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  • richardmitnick 12:41 pm on August 21, 2021 Permalink | Reply
    Tags: "Addressing a Gap in Our Knowledge of Black Holes", , , , , , VIRGO Gravitational Wave interferometer   

    From AAS NOVA : “Addressing a Gap in Our Knowledge of Black Holes” 


    From AAS NOVA

    Artist’s by now iconic conception of two merging black holes similar to those detected by LIGO. Credit: Aurore Simonnet /Caltech MIT Advanced aLIGO(US)/Sonoma State University (US).

    One way for black holes to form is in supernovae, or the deaths of massive stars. However, our current knowledge of stellar evolution and supernovae suggests that black holes with masses between 55 and 120 solar masses can’t be produced via supernovae. Gravitational-wave signals from black hole mergers offer us an observational test of this “gap” in black hole masses.

    Black Hole Boundaries

    You need a massive star to go supernova to produce a black hole. Unfortunately, extremely massive stars explode so violently they leave nothing behind! This scenario can occur with pair-instability supernovae, which happens in stars with core masses between 40 and 135 solar masses. The “pair” in “pair-instability” refers to the electron–positron pairs that are produced by gamma rays interacting with nuclei in the star’s core. Energy is lost in this process, meaning that there’s less resistance to gravitational collapse.

    As the star collapses further, two things can happen. If the star is sufficiently massive, its core ignites in an explosion that tears the star apart, leaving no remnant. If the star is less massive, the core ignition causes the star to pulse and shed mass till it leaves the pair-production stage and its core collapses normally into black hole. The most massive black hole that can be produced in this scenario is roughly 55 solar masses, forming the lower end of the black hole mass gap.

    On the other side of the mass gap, it’s theoretically possible for certain massive stars to collapse normally without entering the pair-production state, thus evolving into black holes with masses greater than 120 solar masses. The unique thing about these massive stars is that they are low metallicity, containing practically no elements that are heavier than helium.

    So the bottom line is that we’re unlikely to observe any black holes with masses between 55 and 120 solar masses. But how can we test this prediction? Gravitational-wave signals are an option! Properties of merging black holes are coded into the gravitational waves produced by the merger, including the black hole masses. So, a recent study led by Bruce Edelman (University of Oregon (US)) looked at our current catalog of black hole merger signals to see if the mass gap would emerge from the data.

    Mind the Gap, If There Is a Gap

    Edelman and collaborators used two established model distributions of black hole masses to approach the problem. They also altered the models so the gap was explicitly allowed and so higher black hole masses could be explored without artificially inflating the rate of mergers above the gap. Edelman and collaborators then fit their models to data from 46 binary black hole mergers observed by the Laser Interferometer Gravitational-Wave Observatory and the Virgo interferometer.

    Masses in the Stellar Graveyard GWTC-2 plot v1.0 BY LIGO-Virgo. Credit: Frank Elavsky and Aaron Geller at Northwestern University(US)

    Caltech/MIT Advanced aLigo at Hanford, WA(US), Livingston, LA(US) and VIRGO Gravitational Wave interferometer, near Pisa(IT).

    Interestingly, the existence of the gap is rather ambiguous! One factor is the inclusion of the merger associated with the signal GW190521, which was likely a high mass merger whose component black holes straddle the mass gap. If the gap doesn’t exist, it’s possible that the unexpected black holes are formed by the merging of smaller black holes. On the whole, this result points to many avenues of study when it comes to pair-instability supernovae and black hole formation!


    “Poking Holes: Looking for Gaps in LIGO/Virgo’s Black Hole Population,” Bruce Edelman et al 2021 ApJL 913 L23.

    See the full article here .


    Please help promote STEM in your local schools.

    Stem Education Coalition


    AAS Mission and Vision Statement

    The mission of the American Astronomical Society is to enhance and share humanity’s scientific understanding of the Universe.

