An artist's impression of the last moments before the merger of two black holes. Credit: LIGO-VIRGO COLLABORATION

Jonathan Amos  -- BBC Science Correspondent
2 September 2020

Imagine the energy of eight Suns released in an instant.

This is the gravitational "shockwave" that spread out from the biggest merger yet observed between two black holes.
The signal from this event travelled for some seven billion years to reach Earth but was still sufficiently strong to rattle laser detectors in the US and Italy in May last year.
Researchers say the colliding black holes produced a single entity with a mass 142 times that of our Sun.
This is noteworthy. Science has long traced the presence of black holes on the sky that are quite a bit smaller or even very much larger. But this new observation inaugurates a novel class of so-called intermediate-sized black holes in the range of 100-1,000 Sun (or solar) masses.
The analysis is the latest to come out of the international LIGO-VIRGO collaboration, which operates three super-sensitive gravitational wave-detection systems in America and Europe.

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The link for the article in PRL 

© Carl Knox, ARC Centre of Excellence for Gravitational Wave Discovery (OzGrav)

23 June 2020

Scientists puzzle over the mysterious astrophysical object: have they discovered the heaviest neutron star or the lightest black hole ever observed?

An international team of scientists including Lancaster University have discovered a compact object lying between neutron stars and black holes in terms of mass.

When the most massive stars die, they collapse under their own gravity and leave behind black holes; when stars that are a bit less massive die, they explode in a supernova and leave behind dense, dead remnants of stars called neutron stars.

For decades, astronomers have been puzzled by a gap that lies between neutron stars and black holes: the heaviest known neutron star is no more than 2.5 times the mass of our sun, or 2.5 solar masses, and the lightest known black hole is about 5 solar masses.

The question remained: does anything lie in this so-called mass gap?

Now, in a new study from the National Science Foundation's Laser Interferometer Gravitational-Wave Observatory (LIGO) and the Virgo detector in Europe, scientists have announced the discovery of an object of 2.6 solar masses, placing it firmly in the mass gap.

The object was found on August 14, 2019, as it merged with a black hole of 23 solar masses, generating a splash of gravitational waves detected back on Earth by LIGO and Virgo. A paper about the detection has been accepted for publication in The Astrophysical Journal Letters.

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See the same news from Virgo website

See the same news from LIGO website

Marek Abramowicz, Michał Bejger, Éric Gourgoulhon & Odele Straub

Our existence in the Universe resulted from a rare combination of circumstances. The same must hold for any highly developed extraterrestrial civilisation, and if they have ever existed in the Milky Way, they would likely be scattered over large distances in space and time. However, all technologically advanced species must be aware of the unique property of the galactic centre: it hosts Sagittarius A* (Sgr A*), the closest supermassive black hole to anyone in the Galaxy. A civilisation with sufficient technical know-how may have placed material in orbit around Sgr A* for research, energy extraction, and communication purposes. In either case, its orbital motion will necessarily be a source of gravitational waves. We show that a Jupiter-mass probe on the retrograde innermost stable circular orbit around Sgr A* emits, depending on the black hole spin, at a frequency of fGW = 0.63–1.07 mHz and with a power of PGW = 2.7 × 10^36–2.0 × 10^37 erg/s. We discuss that the energy output of a single star is sufficient to stabilise the location of an orbiting probe for a billion years against gravitational wave induced orbital decay. Placing and sustaining a device near Sgr A* is therefore astrophysically possible. Such a probe will emit an unambiguously artificial continuous gravitational wave signal that is observable with LISA-type detectors.

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A visualization of a collision between two differently sized black holes.

Credit: N. Fischer, H. Pfeiffer, A. Buonanno (Max Planck Institute for Gravitational Physics), Simulating eXtreme Spacetimes (SXS) Collaboration

Davide Castelvecchi,  20 APRIL 2020

Gravitational-wave astronomers have for the first time detected a collision between two black holes of substantially different masses — opening up a new vista on astrophysics and on the physics of gravity. The event offers the first unmistakable evidence from these faint space-time ripples that at least one black hole was spinning before merging, giving astronomers rare insight into a key property of these these dark objects.

“It’s an exceptional event,” said Maya Fishbach, an astrophysicist at the University of Chicago in Illinois. Similar mergers on which data have been published all took place between black holes with roughly equal masses, so this new one dramatically upsets that pattern, she says. The collision was detected last year, and was unveiled on 18 April by Fishbach and her collaborators at a virtual meeting of the American Physical Society, held entirely online because of the coronavirus pandemic.

