Detecting gravitational waves from collisions between two black holes or two neutron stars is becoming almost mundane, but now astronomers have detected the final piece of the trifecta – a black hole swallowing a neutron star. Two separate events rolled in just days apart, with the black holes gobbling up the stars like Pac-Man rather than Cookie Monster.
Gravitational waves are ripples in the very fabric of spacetime itself, caused by some of the most energetic cataclysms in the universe, such as collisions between massive objects like black holes and neutron stars. More than 50 of these events have been detected since the Nobel Prize-winning first detection in 2015.
And now, scientists have confirmed two new milestone detections. The first took place on January 5, 2020, when a black hole with a mass around nine times that of the Sun swallowed up a neutron star with 1.9 solar masses. It appeared to take place about 900 million light-years away, although its location in the sky is hard to pin down precisely because one of the three detectors in the LIGO-Virgo Collaboration was offline at the time.
The second event was detected just 10 days later on January 15, involving a 6-solar-mass black hole and a 1.5-solar-mass neutron star located about 1 billion light-years from Earth. This time all three detectors were up and running, so the location could be traced more precisely – although that’s still a swathe of sky about 3,000 times larger than the full moon.
In the past, collisions between pairs of neutron stars have been known to produce stunning signals in other wavelengths, such as visible light, radio, X-rays and gamma rays. So after these detections, astronomers rushed to scan the skies for any flashes of light or other signals. None were spotted, but the team says that’s not surprising, since the great distances involved mean any light would be extremely faint.
Another factor, according to the astronomers, is that the black holes may have been neat eaters. They seem to have gobbled up the neutron stars whole “like Pac-Man,” as astronomer Susan Scott put it.
"These were not events where the black holes munched on the stars like the Cookie Monster and flung bits and pieces about,” says Patrick Brady, an author of the study. “That 'flinging about' is what would produce light, and we don't think that happened in these cases.”
This is the first confirmed detection of a collision between a black hole and a neutron star, but there have been other candidates. The first occurred on April 26, 2019, but scientists later concluded that the signal was most likely just detector noise. A second candidate was detected in August 2019, but it’s still uncertain exactly what kind of objects were involved.
The next observing run is set to begin in mid-2022, after the detectors undergo upgrades. No doubt we’ll have some incredible new discoveries to look forward to after that.
The research was published in the Astrophysical Journal Letters. An animation of the eerily neat merger can be seen in the video below.
Sources: Caltech, Northwestern University, Australian National University