    The Society, through its publications, disseminates and archives the results of astronomical research. The Society also communicates and explains our understanding of the universe to the public.
    The Society facilitates and strengthens the interactions among members through professional meetings and other means. The Society supports member divisions representing specialized research and astronomical interests.
    The Society represents the goals of its community of members to the nation and the world. The Society also works with other scientific and educational societies to promote the advancement of science.
    The Society, through its members, trains, mentors and supports the next generation of astronomers. The Society supports and promotes increased participation of historically underrepresented groups in astronomy.
    The Society assists its members to develop their skills in the fields of education and public outreach at all levels. The Society promotes broad interest in astronomy, which enhances science literacy and leads many to careers in science and engineering.

    Adopted June 7, 2009

    The society was founded in 1899 through the efforts of George Ellery Hale. The constitution of the group was written by Hale, George Comstock, Edward Morley, Simon Newcomb and Edward Charles Pickering. These men, plus four others, were the first Executive Council of the society; Newcomb was the first president. The initial membership was 114. The AAS name of the society was not finally decided until 1915, previously it was the “Astronomical and Astrophysical Society of America”. One proposed name that preceded this interim name was “American Astrophysical Society”.

    The AAS today has over 7,000 members and six divisions – the Division for Planetary Sciences (1968); the Division on Dynamical Astronomy (1969); the High Energy Astrophysics Division (1969); the Solar Physics Division (1969); the Historical Astronomy Division (1980); and the Laboratory Astrophysics Division (2012). The membership includes physicists, mathematicians, geologists, engineers and others whose research interests lie within the broad spectrum of subjects now comprising contemporary astronomy.

    In 2019 three AAS members were selected into the tenth anniversary class of TED Fellows.

    The AAS established the AAS Fellows program in 2019 to “confer recognition upon AAS members for achievement and extraordinary service to the field of astronomy and the American Astronomical Society.” The inaugural class was designated by the AAS Board of Trustees and includes an initial group of 232 Legacy Fellows.

  • richardmitnick 4:20 pm on August 24, 2020 Permalink | Reply
    Tags: "A Lopsided Merger", , , , , , , , , The detection of GW190412 shows just how much more we have to learn from merging black holes., The LIGO and Virgo detectors have spotted the merger of a remarkably asymmetric black hole duo whose components weigh 8 and 30 solar masses respectively., Until now most detections showed pairs of black holes with roughly comparable masses., VIRGO Gravitational Wave interferometer   

    From “Physics”: “A Lopsided Merger” 

    About Physics

    From “Physics”

    August 24, 2020
    Stephen Taylor
    Department of Physics and Astronomy, Vanderbilt University

    The merger of two black holes with significantly different masses allows researchers to better characterize black hole parameters and to perform new tests of general relativity.

    APS/Alan Stonebraker.
    Figure 1: The LIGO and Virgo detectors have spotted the merger of a remarkably asymmetric black hole duo, whose components weigh 8 and 30 solar masses, respectively.

    Since LIGO and Virgo first observed the merger of two black holes in September 2015 [1*] (see Viewpoint: The First Sounds of Merging Black Holes), gravitational-wave detection has become a regular occurrence. Yet the warped Universe continues to yield a bounty of diverse discoveries. Now, the LIGO and Virgo collaborations report that, just two weeks into their third observation run, the LIGO and Virgo detectors spotted a black hole merger, dubbed GW190412, that was remarkably different from those previously measured: With 8 and 30 solar masses, respectively, this “Laurel & Hardy” duo is the first truly asymmetric black hole binary system ever spotted (Fig. 1) [2].

    MIT /Caltech Advanced aLigo

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

    VIRGO Gravitational Wave interferometer, near Pisa, Italy

    VIRGO Gravitational Wave interferometer, near Pisa, Italy

    The detection of such an asymmetric binary has allowed a trove of new scientific possibilities, from placing strong constraints on the black holes’ spins, to suggesting new astrophysical scenarios for the formation of such lopsided systems, to testing Einstein’s theory of general relativity in previously unexplored regimes.