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This simulation shows the radiation emitted from a binary black hole system. In principle, we should have neutron star binaries, black hole binaries, and neutron star-black hole systems, covering the entire allowable mass range. In practice, we saw a longstanding 'gap' in such binaries between about 2.5 and 5 solar masses. With the newest LIGO data, that gap seems to disappear.NASA'S GODDARD SPACE FLIGHT CENTER

Ethan Siegel

Mar 20, 2020

On Monday, March 16, 2020, astrophysicist Carl Rodriguez expressed a sentiment echoed by gravitational wave physicists all across the world: NOT NOW LIGO! Just minutes earlier, the LIGO collaboration sent out an alert suggesting that it had just detected another gravitational wave event, the 56th candidate detection since starting up its latest data-taking run in April of 2019. This one appears to indicate the merger of two black holes, like so many others before it.

Unlike most of the others, however, this one might be the nail-in-the-coffin of the idea of a "mass gap" between neutron stars and black holes. Before LIGO turned back on last April, all of its events, combined with otherwise-known neutron stars and black holes, showed two distinct populations: low-mass neutron stars (below 2.5 solar masses) and high-mass black holes (5 solar masses and up). This latest event, however, falls right into the mass gap range, and could demolish the idea once and for all.

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An artist's illustration of two black holes merging and creating ripples in spacetime known as gravitational waves.  (Image: © LIGO/T. Pyle)

By Paul Sutter 6.12.2019

Scientists are on the verge of being able to detect the "memory" left behind by gravitational waves.

Paul M. Sutter is an astrophysicist at The Ohio State University, host of Ask a Spaceman and Space Radio, and author of "Your Place in the Universe." Sutter contributed this article to Space.com's Expert Voices: Op-Ed & Insights.

Gravitational waves slosh throughout the universe as ripples in space-time produced by some of the most cataclysmic events possible.

With facilities like the Laser Interferometer Gravitational-Wave Observatory (LIGO) and Virgo, we can now detect the strongest of those ripples as they wash over the Earth. But gravitational waves leave behind a memory — a permanent bend in space-time — as they pass through, and we are now on the verge of being able to detect that too, allowing us to push our understanding of gravity to the limits.

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NASA’S GODDARD SPACE FLIGHT CENTER/CI LAB

Astronomers still hope to catch a star going supernova and a bumpy neutron star, among others
BY LISA GROSSMAN 1:14PM, MAY 6, 2019

BANG, CRASH Physicists using the LIGO and Virgo observatories are catching all sorts of cosmic collisions, including of pairs of neutron stars (illustrated). But scientists hope to bag even more exotic quarry.

Seekers of gravitational waves are on a cosmic scavenger hunt.

Since the Advanced Laser Interferometer Gravitational-wave Observatory turned on in 2015, physicists have caught these ripples in spacetime from several exotic gravitational beasts — and scientists want more.

This week, LIGO and its partner observatory Virgo announced five new possible gravitational wave detections in a single month, making what was once a decades-long goal almost commonplace (SN Online: 5/2/19).

“We’re just beginning to see the field of gravitational wave astronomy open,” LIGO spokesperson Patrick Brady from the University of Wisconsin–Milwaukee said May 2 in a news conference. “Opening up a new window on the universe like this will hopefully bring us a whole new perspective on what’s out there.”

The speed and pitch of gravitational wave signals allow astronomers to make out what’s stirring up the waves. Here are the sources of gravitational waves that scientists that already have in their nets, and what they’re still hoping to find.

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Scientific simulation of a black hole consuming a neutron star.Credit: A. Tonita, L. Rezzolla, F. Pannarale

NATURE 26 APRIL 2019

Davide Castelvecchi

Gravitational waves may have just delivered the first sighting of a black hole devouring a neutron star. If confirmed, it would be the first evidence of the existence of such binary systems. The news comes just a day after astronomers had detected gravitational waves from a merger of two neutron stars for only the second time.

At 15:22:17 UTC on 26 April, the twin detectors of the Laser Interferometer Gravitational-wave Observatory (LIGO) in the United States and the Virgo observatory in Italy reported a burst of waves of an unusual type. Astronomers are still analysing the data and doing computer simulations to interpret them.

But they are already considering the tantalizing prospect that they have made a long-hoped-for detection that could produce a wealth of cosmic information, from precise tests of the general theory of relativity to measuring the Universe’s rate of expansion. Astronomers around the world are also racing to observe the phenomenon using different types of telescope.

