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Gravity

Gravity

Black holes merging with neutron stars have been spotted by LIGO–Virgo for the first time

29 Jun 2021 Hamish Johnston
Tidal disruption of a neutron star
Final moments: artist’s impression of a neutron star as it spirals into a black hole. (Courtesy: NSBH)

Gravitational waves from two separate mergers of a black hole with a neutron star have been seen by the LIGO observatories in the US and the Virgo observatory in Italy. Although hints of similar mergers have been spotted by the detectors before, these are the first confirmed events of this kind. One signal was detected on 5 January 2020 and the other was observed less than two weeks later, on 15 January.

LIGO–Virgo has already detected the mergers of pairs of black holes and pairs of neutron stars, so these observations complete the set of possible mergers of these objects. “We finally have the final piece of the puzzle: black holes swallowing neutron stars whole,” says LIGO–Virgo team member Vivien Raymond, from Cardiff University’s Gravity Exploration Institute. “This observation really completes our picture of the densest objects in the universe and their diet.”

Gravitational waves are ripples in space–time that are generated when pairs of massive objects such as black holes and neutron stars orbit each other in a rapid inspiral before merging. The LIGO and Virgo observatories are kilometre-scale interferometers that can measure the minuscule expansion and contraction of space–time that occurs when a gravitational wave passes through Earth.

The first of the two events has been named GW200105, and scientists believe that it involved the merger of a 9 solar mass black hole with a 1.9 solar mass neutron star. Despite being seen in only two of the three LIGO–Virgo detectors (LIGO Livingston and Virgo, LIGO Hanford was offline at the time), the signal was strong enough to meet the threshold of a detection. Scientists calculate that the merger occurred about 900 million light-years away.

Difficult to pinpoint

The GW200105 signal was much weaker in Virgo than in LIGO Livingston, which meant that scientists were not able to pinpoint its origin – it could have been anywhere in a region of sky about 34,000 times the size of a full Moon.

The second event has been dubbed GW200115 and it occurred about 1 billion light-years away and involved a 6 solar-mass black hole and a 1.5 solar mass neutron star. Because it was spotted by all three detectors, researchers were able to narrow down its location in the sky to an area of about 3000 times the size of a full Moon.

When gravitational waves are detected by LIGO–Virgo, a notice goes out to other astronomers who then train their telescopes on that region of the sky and look for electromagnetic radiation from the merger. However, unlike the merger of two neutron stars that was observed by LIGO–Virgo in 2017, no electromagnetic radiation from GW200115 and GW200105 was spotted by other telescopes.

No light show

According to the LIGO–Virgo team, this lack of other observations is expected for several reasons. For one thing, the black hole is expected to swallow the neutron star whole, with little matter being flung out to generate an electromagnetic signal. This is unlike neutron-star mergers, in which the resulting object explodes spectacularly. Furthermore, the great distances to the mergers means that any light produced by the events would be very dim indeed.

“These were not events where the black holes munched on the neutron stars like the cookie monster and flung bits and pieces about. That ‘flinging about’ is what would produce light, and we don’t think that happened in these cases,” says LIGO spokesperson Patrick Brady at the University of Wisconsin-Milwaukee.

The LIGO detectors spotted their first gravitational waves in 2015, from the merger of two black holes. In 2017 the Virgo detector spotted its first signal and since then all three observatories have been upgraded. A fourth observatory – KAGRA in Japan – joined the search for gravitational waves in February 2020.

The observations are described in Astrophysical Journal Letters.

How gravitational waves are detected

In the video below, LIGO–Virgo team member Nergis Mavalvala of the Massachusetts Institute of Technology explains how gravitational waves are detected.

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