    Gravitational waves naturally arise in the theory of general relativity, which connects gravity to the warping of spacetime in the presence of matter. A pair of compact objects like neutron stars or black holes churn up spacetime as they orbit one another, inspiral, and eventually merge, creating spacetime ripples that radiate outwards at the speed of light. Billions of years later, the waves reach our gravitational-wave detectors, where they slightly deform the detector’s orthogonal, kilometers-long arms. Such a pattern of deformations encodes the merging system’s dynamics, including the orbital geometry and the properties of each neutron star or black hole.

    Until now, most detections showed pairs of black holes with roughly comparable masses: Even the most asymmetric detected mergers involved mass ratios of less than 2 [3]. As predicted by general relativity, the gravitational-wave signal in these symmetric cases is dominated by a single frequency—the second harmonic of the binary orbital frequency [4]. Furthermore, the signals from all detections but two (GW151226 and GW170729) have been consistent with the binary having an effective spin of zero. This parameter is a mass-weighted sum of each black hole’s spin component perpendicular to the orbital plane. A vanishing effective spin for same-weight black holes means that the two bodies either aren’t spinning or they are spinning in opposite directions. A nonzero effective spin would significantly affect the merger dynamics.

    At first glance, GW190412 is in line with previous discoveries—it is a pair of black holes with individual masses consistent with previous detections and with the black hole formation pathways considered by theory. At the same time, the asymmetry of the binary’s two masses makes it a beautifully divergent system compared to anything seen before (Video 1). LIGO and Virgo used a variety of different waveform models to determine that one of the black holes was about 3.5 times as massive as the other. In accord with general relativity, this asymmetry means that, in addition to the main second-harmonic emission frequency, higher overtones of gravitational-wave emission, specifically the mode at 3 times the binary orbital frequency, were detectable. Gravitational waves have often been referred to as the music of the cosmos, and in this case the analogy is quite apt: The collision of these black holes out in the vastness of the Universe produced a signal containing a “perfect fifth”—a musical interval, like that between the G and C notes, corresponding to two frequencies with a 3:2 ratio [5].

    Video 1: Numerical simulations of the inspiral and merger of two black holes with a mass ratio of 3.5 are consistent with the GW190412 observation. N. Fischer, H. Pfeiffer, A. Buonanno/Max Planck Institute for Gravitational Physics/Simulating eXtreme Spacetimes project [SXS].

    The lopsided black hole duo allowed the researchers to vet general relativity in previously unexplored regimes. Namely, by testing the theory’s predictions for the multipole moments associated with the higher-harmonic emissions of an asymmetric merger. All parameters associated with deviations from general relativity were consistent with being zero—even as the nuances of his theory are tested in novel ways, Einstein continues to be correct.

    By fitting the more complex signal containing the higher overtones, the researchers drastically improved, compared to previous detections, constraints on system parameters such as the binary’s distance from Earth, its mass ratio, and the black holes’ spin. In fact, the LIGO and Virgo scientists report the most precise determination of a black hole spin ever extracted from gravitational-wave data, finding that the larger black hole’s event horizon spins at about 43% of the speed of light. GW190412 is now the third signal to show evidence of a nonzero spin [6, 7], showing that there is great potential to determine this feature cleanly and directly using gravitational-wave analysis. Other common spin characterization techniques are indirect, as they infer a black hole’s spin from the x-ray emission of material falling into the black hole and must thus rely on hard-to-test models of accretion dynamics. For GW190412, the signal analysis also hinted at a difficult-to-observe effect: a mild precession of the orbital spins suggesting that the spins of each black hole were not aligned with the axis of the orbital motion.

    GW190412’s peculiarity among its peers—the measurable spin and the asymmetric masses—makes it a valuable addition that informs black hole demographics. From previously detected mergers, researchers showed that the probability of finding black hole binaries with certain sizes can be described by a power law. The inclusion of this asymmetric duo yields much tighter constraints on the distribution of expected mass ratios. While previous analysis suggests that a system at least as asymmetric as GW190412 should only arise in 1.7% of the cases [8], the population distribution law that accounts for the new detection revises that probability to 25%.

