On Jan. 5, 2020, astrophysicists heard a chirp from a distant part of the cosmos, some 900 million light-years away. The fleeting sound was unlike any they'd heard before and was caused by a great ripple in space-time -- a gravitational wave -- that spread out across the universe from over 900 million light-years away, washing over the Earth and pinging detectors. Chirp.
Then, 10 days later, they heard another, similar sound. A cosmic twin. Gravitational waves had once again pinged Earth's detectors. Chirp.
After careful analysis, the two signals have been identified as emanating from extreme, never-before-seen events in deep space: the collision between a black hole and a neutron star.
The pair of collisions (or, less poetically, "mergers") are detailed in a new study published in the Astrophysical Journal Letters on Tuesday, featuring over 1,000 scientists from the LIGO/Virgo and KAGRA collaborations, a multinational effort to . The two newly described events are named GW200105 and GW200115, for the dates they were discovered, and provide the first definitive evidence of an elusive merger.
Prior to the dual detection, astronomers had only found black holes merging with black holes and neutron stars merging with neutron stars.
"We've been waiting and expecting, at some stage, to detect a system with one of each," said Susan Scott, an astrophysicist at Australian National University and a member of OzGrav and the LIGO collaboration.
Now they have.
Over the last two years, there had been suggestions that a-- but one of the objects appeared a little unusual. It was too big to be a neutron star and too small to be a black hole. The unknown object remains a mystery, which means GW200105 and GW200115 will go down in history.
"These are the first really confident detections of the merger of a neutron star with a black hole," adds Rory Smith, an astrophysicist at Monash University in Australia and a member of the LIGO collaboration.
A quick interlude before we continue.
Black holes and neutron stars are strange objects. They are the relics of dead stars and form at the end of a star's life. The size of a star affects how its life comes to a close. If it's a small star (small being "about 10 times more massive than our sun"), it collapses into an incredibly dense "zombie star," known as a neutron star. If it's a big star, it collapses into a black hole. Both are well-known and well-studied objects, but they still contain many mysteries.
For one, we can't see inside them. This is a much-discussed trait of black holes. Their gravity is so strong that when light gets pulled in -- past the so-called event horizon -- it never comes back out. But scientists also don't know what is going on in the heart of a neutron star. They suspect some truly weird physics could be occurring within both objects. The laws of physics seem to break down inside them.
Observing the objects via gravitational waves is a "kind of stellar paleontology," according to Smith, because it can tell us about their evolutionary history and the environments they form in.
The chirps are central to this. When US-based LIGO and Italy-based Virgo detect a "chirp," they are looking back through time. Within the chirp is a bunch of information that can tell astrophysicists how massive the colliding objects are and the way they spin. This information is critical to understanding how the two objects came to be locked in a death dance with each other.
"By studying these systems, we get to know a lot more about the life and death of black holes and neutron stars in these binary systems," Scott said.
GW200105, the chirp detected on Jan. 5, 2020, and GW200115, the chirp detected on Jan. 15, 2020, are similar events but the objects that collided have slightly different properties. Those stuffy scientific names are rather confusing, so we've dubbed them Lenny (GW200105) and Carl (GW200115).
The researchers say that Lenny is the result of a black hole about nine times as massive as the sun colliding with a neutron star about 1.9 times as massive as the sun. Carl came via a black hole about six times as massive as the sun merging with a neutron star about 1.5 times as massive as the sun. Lenny and Carl are completely different beasts in the present day. The mergers occurred almost a billion years ago far from the Earth, and the chirps only recently reached us.
When we say "collide" or "merge" here, we're not entirely sure what happened when the two objects finally came together. For a long time, they circled each other, trapped by the other's gravity. Eventually, they came together. Scott describes Lenny and Carl as "a bit like Pac-Man," with the black hole swallowing up the neutron star.
There's also the possibility the black hole "shreds" the neutron star in a process known as tidal disruption. In this scenario, the black hole would rip material from the surface of the neutron star and steal it, creating a disk of debris around the event horizon. "That should produce an electromagnetic signal," Scott said.
And a shredded neutron star is a goldmine for astrophysicists. You can't make the material present in a neutron star in a laboratory and study it, so these types of events may open a window to understand what's happening inside them.
"By watching how a neutron star is pulled apart by a black hole, we are beginning to learn about how matter behaves in its most dense state," said Eric Thrane, an astrophysicist at Monash University and member of the LIGO collaboration. With enough gravitational wave detections, we may be able to decode their properties.
That makes Lenny and Carl the first of many black hole-neutron star mergers helping shed light on the most extreme objects in our universe.
"These observations may one day reveal new laws of nature," Smith said.