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Mollweide projection of bayestar.fits bayestar.png. Submitted by LIGO/Virgo EM Follow-Up on Apr 8, 2019 18:52:21 UTC

A week into the 3rd @LIGO @ego_virgo Observing Run and our first candidate has been posted to GraceDB - the #GravitationalWave candidate event database: say hello to S190408an! More info, including a map of its likely sky location, at https://gracedb.ligo.org/superevents/S190408an/view/ …

Stay tuned this week for more about our #GravitationalWave candidate events: how often to expect them, how to access the alert info and what it means. And remember, we're expecting *lots* of #O3 events and the alerts will be public! https://emfollow.docs.ligo.org/userguide/

GraceDB — Gravitational Wave Candidate Event Database

The Virgo gravitational-wave detector near Pisa, Italy, has roughly doubled its sensitivity since 2017.Credit: Cappello/Ropi via ZUMA

Detailed data on space-time ripples are set to pour in from LIGO and Virgo’s upgraded detectors.

Davide Castelvecchi  --  02 APRIL 2019

The hunt for gravitational waves is on again — this time assisted by the quirks of quantum mechanics.

Three massive detectors — the two in the United States called LIGO and one in Italy known as Virgo — officially resumed collecting data on 1 April, after a 19-month shutdown for upgrades. Thanks in part to a quantum phenomenon known as light squeezing, the machines promise not only to spot more gravitational waves — ripples in space-time that can reveal a wealth of information about the cosmos — but also to make more detailed detections. Researchers hope to observe as-yet undetected events, such as a supernova or the merging of a black hole with a neutron star.

The run, which will last until next March, also marks a major change in how gravitational-wave astronomy is done. For the first time, LIGO and Virgo will send out public, real-time alerts on wave detections to tip off other observatories — and anyone with a telescope — on how to find the events, so that they can be studied with traditional techniques, from radio- to space-based X-ray telescopes. The alerts will also be available through a smartphone app. “Astronomers are really hungry,” says David Reitze, a physicist at the California Institute of Technology in Pasadena and director of the Laser Interferometer Gravitational-wave Observatory (LIGO), which made the first historic detection of gravitational waves in 2015.

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A worker inspects quartz fibers that suspend a mirror inside the Virgo gravitational-wave observatory. EGO/Virgo Collaboration/Perciballi

Business Insider

Dave Mosher Mar. 28, 2019

One of the most remarkable experiments in history — a pair of giant machines that listen for ripples in spacetime called gravitational waves— will wake up from a half-year nap on Monday. And it will be about 40% stronger than before.

That experiment is called the Laser Interferometer Gravitational-Wave Observatory (LIGO); it consists of two giant, L-shaped detectors that together solved a 100-year-old mystery posed by Albert Einstein.

In 1915, Einstein predicted the existence of ripples in the fabric of space However, he didn't think these gravitational waves would ever be detected — they seemed too weak to pick up amid all the noise and vibrations on Earth. For 100 years, it seemed Einstein was right.

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Any distant gravitational source can emit gravitational waves and send out a signal that deforms the fabric of space, which manifests as gravitational attraction. But while gravitational forces fall off as the distance squared, the gravitational wave signal only falls off proportionally to the distance.

EUROPEAN GRAVITATIONAL OBSERVATORY, LIONEL BRET/EUROLIOS

Ethan Siegel - Senior Contributor
Mar 2, 2019

One of the things we often just accept about the world is that physical effects get weaker the farther away we get from them. Light sources appear dimmer, the gravitational force gets weaker, magnets deflect by smaller amounts, etc. The most common way this arises is through an inverse-square law, meaning that if you double the distance between you and the source that creates the effect you're measuring, the effect will be one quarter of what it was previously. But this isn't true for gravitational waves, and that puzzles reader Jack Dectis, who asks:

You have stated:
1) The strength of gravity varies with the square of the distance.
2) The strength of gravity waves, as detected by LIGO, varies directly with the distance.
So the question is, how can those two be the same thing?

This is a real surprise to almost everyone when they hear about it, even professional physicists. But it's true! Here's the science of why.

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The observatories are also releasing their first catalog of gravitational-wave events

On Saturday, December 1, scientists attending the Gravitational Wave Physics and Astronomy Workshop in College Park, Maryland, presented new results from the National Science Foundation's LIGO (Laser Interferometer Gravitational-Wave Observatory) and the European- based VIRGO gravitational-wave detector regarding their searches for coalescing cosmic objects, such as pairs of black holes and pairs of neutron stars. The LIGO and Virgo collaborations have now confidently detected gravitational waves from a total of 10 stellar-mass binary black hole mergers and one merger of neutron stars, which are the dense, spherical remains of stellar explosions. Six of the black hole merger events had been reported before, while four are newly announced.

