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GW190521: a Binary Black Hole Merger with a Total Mass of 150 M

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GW190521: A Binary Black Hole Merger with a Total Mass of 150 M☉ LIGO Scientific Collaboration and Virgo Collaboration Discovery paper - Phys. Rev. Lett. 125, 101102 (2020) https://dcc.ligo.org/LIGO-P2000020/public (Astro)physical implications - Astrophys. J. Lett. 900, L13 (2020) https://dcc.ligo.org/LIGO-P2000021/public Data release - https://dcc.ligo.org/LIGO-P2000158/public 1 Gravitational waves Gravitational waves (GWs) are ripples in spacetime caused by accelerating masses Some of the best sources of GWs are colliding black holes and neutron stars Incoming GWs are measured by the Earth- based detector network 2 LIGO Livingston LIGO-Virgo observing runs Average sensitive distance to optimally oriented NS-NS source https://dcc.ligo.org/LIGO-P1200087/public https://dcc.ligo.org/LIGO-G1901322/public 3 GW190521 in LIGO Hanford, LIGO Livingston, Virgo 4 GW190521 signal morphology GW150914 GW150914 Hanford (H1) Livingston (L1) GW150914 Normalized energy Normalized energy Normalized energy GW190521 GW190521 GW190521 Hanford (H1) Livingston (L1) Virgo (V1) 5 LIGO-Virgo sensitivity to binary black hole mergers ● LIGO-Virgo detectors measure the stretching and squeezing of spacetime caused by a passing Inspiral GW Ringdown Merger ● Higher mass binaries correspond to lower frequency GW150914 GW emission GW190521 less sensitive GW170729 ● GW signals from highest mass binaries in LIGO-Virgo detectors more sensitive lack the inspiral (characteristic “chirp”) 6 GW190521 in the context of all LIGO-Virgo detections Supermassive Black Holes ● So far, LIGO-Virgo has reported 12 binary black hole mergers 105 ● The final (remnant) mass of GW190521 is Intermediate Mass Black Holes well above 100 solar masses, marking it as the first intermediate mass black hole (IMBH) detected by LIGO-Virgo ● The primary mass lies outside of the stellar mass graveyard (> 65 solar masses) graveyard Stellar mass LIGO-Virgo component mass LIGO-Virgo remnant mass Solar 7 masses LIGO-Virgo IMBH search sensitivity LIGO-Virgo sensitive distance in O1-O2 Sensitive distance (Dsen): a measure of how well we can see a given source type on average IMBH rate upper limit from O1-O2: Astrophysical merger rate density of GW190521-like events: (based on one detected event) GW190521, Dsen = 1.7 Gpc (in O3) 8 Searches for GW transients in early O3 Coherent Wave Burst (cWB) GstLAL, MBTA, PyCBC, SPIIR Search for generic transient signals Model-based searches for Cross-correlation in the time-frequency (wavelet) domain compact binary coalescences (CBC) x x 9 Low-latency discovery of GW190521 Pipeline Search type GW190521 results cWB Generic transient Detected with FAR < 1 / 28 yr GstLAL CBC Detected with FAR above alert threshold MBTA CBC Not detected PyCBC Live CBC Detected with FAR ~ 1 / 8 yr FAR: rate of false alarms SPIIR CBC Detected with FAR above alert threshold ranked higher than the candidate in a given search 6-hr-latency localization using dominant-multipole waveforms w/o precession: GraceDB page 10 GW190521 rediscovery with offline (archival) search ● Improve calibration of the data Pipeline Search type GW190521 results ● Investigate data quality issues, cWB Generic transient Detected with veto contaminated data FAR ~ 1 / 4900 yr ● Different analysis GstLAL CBC Detected with ○ Candidate ranking FAR ~ 1 / 829 yr ○ Analyzed interferometers ○ Searched mass range PyCBC CBC Detected* with ○ Duration of data used FAR ~ 1 / yr to measure the FAR * Masses/spins of GW190521’s components at the limit of PyCBC’s O3 search range 11 A special signal ● Very short duration (~0.1 s) ??? Massive source ● Low peak frequency (~60 Hz) ● Standard scenario: quasi-circular BBH merger GW190521 Very short inspiral signal ● Alternative scenarios may be explored: Eccentric Binary, Head-on merger GW150914 Cosmic String 12 Inferring the source properties, assuming quasi-circular BBH Compare the data to waveform templates for binary black holes: ● Three different template models (NRSurPHM, SEOBPHM, IMRPhenomPHM) ● Numerical simulations ● All include: higher-order multipoles and orbital precession All models lead to consistent source parameters: ● Discovery paper: results using NRSurPHM. ● Properties & Implications paper: all results. 