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Thread: More black hole mergers detected

  1. #1
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    More black hole mergers detected

    phys.org

    An international team of scientists have detected ripples in space and time, known as gravitational waves, from the biggest known black-hole collision that formed a new black hole about 80 times larger than the Sun and from another three black-hole mergers.

    ...

    Professor Susan Scott, who is Leader of the General Relativity Theory and Data Analysis Group at ANU, said the team discovered the four collisions by re-analysing data from Advanced LIGO's first two observing runs.

    Scientists detected the event that formed the biggest known black hole from a merger of a binary system of two black holes on 29 July 2017. The event occurred about nine billion light years away.

    "This event also had black holes spinning the fastest of all mergers observed so far. It is also by far the most distant merger observed," Professor Scott said.

    The three other black-hole collisions were detected between 9 and 23 August 2017, were between three and six billion light years away and ranged in size for the resulting black holes from 56 to 66 times larger than our Sun.

    ...

    After the initial observing runs were concluded, scientists recalibrated and cleaned the collected data.

    "This increased the sensitivity of the detector network allowing our searches to detect more sources," Professor Scott said.

    "We have also incorporated improved models of the expected signals in our searches."
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    This threw me for a bit... somehow InSight went to Mars under my personal radar, now I missed that they started up the GW detectors again?!

    Three events in 2 weeks, that would keep them busy if detected "live" at that rate!
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    I saw no mention of any optical counterparts, which has happened for neutron star mergers -- BH mergers are not expected to have EM emissions.

    I'm curious if this means that there were no optical results or if no viewing took place during those extremely brief events?
    We know time flies, we just can't see its wings.

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    Quote Originally Posted by George View Post
    I saw no mention of any optical counterparts, which has happened for neutron star mergers -- BH mergers are not expected to have EM emissions.

    I'm curious if this means that there were no optical results or if no viewing took place during those extremely brief events?
    I haven't seen if they picked up the event at one or both LIGO sites. If they only picked it up at one, they may have almost no information about where in the sky it was. And, IIRC, even with two sites, location is only broadly defined. So they might not know what part of the sky it was in.

    This analysis is well after the fact. No one at the time knew to look, even if there is location information now.

    Still, I suspect they went back to those dates and looked for any optical events detected by Swift or other instruments at the appropriate time. But I haven't heard of any such analysis.
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    Quote Originally Posted by Swift View Post
    I haven't seen if they picked up the event at one or both LIGO sites. If they only picked it up at one, they may have almost no information about where in the sky it was. And, IIRC, even with two sites, location is only broadly defined. So they might not know what part of the sky it was in.
    Right. I know one of the reasons that there's a push to bring more LIGO-type sites online is that it would allow much more precise localization of gravitational wave events, precisely so that we can then bring optical and radio telescopes to bear.
    Conserve energy. Commute with the Hamiltonian.

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    Thanks Swift! The newer grav-wave detectors that will assist in refining their location is good news as well.
    We know time flies, we just can't see its wings.

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    LIGO-P1800307-v6: GWTC-1: A Gravitational-Wave Transient Catalog of Compact Binary Mergers Observed by LIGO and Virgo during the First and Second Observing Runs - List of gravitational wave observations The LIGO and Virgo teams have decided that eleven of their observed events are real gravitational-wave events. Of them, ten are observations of black-hole mergers and one an observation of a neutron-star merger. None of them were mergers of a neutron star and a black hole.

    The black holes had masses from 8 to 50 solar masses, and the more massive one of each pair was 1.2 to 1.8 times more massive than the less massive one. Their distances were from 320 to 2750 megaparsecs, or from 1 to 9 billion light years.

    The first one observed was GW150914, and it is more-or-less typical of black-hole mergers. Its two black holes had masses 36 and 31 times the Sun's mass. It radiated about 3 solar masses of gravitational-wave energy, with a peak luminosity roughly comparable to that of all the stars in the observable Universe. Its distance was 430 megaparsecs or 1.4 billion light years, it was not accompanied by any gamma-ray burst, and its galaxy is unknown.


    The only neutron-star merger of these events is GW170817, and its two neutron stars had masses 1.3 and 1.5 solar masses. It radiated some 0.04 solar masses of gravitational-wave energy, and it is not very certain whether the two neutron stars produced a neutron star or a black hole. About 1.7 seconds later, a gamma-ray burst was observed in essentially the same spot, GRB 150101B. The material spewed off by the merger was then observed in radio, infrared, visible light, ultraviolet, and X-rays, and its expansion speed is around 0.1 c, as one might expect from this kind of event. No neutrinos were observed, however. The stars' home galaxy was found, NGC 4993, in a search where the remnant's glow was an astronomical transient originally called SSS17a and then AT 2017gfo. That galaxy's distance is about 44 megaparsecs or 144 million light years, consistent with the G-wave estimated distance of 40 megaparsecs.

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    As G-wave observatories record more events, then we may be in a better position to test general relativity itself, to obtain better constraints on departures from it. Currently, the most successful tests so far have been post-Newtonian tests of it, along with equivalence-principle and fifth-force tests. The surviving alternatives to GR, like generalized Brans-Dicke, are essentially GR + extra stuff, like extra fields that make the gravitational constant vary.

    For lowest-order G-waves, GR predicts mass-quadrupole ones, while alternatives sometimes predict additional G-wave sources, like binding-energy dipoles. This effects how much G-wave energy is emitted as a function of orbit size, and thus the G-wave inspiral rate. I'll make some crude estimates.

