The first conclusive discovery of an intermediate-mass black hole

Hyperaxion Sep 3, 2020

The signal, first detected in 2019, points to the existence of a black hole that, according to current theories, should not exist.

On May 21, 2019, the gravitational wave detectors LIGO, in the United States, and Virgo, in Italy, picked up the most intense signal since the beginning of their activities.

Since then, astronomers around the world have been trying to understand what caused the phenomenon – and most theories involve the merger of two black holes.

The first conclusive discovery of an intermediate-mass black hole
(Credit: LIGO-Virgo / Caltech / MIT).

A team led by the California Institute of Technology (Caltech) and the Massachusetts Institute of Technology (MIT) recently published two articles in The Astrophysical Journal and Physical Review Letters, where they argue that this may have been the first detection of an intermediate-mass black hole, an object with a mass between 100 and 1000 times that of the Sun.

The detected signal, called GW190521, was generated by a distant source when the Universe was approximately half its current age and lasted for a short time, less than a tenth of a second.

“This doesn’t look much like a chirp, which is what we typically detect,” said Nelson Christensen, a member of the Virgo team. “This is more like something that goes ‘bang,’ and it’s the most massive signal LIGO and Virgo have seen.”

According to the scientists, the gravitational waves are the result of the merger of two black holes with approximately 85 and 66 times the mass of the Sun, which created a black hole with 142 solar masses.

The event released an enormous amount of energy, equivalent to almost eight times the mass of the Sun, which spread throughout the Universe in the form of gravitational waves.

All black holes observed so far fall into one of two categories: stellar-mass black holes, with up to dozens of solar masses, and supermassive black holes, which have hundreds of thousands to billions of solar masses.

However, GW190521 suggests something unprecedented: the existence of an intermediate-mass range.

The merger of two black holes generated the detected gravitational waves
The merger of two black holes generated the detected gravitational waves. (Credit: LIGO-Virgo / Caltech / MIT).

The formation of black holes

According to current physics, a star remains stable as long as the outward pressure of the photons and gas at its core cancels out the inward pressure of gravity.

Over time, however, the star’s core begins to produce heavy elements, the outward pressure decreases, and sustaining its outer layers becomes impossible.

When the outward pressure is less than gravity, the star collapses under its own weight in an explosion called a core-collapse supernova, which can give rise to a black hole.

This process explains how stars of up to 130 solar masses can produce black holes with up to 65 solar masses.

But this is not how heavier stars become black holes – in their case, the formation of black holes is attributed to a phenomenon known as pair instability.

When the photons in the core of these stars become extremely energetic, they can turn into a pair of electrons and antielectrons, particles that generate less pressure.

As a result, the star becomes unstable, has a gravitational collapse and explodes, leaving nothing behind.

Due to pair instability, when stars with more than 200 solar masses collapse, they end up becoming black holes with at least 120 solar masses.

Can you notice the gap? Theoretically, a collapsed star should not be able to produce a black hole with 65 to 120 times solar masses – this is known as the pair instability mass gap.

But this model is contradicted by the GW190521 detection, as the signal points to the existence of a black hole within the mass gap – one of the black holes in the merger had 85 solar masses.

“The fact that we’re seeing a black hole in this mass gap will make a lot of astrophysicists scratch their heads and try to figure out how these black holes were made,” Christensen said.

A possible explanation for what happened is a hierarchical merger: the two black holes that generated the gravitational waves would have formed from the merger of two smaller black holes.

“This event opens more questions than it provides answers,” said Alan Weinstein, a member of the LIGO team and professor of physics at Caltech. “From the perspective of discovery and physics, it’s a very exciting thing.”

Other explanations

While the LIGO and Virgo detectors pick up gravitational waves that are passing through Earth, automated searches scan the data for other signals.

These searches can occur by two methods: in the first, an algorithm identifies patterns that may have been produced by binary systems, while in the second, the analysis is more generic, aiming at “unusual” events, such as mergers.

In the case of GW190521, it was the second method that detected the signal, suggesting that gravitational waves are most likely the result of a binary merger.

Even so, experts stress that other possibilities must be considered, taking into account the peculiarity of the event.

“We have all been hoping for something new, something unexpected, that could challenge what we’ve learned already. This event has the potential for doing that,” Weinstein said.

Related topics:

black hole

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