13 Masses ● Most massive binary ever detected ● Most massive colliding black holes ● Masses within the pair instability supernova gap 14 90% credible regions Spins Orbital angular momentum BH1 BH2 ● Spin magnitude (cosmic censorship) ● Orientation (or tilt) 15 Spins Orbital angular momentum BH1 BH2 BH1 BH2 ● Spin magnitude (cosmic censorship) ● Orientation (or tilt) 16 Spins Orbital angular momentum ● Spin magnitude (cosmic censorship) ● tilt = 0: constant orbital plane 17 Spins Orbital angular momentum ● Spin magnitude (cosmic censorship) ● tilt = 0: constant orbital plane ● tilt not 0: orbital precession 18 Spins ● Spin magnitudes: ○ Strong support at ○ Also at ● Support for misaligned spins 19 Distance and inclination angle ● GWs: sum of many emission modes (multipoles) ● : : Loud signal, only quadrupole multipole ● : Weaker signal, many multipoles. ● No statistical evidence for modes beyond quadrupole Low latency (6h) estimate: No precession or higher multipoles 20 Final black hole: mass and spin ● The binary merger produces a final BH ● No support for ○ Most massive BH observed via GWs ○ First conclusive observation of an intermediate-mass black hole ● Final spin consistent with the remnant BH of a non-spinning binary 21 Looking at the final black hole: Testing GR ● Two ways to measure the final black hole properties ○ Measure the binary parameters + infer the remnant 22 Looking at the final black hole: Testing GR ● Two ways to measure the final black hole properties ○ Measure the binary parameters + infer the remnant ○ Directly look at the remnant signal ● Black holes characterised by mass and spin ● Material ● Mass ● Size ● Spin 23 Looking at the final black hole: Testing GR ● Two ways to measure the final black hole properties ○ Measure the binary parameters + infer the remnant ○ Directly look at the remnant signal ● Black holes characterised by mass and spin ● Comparing both measurements: test of General Relativity ● Both agree Redshifted mass 24 Astrophysical implications IMBHs: the missing link between stellar and supermassive black holes 25 Primary component in the pair-instability (PI) mass gap Efficient pair production in massive stars drives (pulsational) pair instability: opens a gap in ~ 65 - 120 M⊙ range Large uncertainties on mass gap boundaries: ● Nuclear reaction rates, e.g. 12C(α,γ)16O ● Collapse of hydrogen envelope ● Stellar rotation ● Convection model CHALLENGE FOR STELLAR EVOLUTION 26 Dynamical scenarios: Hierarchical mergers ● Multiple mergers of black holes are possible in dense star clusters and galactic nuclei: ○ mass in the PI gap First-generation (1g) black holes ○ consistent with χ ~ 0 and large χ eff p ○ uncertain rates credit: NASA / ESA / Hubble ● GW170729 final black hole in the PI gap Second generation ● Results of hierarchical Bayesian inference on (2g) a population model depend strongly on assumed properties of 1g mergers → no conclusive evidence for GW190521 to be a 2g merger 27 Dynamical scenarios: Stellar mergers in young star clusters Giant star ● Merger of a massive star with He core and a with He core MS main sequence (MS) star produces a star star with oversized H envelope ● It could collapse to a black hole in the PI gap credit: NASA, ESA, F. Paresce, R. O’Connell ● In a dense star cluster, the black hole acquires companions dynamically: Stellar merger product ○ primary mass