From September 12, 2015, to January 19, 2016, during the first LIGO observing run since undergoing upgrades in a program called Advanced LIGO, gravitational waves from three binary black hole mergers were detected. The second observing run, which lasted from November 30, 2016, to August 25, 2017, yielded one binary neutron star merger and seven additional binary black hole mergers, including the four new gravitational-wave events being reported now. The new events are known as GW170729, GW170809, GW170818, and GW170823, in reference to the dates they were detected.

All of the events are included in a new catalog, also released Saturday, with some of the events breaking records. For instance, the new event GW170729, detected in the second observing run on July 29, 2017, is the most massive and distant gravitational-wave source ever observed. In this coalescence, which happened roughly 5 billion years ago, an equivalent energy of almost five solar masses was converted into gravitational radiation.

GW170814 was the first binary black hole merger measured by the three-detector network, and allowed for the first tests of gravitational-wave polarization (analogous to light polarization).

The event GW170817, detected three days after GW170814, represented the first time that gravitational waves were ever observed from the merger of a binary neutron star system. What's more, this collision was seen in gravitational waves and light, marking an exciting new chapter in multi-messenger astronomy, in which cosmic objects are observed simultaneously in different forms of radiation.

One of the new events, GW170818, which was detected by the global network formed by the LIGO and Virgo observatories, was very precisely pinpointed in the sky. The position of the binary black holes, located 2.5 billion light-years from Earth, was identified in the sky with a precision of 39 square degrees. That makes it the next best localized gravitational-wave source after the GW170817 neutron star merger.

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A still image of a visualization of the merging black holes that LIGO and Virgo have observed so far. As the horizons of the black holes spiral together and merge, the emitted gravitational waves become louder (larger amplitude) and higher pitched (higher in frequency). The black holes that merge range from 7.6 solar masses up to 50.6 solar masses, with about 5% of the total mass lost during each merger.TERESITA RAMIREZ/GEOFFREY LOVELACE/SXS COLLABORATION/LIGO-VIRGO COLLABORATION

Dec 4, 2018,
Ethan Siegel Senior Contributor
Science

On September 14th, 2015, just days after LIGO first turned on at its new-and-improved sensitivity, a gravitational wave passed through Earth. Like the billions of similar waves that had passed through Earth over the course of its history, this one was generated by an inspiral, merger, and collision of two massive, ultra-distant objects from far beyond our own galaxy. From over a billion light years away, two massive black holes had coalesced, and the signal — moving at the speed of light — finally reached Earth.

But this time, we were ready. The twin LIGO detectors saw their arms expand-and-contract by a subatomic amount, but that was enough for the laser light to shift and produce a telltale change in an interference pattern. For the first time, we had detected a gravitational wave. Three years later, we've detected 11 of them, with 10 coming from black holes. Here's what we've learned.

There have been two "runs" of LIGO data: a first one from September 12, 2015 to January 19, 2016 and then a second one, at somewhat improved sensitivity, from November 30, 2016 to August 25, 2017. That latter run was, partway through, joined by the VIRGO detector in Italy, which added not only a third detector, but significantly improved our ability to pinpoint the location of where these gravitational waves occurred. LIGO is currently shut down right now, as it's undergoing upgrades that will make it even more sensitive, as it prepares to begin a new data-taking observing run in the spring of 2019.

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The Einstein Telescope, the European vision for a third-generation gravitational-wave detector, would consist of three interferometers formed into a triangle, with 10-kilometer-long arms. To minimize noise, it would be underground and cooled to around 10 K.

PHYSICS TODAY  - 01 OCTOBER 2018

Physics Today 71, 10, 25 (2018); https://doi.org/10.1063/PT.3.4041

Toni Feder

The promise of multimessenger astronomy drives the field, brings together scientific communities.

Word traveled fast when gravitational-wave detectors in the US and Europe announced the detection of a binary black hole merger on 14 September 2015. Then on 17 August 2017 the detection of merging neutron stars marked the beginning of multimessenger cosmic science with gravitational waves. (See Physics Today, April 2016, page 14, and December 2017, page 19.) Once that alert went out, dozens of telescopes were pointed toward the merger; radio astronomers are still watching it. Hundreds of papers followed, including one with more than 3500 authors. The excitement created by those detections has the gravitational-wave community chomping at the bit to lay plans for more powerful observatories.

Scientists in Europe put forward a design for the Einstein Telescope in 2011. (See Physics Today, September 2015, page 20.) Their US counterparts held off because NSF, which funded the bulk of the Laser Interferometer Gravitational-Wave Observatory (LIGO), encouraged them to score a detection before focusing on future observatories. So the US Cosmic Explorer design is less far along. But both future facilities would seek to increase sensitivity by at least a factor of 10.

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