For the first time in history, scientists have detected ripples in spacetime created by a mind-boggling cosmic event: a black hole gobbling up a neutron star.
For years, scientists have been trying to snag a clear detection of a neutron star—a type of hyper-dense stellar corpse—and a black hole that are gravitationally locked in orbit around each other the same system, called an NSBH binary, which has never been seen before. In January 2020, the universe handed them not one, but two, clear observations of black holes swallowing their neutron star companions whole, within just 10 days of each other, providing the first glimpse of these exotic interactions.
More than 1,000 researchers located at research centers spanning the globe participated in the unprecedented milestone, which was announced on Tuesday in The Astrophysical Journal Letters. The first event, named GW200105, was captured on January 5, followed by the second, named GW200115, on January 15. Both collisions took place about one billion light years from Earth.
“We were absolutely thrilled to finally detect a black hole and a neutron star colliding,” said Susan Scott, a distinguished professor at The Australian National University who co-authored the study, in an email.
“These are the two most enigmatic objects in the universe and this is the first time that anyone has seen them smashing together,” she continued. “We had waited more than four years since our initial detection of gravitational waves from two black holes colliding. We had also detected two neutron stars merging so it was wonderful to complete the trifecta.”
Scientists have been searching for signs of NSBM binaries with conventional light-based telescopes for decades, but the hunt for these elusive systems was super-charged by the first successful detection of gravitational waves in 2015, an event that has revolutionized astronomy and earned a Nobel Prize.
Gravitational waves are ripples in space formed during tumultuous cosmic events, such as the explosions of stars or collisions between black holes. The oscillations of these waves are smaller than the width of a proton, which is why they remained observationally out of reach in the century since Albert Einstein first predicted their existence. However, sophisticated new detectors, such as the Laser Interferometer Gravitational-Wave Observatory (LIGO), can now capture these waves, opening an entirely different window into the universe.
One of the exciting benefits of gravitational wave astronomy is that information about the masses of wave-making objects are encoded in the ripples that reach Earth. For this reason, scientists using LIGO and Virgo data were able to determine that GW200105 was created by a black hole of nine solar masses swallowing a neutron star that was 1.9 solar masses. GW20011 was created by a black hole of six solar masses gobbling down a neutron star that was 1.5 solar masses.
LIGO, along with its sister observatory Virgo, have detected dozens of gravitational waves over the past five years, most of which were created by mergers between black holes. Astronomers have been holding out hope that the growing number of wave detectors might one day capture a NSBM merger, given how little is known about these systems.
Both black holes and neutron stars are created by the pyrotechnic deaths of massive stars, though these stellar corpses take very different forms. When stars collapse fully in on themselves, creating a cosmic sinkhole from which even light cannot escape, a black hole is born.
Neutron stars, in contrast, are extremely dense spheres that contain the remnant mass of the dead star within a diameter of around 12 miles, and they do emit light, often in radiant pulses. Whether a dying star becomes a black hole or a neutron star depends on its mass, as neutron stars appear to have an upper limit of about 2.14 times the mass of the Sun (or 2.14 solar masses).
While some past gravitational waves look like they may have been created by NSBH mergers, there have been no unambiguous detections—until now.
“The gravitational waves produced by two black holes colliding are much stronger because black holes are much more massive and more dense,” Scott said. “That means that we can detect this type of system much further out into space than we can for systems of two neutron stars or a neutron star and a black hole, and that is why we detect more of them.”
“In our latest observing run the detectors were more sensitive which means that we could detect gravitational waves from particular types of events further out into space,” she added. “So we were definitely hoping to detect this type of system during the observing run, but to make our first two observations of this type of system within 10 days was very surprising indeed.”
Scientists previously detected gravitational waves kicked off by a collision between two neutron stars, and were then able to track down the flash of light this crash produced, which is known as an optical counterpart to the wave event. While researchers looked for a similar counterpart to the new NSBM mergers, they did not find anything, which is not particularly surprising given the enormous distances to the mergers. These collisions likely did not even produce a flash of light, as the black holes may have simply subsumed the neutron stars without any optical signal at all.
The new study describes the detections and uses them to estimate that NSBH mergers occur within one billion light years of Earth about once per month. But just as the announcement marks the end of a decades-long search, it is also a new beginning that raises many questions about the dynamics, abundance, and evolution of these hitherto hidden systems.
Given their estimated frequency and our increasingly advanced wave detectors, many more of these collisions are likely to be found in the coming years, helping to solve key questions about the dramatic mergers of some of the most extreme objects in the universe.
“We want to understand the lives of black holes and neutron stars throughout the universe,” Scott said. “How do a black hole and a neutron star get together in the first place? Perhaps they are thrown together in the very dense systems at the centre of a galaxy. Or they might form in isolation starting with two giant stars and undergo their entire life cycles together exploding as supernovae and eventually collapsing to a black hole and a neutron star.”
Update: This article has been updated to include comments from Susan Scott, a distinguished professor at The Australian National University and co-author of the new study.