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A long time ago in a galaxy far, far away... a pair of black holes coalesced resulting in largest gravitational wave we've seen

Void forged in merger event also the first of its kind spotted

Video The latest gravitational wave event, announced by astrophysicists today, is a particularly good 'un. Not only is it the largest signal detected yet, it's the first time an intermediate-sized black hole has ever been spotted.

Code-named GW190521, the wave was picked up by the LIGO and Virgo detectors on May 21, 2019. The blip lasted less than one-tenth of a second, and was created when two black holes with 66 and 85 solar masses smashed into one another when the universe was half its current age.

A larger void measuring 142 solar masses was forged in the merger process; the remaining nine solar masses were converted into energy. GW190521 is the heftiest and most distant event recorded so far. But the most intriguing thing about the specimen is its overall mass and the mass of its constituents.

Here's a simulation of the merger:

Youtube Video

Black holes are typically either pretty small, measuring less than 100 times the mass of the Sun, or ginormous at millions or even billions of solar masses. The teenier ones are splattered around galaxies, whilst the supermassive ones are right at the center. Scientists have been on the hunt for medium-sized black holes between 100 and 100,000 solar masses, but have never managed to confirm their existence until now.

"It is the first definite detection," Christopher Berry, a member of the LIGO Scientific Collaboration and a research assistant professor at Northwestern University, told El Reg. "There have been many previous claims, based upon X-ray observations or studying the dynamics of stellar clusters. These have always left some room for doubt: the observations could be interpreted as an intermediate-mass black hole, or also some way else. Now we do know that intermediate-mass black holes do indeed exist."

These mysterious objects are difficult to find because stars can't directly collapse into black holes between 60 and 120 solar masses, something described as a mass gap. Larger stars turn into larger black holes when they die, but there's a limit to how massive they can be before they completely blow themselves apart without leaving a black hole behind at all.

"Massive stars are largely supported by radiation pressure," Berry explained. "When their cores get very hot, the photons of light are energetic enough to produce electron and positron pairs. As you convert photons to particles, the radiation pressure drops. This is bad news for the star.

As you convert photons to particles, the radiation pressure drops. This is bad news for the star

"It will start to collapse. As it does so it will become denser and hotter, triggering explosive nuclear reactions. These completely disrupt the star, leaving nothing behind. So for these massive stars, which would be expected to leave black holes in this mass range, pair instability prevents black hole formation."

The two black holes that created the intermediate-sized one, therefore, might have been formed from previous merger events too. But there are two other possible scenarios that might have created the larger one, said Michela Mapelli, an astrophysicist at the INAF Astronomical Observatory of Padova and member of the Virgo Collaboration.

"A merger between two stars might, under some circumstances, lead to a black hole with a mass in this range too," she told The Register. "Or this black hole did not form from stars or from the merger of other black holes but from some 'more exotic channel', such as the so-called 'primordial black holes': black holes that form from gravitational collapses in the early universe, a short time after the Big Bang."

Unfortunately, the problem of how the two black holes with 66 and 85 solar masses formed won't be solved by studying the gravitational wave produced when they fused together. "Based on what we know about GW190521, we cannot say which one of these three scenarios produced the observed black hole, but future observations by LIGO and Virgo will certainly help us to constrain these scenarios," Mapelli added.

The LIGO and Virgo teams have published two papers on their discovery in the journal of Physical Review Letters [PDF] and the Astrophysical Journal Letters. ®


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