Call it the astrophysical equivalent of King Kong versus Godzilla: Scientists have spotted two instances in which a black hole has consumed a neutron star, the superdense sphere of nuclear matter left behind when a middle-weight star burns out and blows up. The violent mergers between the massive objects were detected through the ripples in space and time, or gravitational waves, that the crashes emitted. Previously, scientists had spotted black hole-black hole or neutron star-neutron star mergers and had anxiously awaited detecting such mixed pairs.
“Most people suspected that there were black holes merging with neutron stars, but this is the first time that we’ve confidently seen exactly that,” says Maya Fishbach, a gravitational wave astronomer at Northwestern University, who helped make the discovery. For both events, however, astronomers saw no visible light or other electromagnetic radiation, leaving them pining for a merger in which a black hole strews a neutron star’s luminous guts across the sky and helps reveal its secrets.
Five years ago, physicists first detected gravitational waves, which were emitted when two massive black holes spiraled together and melded. That discovery was made by the Laser Interferometer Gravitational-Wave Observatory (LIGO)—a pair of massive optical instruments in Louisiana and Washington state, which uses laser beams to measure the stretching of space with mind-boggling precision.
A year later, Europe’s Virgo gravitational wave detector in Italy joined the hunt and within days, the three detectors had spotted two neutron stars twirling together. The merging neutron stars set off a flash called a gamma ray burst and then an explosion known as a kilonova that spewed freshly formed elements into space. Spotted by telescopes across the electromagnetic spectrum, the fireworks enabled astrophysicists to place limits on the properties of neutron star matter, test their theory of gamma ray bursts, and forge a better understanding of the origins of heavy elements.
That neutron star merger whetted many researchers’ appetites for the third type of cosmic calamity that would obviously produce gravitational waves: the merger of a black hole and a neutron star. Some astrophysicists argued such an event would be even more revealing, as the featureless black hole—which is the ultraintense gravitational field left by a massive star that has collapsed to a point—rips apart the more complex neutron star. Such a collision, they hope, might illuminate the structure of neutron stars in unprecedented way.
Now, the LIGO and Virgo teams have spotted two of the long-sought events. The stronger of the signals triggered all three detectors on 15 January 2020 and, the data indicate, originated when a black hole with an estimated heft of six Suns devouring a neutron star weighing 1.5 solar masses. Ten days earlier, researchers spotted a black hole of nine solar masses merging with a neutron star of 1.9 solar masses. Both events occurred roughly 1 billion light-years away, the LIGO and Virgo teams report today in Astrophysical Journal Letters.
LIGO and Virgo researchers have also now ruled out a couple of events they had previously suggested might be black hole-neutron star mergers. For example, a tremor spotted in August 2019 now appears to involve the merger of a black hole with an object 2.6 times as massive as the Sun, too heavy to be a neutron star. It was possibly an oddball, very light black hole.
Unfortunately for astrophysicists, neither black hole-neutron star mergers produced an explosion visible to electromagnetic telescopes that have scanned their locations. That may be because they were so far away, or it might be because the black hole simply swallowed the neutron star whole, says Fishbach, who is part of the LIGO team. A black hole is likely to completely consume the neutron star if the black hole is either much more massive than the neutron star—which can’t weigh more than about 2.2 solar masses—or if the black hole is slowly spinning, says Brian Metzger, a theoretical astrophysicist at Columbia University and the Flatiron Institute.
Such a merger will rip apart the neutron star and create an explosion only if, before falling in, the neutron star can circle the black hole within a distance equal to its own radius—about 12 or 13 kilometers, Metzger explains. But how close the neutron star can circle depends on the mass and spin of the black hole, with the neutron star able to circle closer if the black hole is spinning faster. Theory predicts that a black hole of a dozen solar masses that’s spinning as fast as possible should still rip up the neutron star and create an explosion. A black hole that isn’t spinning will consume the neutron star whole if it weighs just five solar masses.
LIGO has spotted dozen of black hole pairs and its data so far suggest most black holes spin slowly and would likely swallow a partner neutron star whole, Metzger says. “I think that people just kind of assumed that they would be more rapidly spinning.” With that observation comes the realization that explosions from black hole-neutron star mergers may be rare, Metzger says, especially as LIGO and Virgo are more sensitive to heavier black holes.
Still, the data from the two new events suggest that, per cubic light-year of space, black hole-neutron star mergers outnumber black hole-black hole mergers, Fishbach says. (Although less numerous, the louder black hole-black hole signals can be detected much farther away.) So there’s still the possibility that LIGO and Virgo will spot a black hole-neutron star merger with the light show everybody so eagerly awaits, Fishbach says: “There still are some really exciting firsts in our future.”