Elusive mid-sized black hole discovered with mass 142 times that of the Sun formed by merger of 2 black holes

Two black holes likely collided and merged to create the more massive black hole which produced gravitational waves equal to the energy of eight suns, say scientists

                            Elusive mid-sized black hole discovered with mass 142 times that of the Sun formed by merger of 2 black holes
(Getty Images)

Astronomers have witnessed the birth of an "intermediate-mass" black hole, an object which has long eluded scientists. This was made possible by gravitational wave detectors — Laser Interferometer Gravitational-wave Observatory (LIGO) and Virgo — which have chalked up their biggest catch yet, a black hole 142 times the mass of the Sun, resulting from the merger of two black holes of 85 and 65 solar masses.

The cosmic event, its energy detected on Earth in the form of gravitational waves, is the most massive black hole merger observed in gravitational waves, which are ripples in the fabric of space-time, says the research team. It likely produced gravitational waves equal to the energy of eight suns. 

The findings suggest that two black holes likely collided and merged to create a more massive black hole. This final black hole was found in an intermediate-mass range that lies between stellar-mass and supermassive black holes. Intermediate-mass black holes are interesting since they may hold the key to one of the big puzzles in astrophysics and cosmology: the origin of supermassive black holes that sit at the centers of some galaxies. 

The scientists identified the merging black holes by detecting the gravitational waves produced in the final moments of the merger. They detected the signal, which they have labeled GW190521, on May 21, 2019, with the National Science Foundation’s LIGO, a pair of identical, four-kilometer-long interferometers in the US, and Virgo, a three-kilometer-long detector in Italy.

"Researchers have detected a signal from what may be the most massive black hole merger yet observed in gravitational waves. The product of the merger is the first clear detection of an intermediate-mass black hole, with a mass between 100 and 1,000 times that of the Sun," say scientists, who make up the LIGO Scientific Collaboration (LSC) and the Virgo Collaboration.

They add, "The new signal likely represents the instant the two black holes merged. The merger created an even more massive black hole, with a final mass 142 times that of the sun, or 142 solar masses, and released an enormous amount of energy, equivalent to around eight solar masses, spread across the universe in the form of gravitational waves."

Numerical simulation of two black holes that inspiral and merge, emitting gravitational waves (N Fischer, H Pfeiffer, A Buonanno, Max Planck Institute for Gravitational Physics, Simulating eXtreme Spacetimes/SXS Collaboration)

A stellar black hole forms when a massive star undergoes an explosive death called a supernova. A typical stellar-class of black hole has a mass between about 3 and 10 solar masses. Supermassive black holes exist in the center of most galaxies, including our own Milky Way Galaxy. They are extremely heavy, with masses ranging from millions to billions of solar masses.

The scientists explain that the black holes observed to date fit within either of two categories: stellar-mass black holes or supermassive black holes. However, the final 142-solar-mass black hole produced by the GW190521 merger lies within an intermediate-mass range between stellar-mass and supermassive black holes, they add. Until now, only indirect evidence obtained from electromagnetic observations hinted at the existence of intermediate-mass black holes.

"Long have we searched for an intermediate-mass black hole to bridge the gap between stellar-mass and supermassive black holes. Now, we have proof that intermediate-mass black holes do exist," writes Christopher Berry, the CIERA board of visitors research professor in Northwestern’s CIERA or the Center for Interdisciplinary Exploration and Research in Astrophysics. 

"LIGO once again surprises us not just with the detection of black holes in sizes that are difficult to explain, but doing it using techniques that were not designed specifically for stellar mergers. This is of tremendous importance since it showcases the instrument's ability to detect signals from completely unforeseen astrophysical events. LIGO shows that it can also observe the unexpected," emphasizes Pedro Marronetti, program director for gravitational physics at the National Science Foundation. 

The signal, resembling about four short wiggles, is extremely brief, lasting less than one-tenth of a second. From what the researchers can tell, GW190521 was generated by a source that is roughly 5 gigaparsecs away, when the universe was about half its age, making it one of the most distant gravitational-wave sources detected so far.

The findings have been published in two separate papers. One, appearing in Physical Review Letters, details the discovery of the gravitational wave signal, and the other, in the Astrophysical Journal Letters, discusses the signal's physical properties and astrophysical implications. "The signal is the most distant, and therefore the oldest, ever detected (the gravitational wave took seven billion years to reach us)," says the study.

The team detected the signal, which they have labeled GW190521, on May 21, 2019, with the National Science Foundation’s Laser Interferometer Gravitational-wave Observatory (LIGO), a pair of identical, 4-kilometer-long interferometers in the US, and Virgo, a 3-kilometer-long detector in Italy (Getty Images)

The two progenitor black holes, of 65 and 85 solar masses that produced the final black hole, also seem to be unique in their size. They are so massive that scientists suspect one or both of them may not have formed from a collapsing star, as most stellar-mass black holes do. Explaining further, astronomers say that models identify a range of masses between about 65 and 120 solar masses, called the "pair-instability mass gap", in which it is thought that black holes cannot be formed by a collapsing star.

"Based on our current knowledge, the gravitational collapse of a star cannot form black holes in the approximate range of 65 to 120 solar masses, since the most massive stars are completely blown apart by the supernova explosion that accompanies the collapse, leaving only gas and dust behind them," the findings state.

However, now, the heavier of the two black holes that produced the GW190521 signal, at 85 solar masses, is the first so far detected within the pair-instability mass gap. “The mass of the larger black hole in the pair puts it into the range where it’s unexpected from regular astrophysics processes. It seems too massive to have been formed from a collapsed star, which is where black holes generally come from,” emphasizes Peter Shawhan, a professor of physics at the University of Maryland (UMD), an LSC principal investigator, and the LSC observational science coordinator.

So how did the two merging black holes observed by LIGO and Virgo originate? Scientists think that these black holes may have themselves formed from the earlier mergers of two smaller black holes, before migrating together and eventually merging. “This event opens more questions than it provides answers. From the perspective of discovery and physics, it's a very exciting thing,” says LIGO member Alan Weinstein, professor of physics at Caltech. 

"Right from the beginning, this signal, which is only a tenth of a second long, challenged us in identifying its origin. But, despite its short duration, we were able to match the signal to one expected of black-hole mergers, as predicted by Einstein's theory of general relativity. We realized we had witnessed, for the first time, the birth of an intermediate-mass black hole from a black-hole parent that most probably was born from an earlier binary merger," writes Alessandra Buonanno, a College Park professor at UMD and an LSC principal investigator.

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