New type of black hole detected in massive collision that sent gravitational waves with a 'bang'

This collision created the first intermediate-mass black hole ever discovered, and it has a mass of 142 times that of our sun.

The "bang" of energy created by this black hole merger released through gravitational waves, or ripples in the space-time continuum, equaled the energy of eight suns. And those ripples took 7 billion years to travel across space to reach us on Earth.

This newly discovered black hole has what's called an "intermediate" mass because its mass is between 100 and 1,000 times that of the sun. It's more than that of stellar mass (the mass of stars) and less than that of supermassive black holes.

The signal astronomers traced through gravitational waves likely happened the moment the two black holes came together.

The gravitational waves were traced on Earth on May 21, 2019, by the dual detectors of the National Science Foundation's Laser Interferometer Gravitational-wave Observatory in the US, as well as the Virgo gravitational-wave detector in Italy.

Two studies published on Wednesday, one about the gravitational waves event in the journal Physical Review Letters and another detailing the implications of the event in the Astrophysical Journal Letters.

"One of the great mysteries in astrophysics is how do supermassive black holes form?" said Christopher Berry, LIGO Science Collaboration Editorial Board reviewer for the discovery paper, in a statement.

"They are the million solar-mass elephants in the room," said Berry, who is also the board of visitors research professor at Northwestern University's Center for Interdisciplinary Exploration and Research in Astrophysics.

"Do they grow from stellar-mass black holes, which are born when a star collapses, or are they born via an undiscovered means? Long have we searched for an intermediate-mass black hole to bridge the gap between stellar-mass and supermassive black holes.

The search is over. "Now, we have proof that intermediate-mass black holes do exist," he said.

The gravitational wave event only lasted about one-tenth of a second. The waves originated from 7 billion light-years away -- the most distant source of them so far.

With this detection, LIGO has observed the unexpected and led to another breakthrough.

"Gravitational-wave observations are revolutionary," Berry said. "With these gravitational-wave breakthroughs, it won't be long until we have enough data to uncover the secrets of how black holes are born and how they grow."

The gravitational wave event was named GW190521. And the four little wiggles picked up by the detectors represent a literal bang that traveled across space to reach us on Earth 7 billion years later. This differs from the signal picked up during LIGO's first detection of gravitational waves in 2015.

"This doesn't look much like a chirp, which is what we typically detect," said Nelson Christensen, Virgo team collaborator and director of the Artemis Laboratory at the Observatoire de la Côte d'Azur, in a statement. "This is more like something that goes 'bang,' and it's the most massive signal LIGO and Virgo have seen."

Why this black hole is different

There are two main categories of black holes.

Stellar-mass black holes form when massive stars die and they range from a few times the mass of the sun to tens of times its mass. And supermassive black holes, such as those found at the center of galaxies like our own, can be anywhere from hundreds, thousands or even billions of times the mass of our sun.

Then, there is this new intermediate black hole, which is in between the two. It was formed by two massive black holes that were likely created by collapsing stars. Of the two black holes that merged, the heavier one was 85 solar masses and the other black hole was about 66 solar masses.

Stars collapse beneath their own weight when their cores evolve and no longer have enough pressure to support the star's outer layers. The result can create a black hole.

But a star that collapses shouldn't be able to produce a black hole between the range of 65 to 120 solar masses, which is called the pair-instability mass gap. This is because the most massive stars are obliterated by the supernova that comes hand in hand with their collapse.

The larger of the two black holes in this merger, with 85 solar masses, is the first detected black hole in this range. But how did it form?

"There are many ideas about how to get around this -- merging two stars together, embedding the black hole in a thick disc of material it can swallow, or primordial black holes created in the aftermath of the Big Bang," Berry said. "The idea I really like is a hierarchical merger where we have a black hole formed from the previous merger of two smaller black holes."

The possibility of the hierarchical merger, where each black hole in this merger likely formed from the merger of two smaller black holes, is included by the researchers in the second study.

"After so many gravitational-wave observations since the first detection in 2015, it's exciting that the universe is still throwing new things at us, and this 85-solar-mass black hole is quite the curveball," said Chase Kimball, second study coauthor, LIGO Scientific Collaboration member and a Northwestern astronomy postdoctoral student, in a statement.

New questions

"This event opens more questions than it provides answers," said Alan Weinstein, LIGO member and a professor of physics at the California Institute of Technology, in a statement. "From the perspective of discovery and physics, it's a very exciting thing."

In fact, the researchers are considering that this incredibly short signal may have been created by an entirely different source rather than the merging of black holes.

"What if something entirely new produced these gravitational waves?" said Vicky Kalogera, principal investigator of Northwestern's LIGO Scientific Collaboration group, director of CIERA and the Daniel I. Linzer distinguished university professor of physics and astronomy, in a statement.

"For instance, perhaps the gravitational waves were emitted by a collapsing star in our galaxy. The signal also could be from a cosmic string produced just after the universe inflated in its earliest moments -- although neither of these exotic possibilities matches the data as well as a binary merger."

More gravitational wave signals may be pulled from the detectors' latest observational period. And next year, they will be joined by the Japanese KAGRA detector, which could help make even more gravitational wave detections possible than before.

"We're really in the dawn of gravitational-wave astronomy," Kimball said. "It's hard to pick a better time to come up as an astrophysicist."