided the most authoritative views from researchers running the two gravitational-wave detectors.

The April 26 detection of a cosmic collision known as S190426c is the most intriguing event. The subtle signal of a far-off disturbance in the gravitational force was picked up by LIGO’s twin detectors at Hanford in Eastern Washington and at Livingston in Louisiana. The Virgo detector in Italy also detected the signal.

The signal is consistent with what might be expected if a black hole were to swallow a neutron star, roughly 1.2 billion light-years from Earth. Such an event has never been observed before.

“Unfortunately, the signal is rather weak,” Patrick Brady, a physicist at the University of Wisconsin at Milwaukee and spokesperson for the LIGO Scientific Collaboration, said in a news release. “It’s like listening to somebody whisper a word in a busy café; it can be difficult to make out the word or even to be sure that the person whispered at all. It will take some time to reach a conclusion about this candidate.”

One day earlier, the Virgo detector and the LIGO Livingston detector picked up the signal of a neutron star merger that occurred about 500 million-light years away. The LIGO Hanford detector was offline at the time, which reduced the detector networks ability to focus in on the origin of the signal, dubbed S190425z.

Only one such neutron star collision has been reported previously, and that set off a grand campaign to document the event with multiple astronomical instruments. Last month’s neutron star events sparked a similar effort, but researchers say neither of the gravitational-wave sources has been spotted by other means.

The current LIGO-Virgo observing run, which began on April 1, has also turned up three likely black hole mergers, which adds to 10 previous smashups of that sort.

LIGO’s two detectors pick up gravitational disturbances in the fabric of spacetime that are given off by faraway black hole crashes and other cosmic cataclysms. Such disturbances show up as tiny shifts in spatial dimensions, just barely affecting the paths of laser beams that shoot back and forth through 2.5-mile-long (4-kilometer-long) tunnels at the Hanford and Livingston detectors.

Scientists made their first-ever detection of gravitational waves from a black hole merger in 2015, which earned the Nobel Prize for physics in 2017. Since then, the Advanced Virgo detector has joined the hunt, and LIGO’s equipment has been upgraded as well. The fact that so many detections are now being made in such a short time span suggests that the nearly four-year-old field of gravitational-wave astronomy will exceed expectations in the months to come.

“The latest LIGO-Virgo observing run is proving to be the most exciting one so far. We’re already seeing hints of the first observation of a black hole swallowing a neutron star. If it holds up, this would be a trifecta for LIGO and Virgo — in three years, we’ll have observed every type of black hole and neutron star collision,” said Caltech’s David Reitze, executive director of LIGO. “But we’ve learned that claims of detections require a tremendous amount of painstaking work — checking and rechecking — so we’ll have to see where the data takes us.”

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