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Meet GW190521—a black-hole merger for the record books:

The LIGO/VIRGO collaboration has picked up a gravitational wave signal from another black-hole merger—and it's one for the record books.

The merger is the most massive and most distant yet detected by the collaboration, its signal traveling across the Universe for a billion years before reaching Earth. The merger also produced the most energetic signal detected thus far, showing up in the data as more of a "bang" than the usual "chirp." And the new black hole resulting from the merger is the rarest of all in terms of its intermediate mass (about 150 times as heavy as our Sun), making this the first direct observation of an intermediate-mass black hole.

[...] Details of this latest discovery, dubbed GW190521, appeared today in two concurrent papers published in Physical Review Letters and Astrophysical Journal Letters. The former details the discovery of the gravitational wave signal, while the latter discusses the signal's physical properties and its astrophysical implications.

[...] What makes this event so unusual is that 142 solar masses falls smack in the middle of what's known as a "mass gap" for black holes. Most such objects fall into two groups: stellar-mass black holes (ranging from a few solar masses to tens of solar masses) and supermassive blackholes like the one in the middle of our Milky Way galaxy (ranging from hundreds of thousands to billions of solar masses). The former are the result of massive stars dying in a core-collapse supernova, while the latter's formation process remains something of a mystery.

The fact that one of the progenitor black holes here weighs in at 85 solar masses is also highly unusual, since this is at odds with current models of stellar evolution. The kinds of stars that would give rise to black holes between 65 and 135 solar masses would not go supernova and thus would not end up as black holes. Rather, such stars would become unstable and slough off a significant chunk of their mass. Only then would they go supernova—but the result would be a black hole of less than 65 solar masses.

"From our understanding of how stars age and evolve we expect to find black holes with either less than 65 solar masses or more than 120 solar masses, but none in between," said Frank Ohme, who leads an Independent Max Planck Research Group at AEI Hannover. "The 85 solar-mass black hole in the GW190521 origin system falls right in that gap where it shouldn't be. This can mean two things: our understanding of stars' evolution is incomplete or something different has happened here."

Journal References:
R. Abbott, et al.GW190521: A Binary Black Hole Merger with a Total Mass of M ⊙ [open], Physical Review Letters (DOI: 10.1103/PhysRevLett.125.101102)
R. Abbott, T. D. Abbott, S. Abraham, et al. Properties and Astrophysical Implications of the 150 M ⊙ Binary Black Hole Merger GW190521 - IOPscience, The Astrophysical Journal Letters (DOI: 10.3847/2041-8213/aba493)

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