PUBLISHED August 16, 2019

This artist's depiction illustrates a black hole devouring a neutron star. As the neutron star circles the black hole, the black hole's immense gravity shreds it to pieces, a phenomenon called tidal disruption.

Illustration by Dana Berry, NASA

Some 900 million years ago, a black hole released a terrible belch that echoed through the cosmos. On August 14, the resulting ripples in the fabric of spacetime passed through Earth—giving us the best evidence yet of a never-before-seen type of cosmic collision that could offer new insights on how the universe works.

However, astronomers also expect such systems to create ripples known as gravitational waves if and when the black hole and neutron star merge. These spacetime ripples were predicted more than a century ago by Einstein’s general theory of relativity, which suggested that the collision of two extremely massive bodies would cause the very fabric of the universe to wrinkle.

Gravitational waves were detected for the first time in 2015, when the LIGO observatory picked up the signal of two black holes becoming one. Since then, LIGO and its European counterpart, the Virgo observatory, have detected additional black hole mergers, as well as the collision of two neutron stars. LIGO and Virgo both detected S190814bv, and if it is in fact a neutron star-black hole merger, it’d be the third distinct kind of collision picked up with gravitational waves.

Gravitational Waves 101

Though detectors also picked up signs of a neutron star-black hole merger on April 26, researchers say that S190814bv is far more compelling. The April event has a one-in-seven chance of being noise from Earth, and false alarms akin to the April signal are expected to pop up once every 20 months. But S190814bv almost certainly came from beyond our planet, and to see a false alarm resembling S190814bv, the LIGO team estimates that you’d have to wait longer than the age of the universe.

“This is something to get much more excited about,” says LIGO team member Christopher Berry, a physicist at Northwestern University. “It’s much more likely to turn up a real one, so that means it’s worth investing more time and effort.”

Cosmic shredder

LIGO and Virgo also tracked the origin of S190814bv down to an oval patch of sky about 11 times wider than the full moon—making it possible for telescopes to follow up for unusual flashes of light. Instruments all over the world and in orbit have paused their regularly scheduled observations to join the hunt, posting their early results in real time.

“It’s very exciting,” says Aaron Tohuvavohu, the observatory duty scientist for NASA’s Swift telescope, which has been searching for flashes of x-rays and ultraviolet light in the same patch of sky as the gravitational wave signal. “I didn’t sleep all night, and I’m very happy to do that.”

If Swift and other telescopes do see the afterglow from the collision that LIGO and Virgo felt, it would be a huge deal for astronomy, since the light would let scientists see the innards of a neutron star for the first time, and possibly test the limits of relativity in new ways.

“That would be fantastic [and] dream-like for a theorist,” says LIGO team member Vicky Kalogera, a physicist at Northwestern University.

However, it’s not a given that telescopes will see anything. Current theory predicts that collisions of neutron stars and black holes won’t always give off light, depending on how the two objects’ masses compare.

The closer the masses of the black hole and the neutron star, the longer it takes for the star to spiral into the black hole. This lets the pair orbit each other more closely, which gives the black hole more opportunity to gravitationally shred the neutron star. Before this glowing confetti falls into the black hole, it can give off light that telescopes can pick up.

But if the black hole is much more massive than the neutron star, it can swallow up the star whole with little muss or fuss, giving off no light. Kalogera says that scientists are still combing through the data on S190814bv to put limits on the black hole’s mass, which should clarify the situation for this event.

Sizing up the situation

Another, stranger possibility is that the smaller object in S190814bv isn’t a neutron star at all.

LIGO and Virgo classify the mergers they see by the estimated masses of the objects in each collision. Anything below three times the mass of our sun is considered a neutron star. Anything more than five times the mass of our sun is considered a black hole. In this case, the smaller object in S190814bv is estimated to be less than three solar masses.

