Black holes, neutron stars, and gravitational waves are deeply weird phenomena. Now, scientists are hyped about a new discovery that may involve all three.
Gravitational waves are ripples in the curvature of spacetime created by disturbances such as black hole collisions or the explosions of dying stars. Since scientists first detected a gravitational wave in 2016—an achievement that earned the Nobel Prize in Physics—several other waves have been recorded, all of which were caused by mergers between black holes or collisions between neutron stars.
What has not yet been observed, however, is a collision between a black hole and a neutron star—until now… probably.
On Wednesday, a gravitational wave called S190814bv was detected by the US-based Laser Interferometer Gravitational-Wave Observatory (LIGO) and its Italian counterpart Virgo. Based on its known properties, scientists think there is a 99% probability that the source of the wave is a black hole that ate a neutron star.
“We’ve never detected a neutron star and a black hole together,” said Ryan Foley, an astronomer at UC Santa Cruz, in a phone call. “If it turns out to be right, then we’ve confirmed a new type of star system. It’s that fundamental.”
Both black holes and neutron stars are made of stars that exploded and collapsed into stellar corpses. A black hole swallows anything that passes its outer edge, called the event horizon, including light. As a result, black hole mergers are largely invisible to light-based observatories, though several have been detected since the advent of gravitational wave astronomy.
A neutron star can be thought of as “an atom that’s a couple times the mass of the Sun,” Foley explained. All of that mass is crammed into a sphere with a diameter of about 12 miles, which makes neutron stars super-dense and extremely hot.
In contrast to black hole mergers, neutron star collisions do produce a lot of light. When a gravitational wave from a neutron star crash was detected in 2017, scientists were able to pinpoint bright emissions from the event—called an optical counterpart—in the days that followed the wave detection. This marked the dawn of a technique called “multi-messenger astronomy,” in which scientists use multiple types of signals from space to examine astronomical objects.
Foley was part of the team that tracked down that first optical counterpart, a feat that has not yet been repeated. He and his colleagues are currently scanning the skies with telescopes, searching for any light that might have been radiated by the new suspected merger of a black hole and neutron star.
“We’ve been waiting for two years, so people are really ready,” he said. “The neutron star/black hole system is particularly exciting because we haven’t seen any case of that before, and there’s a lot of interesting stuff we could learn if we did detect it.”
If the team were to pick up light from the event within the coming weeks, they would be witnessing the fallout of a black hole spilling a neutron star’s guts while devouring it. This would provide a rare glimpse of the exotic properties of these extreme astronomical objects and could shed light on everything from subatomic physics to the expansion rate of the universe.
“If you learn about how neutron stars are built, that can tell you about how atoms are built,” Foley said. “This is something that is fundamental to everything in our daily life works.”