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  • richardmitnick 10:49 am on September 3, 2020 Permalink | Reply
    Tags: "Where do black-hole parents meet? LIGO/Virgo may provide answers", Black hole merger GW190412., , ,   

    From University of Birmingham UK: “Where do black-hole parents meet? LIGO/Virgo may provide answers” 

    From University of Birmingham UK

    03 Sep 2020
    Beck Lockwood
    Press Office
    University of Birmingham
    Tel: +44 (0)781 3343348.

    Astrophysicists investigating gravitational-wave data from the far reaches of the Universe believe they may have found an explanation for a curious signal detected from the collision of two black holes.

    The signal, named GW190412, was picked up by the LIGO/Virgo detectors, which are set up to observe gravitational waves – the ripples in space and time caused by huge astronomical objects – and use them to make new discoveries about our Universe.

    MIT /Caltech Advanced aLigo .

    Caltech/MIT Advanced aLigo detector installation Livingston, LA, USA.

    Caltech/MIT Advanced aLigo detector installation Hanford, WA, USA.

    VIRGO Gravitational Wave interferometer, near Pisa, Italy.

    VIRGO Gravitational Wave interferometer, near Pisa, Italy.

    GW190412 is unusual for a number of reasons. Firstly, scientists calculated that one of the two black holes which produced it had an unusually large mass. In addition, unlike other black holes detected previously, the more massive black hole is definitely spinning, and spinning in an unusual way.

    A team of scientists from the University of Birmingham, MIT, and Johns Hopkins University set out to find out more about the black holes that produced GW190412. Their results, published in Physical Review Letters, suggest the unusual characteristics could be explained by one of the black holes being formed as a result of a previous “parent” black hole collision.

    Lead author Dr Davide Gerosa, of the University of Birmingham’s Institute for Gravitational Wave Astronomy, says: “Black holes are usually thought to be born when massive stars run out of nuclear ‘fuel’, and gravity takes over, making their iron core collapse. But what if they could also be formed by the collision of two parent black holes? This would explain the key features of GW190412.”

    The team also modeled the type of environment in which this sort of collision might take place. They suggest it must have occurred in an environment with a large number of black holes, namely a star clusters. The most studied clusters to form black holes are the so-called “globulars” –group of stars in the outskirts of most galaxies. However, GW190412 is exceptional: the team found that globular clusters are not suitable for this event: their density is too low to retain black holes after their merger.

    Dr Salvatore Vitale, Assistant Professor of Physics at MIT and co-author on the paper, says: “Second-generation black holes can merge only in particularly dense astrophysical environments. This is because if only a few black holes were around, the parent black holes would simply not get a chance to meet.”

    Dr Gerosa adds: “All these findings come together to suggest that GW190412 is not just unusual, it must also come from an unusual place in our Universe.”

    See the full article here .


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  • richardmitnick 10:31 am on September 3, 2020 Permalink | Reply
    Tags: "An unexpected origin story for a lopsided black hole merger", , Black hole merger GW190412., , , ,   

    From MIT News: “An unexpected origin story for a lopsided black hole merger” 

    MIT News

    From MIT News

    September 2, 2020
    Jennifer Chu

    A lopsided merger of two black holes may have unusual origins, based on a reanalysis of LIGO data.
    Credits:Image: MIT News.

    A lopsided merger of two black holes may have an oddball origin story, according to a new study by researchers at MIT and elsewhere.

    The merger was first detected on April 12, 2019 as a gravitational wave that arrived at the detectors of both LIGO (the Laser Interferometer Gravitational-wave Observatory), and its Italian counterpart, Virgo.

    MIT /Caltech Advanced aLigo .

    Caltech/MIT Advanced aLigo detector installation Livingston, LA, USA.

    Caltech/MIT Advanced aLigo detector installation Hanford, WA, USA.

    VIRGO Gravitational Wave interferometer, near Pisa, Italy.

    VIRGO Gravitational Wave interferometer, near Pisa, Italy.

    Scientists labeled the signal as GW190412 and determined that it emanated from a clash between two David-and-Goliath black holes, one three times more massive than the other. The signal marked the first detection of a merger between two black holes of very different sizes.

    Now the new study, published today in the journal Physical Review Letters, shows that this lopsided merger may have originated through a very different process compared to how most mergers, or binaries, are thought to form.

    It’s likely that the more massive of the two black holes was itself a product of a prior merger between two parent black holes. The Goliath that spun out of that first collision may have then ricocheted around a densely packed “nuclear cluster” before merging with the second, smaller black hole — a raucous event that sent gravitational waves rippling across space.

