First Likely Detection Of A Black Hole Swallowing A Neutron Star

An artist's impression of a black hole swallowing a neutron star. We think we've discovered the first such example of this, taking place 900 million light-years away. Carl Knox OzGrav ARC Centre of Excellence

Astronomers think they have achieved the trifecta of events gravitational wave detectors were built to catch. Having previously found the ripples produced by the merging of black holes, and the collision of neutron stars, we’ve now found a likely encounter between one of each.

“About 900 million years ago, this black hole ate a very dense star, known as a neutron star, like Pac-man – possibly snuffing out the star instantly,” Professor Susan Scott of the Australian National University said in a statement. The gravitational ripples produced by this event have now reached us, traveling at the speed of light. If we are very lucky, we might get visual confirmation as well. 

When stars at least eight times more massive than the Sun die, they usually become either black holes or neutron stars, depending on their mass, although we recently discovered an apparent exception. If two stars orbiting each other both meet the same fate, their orbits slowly decay until the objects combine, producing a gravitational wave as their circling becomes more frantic.

The Laser Interferometer Gravitational-Wave Observatory (LIGO), which was established in the hope of catching such mergers in action, has proved an enormous success. Where the first black hole merger was anticipated for years and kept under wraps for months, we’re now picking up these signals every month or so. Neutron star collisions can only be detected at shorter distances, but the first discovery transformed our knowledge of how the universe got its heavier elements.

Binary star pairs often involve one much larger than the other, so we would expect to sometimes see a neutron star orbiting, and eventually being consumed by, a black hole. On August 14 LIGO and its Italian counterpart detected a signal that matches what astronomers would expect such an event to look like, following two unconfirmed and less precisely constrained black hole-neutron star events earlier this year.

Gravitational waves’ shape depends on the mass of the object making them, and so far every black hole detected has been at least 5 solar masses, while every neutron star has been less than half that. Scott told IFLScience the masses of the two objects are still being determined more precisely, and until this is done the possible range of each remains a secret. She added there is a possibility the smaller object is a black hole less massive than any we have seen before, but neutron star status is more likely, while the bigger one is definitely a black hole.

The most exciting aspect of such an event would be if astronomers can detect a visible or radio afterglow. It was detection at many electromagnetic wavelengths that turned the neutron star collision into the biggest thing in astronomy for decades, with an estimated 35 percent of working astronomers co-authoring the first batch of papers.

So far, Scott said, we have no glow, but telescopes are being trained on the galaxy from which the gravitational waves came in the hope they will pick something up. Scott added the chances of finding something depend on the mass ratio between the two objects.

A black hole much heavier than a neutron star is expected to consume its dinner too swiftly to leave much to see. In a more equal encounter, Scott explained, “the neutron star will be shredded and we should be able to detect the signal.”

If so, the light we collect could tell us a great deal about the nature of neutron stars. Such a finding would also rule out the small black hole theory.

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