    This is for n-pole radiation (0 = monopole, 1 = dipole, 2 = quadrupole), the G-wave amplitude is proportional to (a/T)n and the G-wave energy-emission rate is proportional to (an/Tn+1)2 for major axis a and period T.

    Since we are working in lowest order, we handle orbits in lowest order, using the Newtonian limit: Kepler's third law: T ~ a3/2. This gives us (amplitude) ~ a-n/2 and (energy emission rate) ~ a-n-3. The orbit energy ~ 1/a, giving the emission timescale t ~ an+2. Thus, (amplitude) ~ t-n/(2(n+2)) and (orbit period) ~ t-3/(2(n+2)).

    So for dipole emission: (amplitude) ~ t-1/6, (orbit period) ~ t-1/2
    For quadrupole emission: (amplitude) ~ t-1/4 and (orbit period) ~ t-3/8

    So one can get a clue about the nature of the G-wave emission by watching the inspiral amplitude and period.
    Last edited by lpetrich; 2018-Dec-14 at 03:38 PM. Reason: Fixed a typo

  9. #9
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    I'll mention why GR has mass quadrupoles as its lowest-order G-waves. A mass monopole is the total mass, of course. A mass dipole is the centroid position times the total mass. A mass magnetic dipole is angular momentum. All of these are conserved, with the centroid having the complication of moving at a constant velocity which may be nonzero. This is much like what one gets with electromagnetism. The lowest-order emitter is an electric dipole, with an electric monopole being total charge, something conserved.

    Another feature of G-waves is their polarization. That is familiar from electromagnetism, but with gravity, polarization is more complicated. An offset between two points gets changed, with the offset change being a linear function of the offset. In general, this requires a matrix, a linear object that takes a vector and returns another vector.

    The matrix can be decomposed into symmetric and antisymmetric parts. Symmetric: transposing it gives the same matrix. Antisymmetric: transposing it gives minus that matrix.
    Symmetric:
    Offset +x, change of offset +y
    Offset +y, change of offset +x
    Antisymmetric:
    Offset +x, change of offset +y
    Offset +y, change of offset -x

    Since space is three-dimensional, this offset matrix has 3*3 = 9 components. The 3 antisymmetric combinations of components give rotations, and are ignored. That leaves the 6 symmetric ones. These, in turn, can be decomposed with the help of the direction of travel of the wave.

    We get a hint from electromagnetism, where the possible directions of the electric field (and magnetic field) are longitudinal, along the direction of motion, and transverse, perpendicular to it. In actual fact, electromagnetic waves have transverse components but no longitudinal ones.

    For G-waves, polarization is more complicated, because of it being direction-to-direction, but the possible components are simpler. There is one all-longitudinal possible component, two longitudinal-transverse possible components, and three all-transverse possible components. Of the latter, one of them is a transverse scaling component and the other two transverse shear or transverse traceless (TT) components. General relativity has only the two TT components, while alternatives may have more components.

    One will have to observe an event with at least three G-wave detectors to test whether TT-only is a good fit, and at least six to get a full picture of G-wave polarization. But two should be enough if one decides to assume TT G-waves only.

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    lpetrich, I have nothing to add to this, but I am very happy to have you spell it out so clearly! Thanks.
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    Of interest here? Just came out, both articles. Hope it's good.

    https://arxiv.org/abs/1812.05121

    The GstLAL template bank for spinning compact binary mergers in the second observation run of Advanced LIGO and Virgo

    Debnandini Mukherjee, et al. (Submitted on 12 Dec 2018)

    We describe the methods used to construct the aligned-spin template bank of gravitational waveforms used by the GstLAL-based inspiral pipeline to analyze data from the second observing run of Advanced LIGO and Virgo. The bank expands upon the parameter space covered during the first observing run, including coverage for merging compact binary systems with total mass between 2 M ⊙ and 400 M ⊙ and mass ratios between 1 and 97.989. Thus the systems targeted include merging neutron star-neutron star systems, neutron star-black hole binaries, and black hole-black hole binaries expanding into the intermediate-mass range. Component masses less than 2 M ⊙ have allowed (anti-)aligned spins between 0.05 while component masses greater than 2 M ⊙ have allowed (anti-)aligned between 0.999 . The bank placement technique combines a stochastic method with a new grid-bank method to better isolate noisy templates, resulting in a total of 677,000 templates.

    =======

    https://arxiv.org/abs/1812.05363

    A New Method to Observe Gravitational Waves emitted by Core Collapse Supernovae

    P. Astone, P. Cerda-Duran, I. Di Palma, M. Drago, F. Muciaccia, C. Palomba, F. Ricci (Submitted on 13 Dec 2018)

    While gravitational waves have been detected from mergers of binary black holes and binary neutron stars, signals from core collapse supernovae, the most energetic explosions in the modern Universe, have not been detected yet. Here we present a new method to analyse the data of the LIGO, Virgo and KAGRA network to enhance the detection efficiency of this category of signals. The method takes advantage of a peculiarity of the gravitational wave signal emitted in the core collapse supernova and it is based on a classification procedure of the time-frequency images of the network data performed by a convolutional neural network trained to perform the task to recognize the signal. We validate the method using phenomenological waveforms injected in Gaussian noise whose spectral properties are those of the LIGO and Virgo advanced detectors and we conclude that this method can identify the signal better than the present algorithm devoted to select gravitational wave transient signal.
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