    GW190412 may then be a second generation, or “hierarchical” merger, standing apart from other first-generation mergers that LIGO and Virgo have so far detected.

    “This event is an oddball the universe has thrown at us — it was something we didn’t see coming,” says study coauthor Salvatore Vitale, an assistant professor of physics at MIT and a LIGO member. “But nothing happens just once in the universe. And something like this, though rare, we will see again, and we’ll be able to say more about the universe.”

    Vitale’s coauthors are Davide Gerosa of the University of Birmingham and Emanuele Berti of Johns Hopkins University.

    A struggle to explain

    There are two main ways in which black hole mergers are thought to form. The first is known as a common envelope process, where two neighboring stars, after billions of years, explode to form two neighboring black holes that eventually share a common envelope, or disk of gas. After another few billion years, the black holes spiral in and merge.

    “You can think of this like a couple being together all their lives,” Vitale says. “This process is suspected to happen in the disc of galaxies like our own.”

    The other common path by which black hole mergers form is via dynamical interactions. Imagine, in place of a monogamous environment, a galactic rave, where thousands of black holes are crammed into a small, dense region of the universe. When two black holes start to partner up, a third may knock the couple apart in a dynamical interaction that can repeat many times over, before a pair of black holes finally merges.

    In both the common envelope process and the dynamical interaction scenario, the merging black holes should have roughly the same mass, unlike the lopsided mass ratio of GW190412. They should also have relatively no spin, whereas GW190412 has a surprisingly high spin.

    “The bottom line is, both these scenarios, which people traditionally think are ideal nurseries for black hole binaries in the universe, struggle to explain the mass ratio and spin of this event,” Vitale says.

    Black hole tracker

    In their new paper, the researchers used two models to show that it is very unlikely that GW190412 came from either a common envelope process or a dynamical interaction.

    They first modeled the evolution of a typical galaxy using STAR TRACK, a simulation that tracks galaxies over billions of years, starting with the coalescing of gas and proceeding to the way stars take shape and explode, and then collapse into black holes that eventually merge. The second model simulates random, dynamical encounters in globular clusters — dense concentrations of stars around most galaxies.

    The team ran both simulations multiple times, tuning the parameters and studying the properties of the black hole mergers that emerged. For those mergers that formed through a common envelope process, a merger like GW190412 was very rare, cropping up only after a few million events. Dynamical interactions were slightly more likely to produce such an event, after a few thousand mergers.

    However, GW190412 was detected by LIGO and Virgo after only 50 other detections, suggesting that it likely arose through some other process.

    “No matter what we do, we cannot easily produce this event in these more common formation channels,” Vitale says.

    The process of hierarchical merging may better explain the GW190412’s lopsided mass and its high spin. If one black hole was a product of a previous pairing of two parent black holes of similar mass, it would itself be more massive than either parent, and later significantly overshadow its first-generation partner, creating a high mass ratio in the final merger.

    A hierarchical process could also generate a merger with a high spin: The parent black holes, in their chaotic merging, would spin up the resulting black hole, which would then carry this spin into its own ultimate collision.

    “You do the math, and it turns out the leftover black hole would have a spin which is very close to the total spin of this merger,” Vitale explains.

    No escape

    If GW190412 indeed formed through hierarchical merging, Vitale says the event could also shed light on the environment in which it formed. The team found that if the larger of the two black holes formed from a previous collision, that collision likely generated a huge amount of energy that not only spun out a new black hole, but kicked it across some distance.

    “If it’s kicked too hard, it would just leave the cluster and go into the empty interstellar medium, and not be able to merge again,” Vitale says.

    If the object was able to merge again (in this case, to produce GW190412), it would mean the kick that it received was not enough to escape the stellar cluster in which it formed. If GW190412 indeed is a product of hierarchical merging, the team calculated that it would have occurred in an environment with an escape velocity higher than 150 kilometers per second. For perspective, the escape velocity of most globular clusters is about 50 kilometers per second.

    This means that whatever environment GW190412 arose from had an immense gravitational pull, and the team believes that such an environment could have been either the disk of gas around a supermassive black hole, or a “nuclear cluster” — an incredibly dense region of the universe, packed with tens of millions of stars.

    “This merger must have come from an unusual place,” Vitale says. “As LIGO and Virgo continue to make new detections, we can use these discoveries to learn new things about the universe.”

    This research was funded, in part by the U.S. National Science Foundation and MIT’s Solomon Buchsbaum Research Fund.

    See the full article here .

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