    The detection of GW190412 shows just how much more we have to learn from merging black holes. An important result would be the extraction of even higher overtones. By breaking important parameter degeneracies, such overtones would improve measurements of the distances of black hole binaries, allowing them to be used as “dark sirens” for inferring the local expansion rate of the Universe [9]. This approach could help researchers settle the debate over the Hubble constant—for which cosmic microwave background and supernovae measurements deliver conflicting values (see Feature: Cosmologists Can’t Agree on the Hubble Constant). And, detections of other asymmetric systems and their spins may shed light on possible formation channels. The misaligned spins of the GW190412 black holes may suggest, for instance, that the duo doesn’t come directly from a stellar binary. Instead, one of them could be the product of a previous black hole merger [10]. A better characterization of black hole spins and of the environment in which the merger took place could deliver conclusive evidence for this hypothesis. Surely, we can expect that upcoming detections will deliver many more insights into the smorgasbord of scenarios that form coalescing black hole pairs. Indeed, during the writing of this Viewpoint, researchers reported an even more asymmetric system, called GW190814, which hosts either the heaviest neutron star or the lightest black hole ever discovered [11].

    References, with links.

    B. P. Abbott et al. (LIGO Scientific Collaboration and Virgo Collaboration), “Observation of gravitational waves from a binary black hole merger,” Phys. Rev. Lett. 116, 061102 (2016).
    R. Abbott et al. (LIGO Scientific Collaboration and Virgo Collaboration), “GW190412: Observation of a binary-black-hole coalescence with asymmetric masses,” Phys. Rev. D 102, 043015 (2020).
    B. P. Abbott et al. (LIGO Scientific Collaboration and Virgo Collaboration), “Binary black hole population properties inferred from the first and second observing runs of advanced LIGO and advanced Virgo,” Astrophys. J. 882, L24 (2019).
    L. Blanchet, “Gravitational radiation from post-Newtonian sources and inspiralling compact binaries,” Living Rev. Relativ. 17, 2 (2014).
    C. Berry, “GW190412—A new flavour of binary black hole,” https://cplberry.com/2020/04/18/gw190412.
    B. P. Abbott et al. (LIGO Scientific Collaboration and Virgo Collaboration), “GW151226: Observation of gravitational waves from a 22-solar-mass binary black hole coalescence,” Phys. Rev. Lett. 116, 241103 (2016).
    K. Chatziioannou et al., “On the properties of the massive binary black hole merger GW170729,” Phys. Rev. D 100, 104015 (2019).
    B. P. Abbott et al. (LIGO Scientific Collaboration and Virgo Collaboration), “GWTC-1: A gravitational-wave transient catalog of compact binary mergers observed by LIGO and Virgo during the first and second observing runs,” Phys. Rev. X 9, 031040 (2019).
    M. Soares-Santos et al. (LIGO Scientific Collaboration and Virgo Collaboration), “First measurement of the hubble constant from a dark standard siren using the dark energy survey galaxies and the LIGO/Virgo binary–black-hole merger GW170814,” Astrophys. J. 876, L7 (2019).
    D. Gerosa, “Astrophysical implications of GW190412 as a remnant of a previous black-hole merger,” arXiv:2005.04243.
    R. Abbott et al. (LIGO Scientific Collaboration and Virgo Collaboration), “GW190814: Gravitational waves from the coalescence of a 23 solar mass black hole with a 2.6 solar mass compact object,” Astrophy. J. 896, L44 (2020).

    *For all notated references see the full article.

    See the full article here .


    Please help promote STEM in your local schools.

    Stem Education Coalition

    Physicists are drowning in a flood of research papers in their own fields and coping with an even larger deluge in other areas of physics. How can an active researcher stay informed about the most important developments in physics? Physics highlights a selection of papers from the Physical Review journals. In consultation with expert scientists, the editors choose these papers for their importance and/or intrinsic interest. To highlight these papers, Physics features three kinds of articles: Viewpoints are commentaries written by active researchers, who are asked to explain the results to physicists in other subfields. Focus stories are written by professional science writers in a journalistic style and are intended to be accessible to students and non-experts. Synopses are brief editor-written summaries. Physics provides a much-needed guide to the best in physics, and we welcome your comments (physics@aps.org).

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