close A composite image of the Messier 81 (M81) galaxy shows what astronomers call a "grand design" spiral galaxy, where each of its arms curls all the way down into its center. Located about 12 million light-years away in the Ursa Major constellation, M81 is among the brightest of the galaxies visible by telescope from Earth. Photograph courtesy NASA/JPL-Caltech/ESA/Harvard-Smithsonian CfA close Sheets of debris from an exploded star swirl in the Large Magellanic Cloud (LMC) galaxy in this Hubble Space Telescope image. At a distance of about 180,000 light-years, the LMC galaxy is a relatively close neighbor of the Milky Way. It can be spotted from the Earth's Southern Hemisphere without a telescope. Photograph courtesy NASA/JPL/Hubble Heritage Team (STScI/AURA) close This NASA/ESA Hubble Space Telescope image reveals the iridescent interior of one of the most active galaxies in our local neighbourhood — NGC 1569, a small galaxy located about eleven million light-years away in the constellation of Camelopardalis (The Giraffe). This galaxy is currently a hotbed of vigorous star formation. NGC 1569 is a starburst galaxy. Photograph by Nasa close An infrared image of the Messier 82 galaxy, nicknamed the "Cigar galaxy," shows the formation's central plane in blue and white, with a halo of smoky dust in red. This red cloud, composed of hydrocarbon dust similar to car exhaust, is being blown out into space by the galaxy's millions of young stars. Photograph courtesy NASA/JPL-Caltech/University of Arizona close A Hubble Space Telescope image shows unprecedented detail of the Antennae galaxies, an intense star-forming region created when two galaxies began to collide some 200 million to 300 million years ago. The bright, blue-white areas show newly formed stars surrounded by clouds of hydrogen, which are colored pink. A similar collision is expected between our galaxy, the Milky Way, and the nearby Andromeda galaxy in several billion years. NASA, ESA, Hubble Heritage Team (STScI/AURA)-ESA/Hubble Collaboration close The Black Eye or Evil Eye galaxy gets its nicknames from the band of light-absorbing dust that appears in front of the star system's bright center in this Hubble Space Telescope image. Messier 64, as the Black Eye galaxy is more formally known, is thought to have taken on its ominous appearance after it collided with another galaxy perhaps a billion years ago. Photograph courtesy NASA and The Hubble Heritage Team (AURA/STScI) close Billows of cosmic dust swirl amid NGC 1316, a giant elliptical galaxy formed billions of years ago when two spiral galaxies merged. Astronomers examined red star clusters within NGC 1316 to determine that the massive galaxy was indeed created by a major celestial collision. Photograph courtesy NASA, ESA, and the Hubble Heritage Team (STScI/AURA) close This image of the Whirlpool galaxy shows the classic features of a spiral galaxy: curving outer arms where newborn stars reside and a yellowish central core, home to older stars. A companion galaxy called NGC 5195, seen here at the tip of one of Whirlpool's arms (right), has been passing by for hundreds of millions of years and exerting gravitational forces on its larger neighbor. Photograph courtesy NASA, ESA, and the Hubble Heritage (STScI/AURA)-ESA/Hubble Collaboration close NASA’s Hubble Space Telescope has picked up the faint, ghostly glow of stars ejected from ancient galaxies that were gravitationally ripped apart several billion years ago. The mayhem happened 4 billion light-years away, inside an immense collection of nearly 500 galaxies nicknamed “Pandora’s Cluster,” also known as Abell 2744. Photograph by NASA Goddard close This 2008 illustration shows a revised look at our galaxy, the Milky Way. Scientists studying infrared images from NASA's Spitzer Space Telescope determined our galaxy's spiral has two major and two minor arms instead of four major arms, as was previously thought. The demoted arms can be seen as faint trails between the major arms, which emanate from the ends of the orange central bar. Illustration courtesy NASA/JPL-Caltech close A color-composite image shows the NGC 300 galaxy, a spiral galaxy like the Milky Way located about seven million light-years from Earth. In this image, young, hot stars are the blue dots that comprise much of the outer arms. Older stars are in the middle and appear yellow-green. Photograph courtesy NASA/JPL/Caltech/R. Hurt (SSC) close The Andromeda galaxy, also known as Messier 31, is the largest neighboring galaxy to the Milky Way. This photo, a mosaic of ten images captured by the Galaxy Evolution Explorer spacecraft in 2003, shows blue-white regions along the galaxy's arms where new stars are forming and a central orange-white area containing older, cooler stars. NASA/JPL/California Institute of Technology close Tap images for captions

Though less massive black holes theoretically might exist, x-ray measurements of the cosmos haven’t yet found any signs of them. Likewise, our best theories for neutron stars say that if they get much bigger than two solar masses, they will collapse into black holes. What if this gap between three and five solar masses simply reflects a gap in our observations, and the smaller object in S190814bv is a pint-size black hole?

“There’s really two mysteries that this event might tell us about,” Berry says. “What is the maximum mass of a neutron star, and what is the minimum mass of a black hole?”

Subtle features of the gravitational waves might let scientists figure out the identity of S190814bv’s smaller object. And if follow-up measurements do pick up an afterglow—which Kalogera says could take weeks—it would all but confirm that the smaller object is a neutron star.