in PI gap ○ consistent with χeff~ 0 and large χp Black hole in PI gap ○ uncertain evolution of stellar collision product ○ rate < 10% of all detectable BBH mergers Credit: Ugo N. Di Carlo 28 Dynamical scenarios: Active galactic nucleus (AGN) disks ● Gas torques from AGN disks favor black hole pairing and hierarchical mergers ○ primary mass in PI gap Credit: NASA/JPL-Caltech ○ large χ p ○ uncertain rates ○ predicts an EM counterpart ● Candidate optical counterpart in AGN J124942.3+344929 (Graham et al. 2020) AGN redshift: z = 0.438 GW190521 redshift: z = 0.82 (+0.28, -0.34) (90% CI) Credit: Imre Bartos 29 Astrophysical implications: Summary ISOLATED BINARY EVOLUTION: DYNAMICAL SCENARIOS: ● Main issue: primary mass in the PI gap ● All dynamical scenarios we considered might produce BBHs ● Uncertainties on stellar evolution with mass in the PI gap and large χp leave open the possibility that the lower edge of the mass gap is >65 M⊙ ● Rates, masses and spins uncertain and model dependent CHALLENGE FOR DYNAMICAL AND STELLAR EVOLUTION MODELS 30 Alternative scenarios HEAD-ON COLLISION or STRONG GRAVITATIONAL LENSING by HIGHLY ECCENTRIC MERGER: galaxies or galaxy clusters: ○ Extremely rare: < 1% of BBH mergers ○ Low expected lensing rate -3 -4 in star clusters ○ Low optical depth (~10 - 10 ) ○ Absence of an identifiable multi-image counterpart CORE-COLLAPSE SUPERNOVA: → no evidence in favor of strong lensing ○ Inconsistent GW signal morphology ○ Non-detection of neutrino signal COSMIC STRINGS: ○ Data strongly favor BBH over cosmic string PRIMORDIAL BLACK HOLES: cusp or kink model: ○ Exciting scenario for dark matter ○ Difficult to constrain 31 Summary and Conclusions ● GW190521 is well described by GR waveforms. ● The GW signal is consistent with a BBH merger source, with total mass of 150 M⊙ . ● The final merged (remnant) black hole is an Intermediate Mass Black Hole (IMBH). ● The more massive of the two BHs in binary is ~ 85 M⊙ , in the Pair Instability
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  • Stochastic Gravitational Wave Backgrounds

    Stochastic Gravitational Wave Backgrounds

    Stochastic Gravitational Wave Backgrounds Nelson Christensen1;2 z 1ARTEMIS, Universit´eC^oted'Azur, Observatoire C^oted'Azur, CNRS, 06304 Nice, France 2Physics and Astronomy, Carleton College, Northfield, MN 55057, USA Abstract. A stochastic background of gravitational waves can be created by the superposition of a large number of independent sources. The physical processes occurring at the earliest moments of the universe certainly created a stochastic background that exists, at some level, today. This is analogous to the cosmic microwave background, which is an electromagnetic record of the early universe. The recent observations of gravitational waves by the Advanced LIGO and Advanced Virgo detectors imply that there is also a stochastic background that has been created by binary black hole and binary neutron star mergers over the history of the universe. Whether the stochastic background is observed directly, or upper limits placed on it in specific frequency bands, important astrophysical and cosmological statements about it can be made. This review will summarize the current state of research of the stochastic background, from the sources of these gravitational waves, to the current methods used to observe them. Keywords: stochastic gravitational wave background, cosmology, gravitational waves 1. Introduction Gravitational waves are a prediction of Albert Einstein from 1916 [1,2], a consequence of general relativity [3]. Just as an accelerated electric charge will create electromagnetic waves (light), accelerating mass will create gravitational waves. And almost exactly arXiv:1811.08797v1 [gr-qc] 21 Nov 2018 a century after their prediction, gravitational waves were directly observed [4] for the first time by Advanced LIGO [5, 6].