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تسجيل مستخدم جديدObserving an intermediate mass black hole GW190521 with minimal assumptions
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On May 21, 2019 Advanced LIGO and Advanced Virgo detectors observed a gravitational-wave transient GW190521, the heaviest binary black-hole merger detected to date with the remnant mass of 142$,$M$_odot$ that was published recently. This observation is the first strong evidence for the existence of intermediate-mass black holes. The significance of this observation was determined by the coherent WaveBurst (cWB) - search algorithm, which identified GW190521 with minimal assumptions on its source model. In this paper, we demonstrate the capabilities of cWB to detect binary black holes without use of the signal templates, describe the details of the GW190521 detection and establish the consistency of the model-agnostic reconstruction of GW190521 by cWB with the theoretical waveform model of a binary black hole.
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The gravitational wave event GW190521 involves the merger of two black holes of $sim 85text{M}_odot$ and $sim 66text{M}_odot$ forming an intermediate-mass black hole (IMBH) of mass $sim 142text{M}_odot$. Both progenitors are challenging to explain wi
thin standard stellar evolution as they belong to the upper black-hole mass gap. We propose a dynamical formation pathway for this IMBH based on multiple hierarchical mergers of progenitors in the core of a dense star cluster. We identified such scenarios from analysis of a set of 58 direct N-body simulations using NBODY6-gpu. In one of our canonical runs aimed at describing the evolution of a star cluster with $N=10^5$ stars and typical globular cluster properties, we observe a stellar black hole undergoing a chain of seven binary mergers in 6 Gyr, attaining a final mass of $97.8text{M}_odot$. We discuss the dynamical interactions that lead to the final IMBH product, as well as the evolution of the black hole population in that simulation. From the analysis of all simulations in our dataset we observe additional smaller chains, tentatively inferring that an IMBH formation through chain mergers is expected in the lifetime of a typical (i.e. median mass) globular cluster with probability $0.01 lesssim p lesssim 0.1$. Using this order-of-magnitude estimate and comoving star formation rates we show our results are broadly consistent with the mean rate implied by GW190521, and we discuss implications for future gravitational wave detections of IMBHs.
The population analysis of compact binaries involves the reconstruction of some of the gravitational wave (GW) signal parameters, such as, the mass and the spin distribution, that gave rise to the observed data. This article introduces VAMANA, which
reconstructs the binary black hole population using a mixture model and facilitates excellent density measurement as informed by the data. VAMANA uses a mixture of weighted Gaussians to reconstruct the chirp mass distribution. We expect Gaussian mixtures to provide flexibility in modeling complex distributions and enable us in capturing details in the astrophysical chirp mass distribution. Each of the Gaussian in the mixture is combined with another Gaussian and a power-law to simultaneously model the spin component aligned with the orbital angular momentum and the mass ratio distribution, thus also allowing us to capture their variation with the chirp mass. Additionally, we can also introduce broadband smoothing by restricting the Gaussian mixture to lie within a threshold distance of a predefined reference chirp mass distribution. Using simulated data we show the robustness of our method in reconstructing complex populations for a large number of observations. We also apply our method to the publicly available catalog of GW observations made during LIGOs and Virgos first and second observation runs and present the reconstructed mass, spin distribution, and the estimated merger rate of binary black holes.
A small cluster of massive stars residing in the Galactic center, collectively known as IRS13E, is of special interest due to its close proximity to Sgr A* and the possibility that an embedded intermediate-mass black hole (IMBH) binds its member star
s. It has been suggested that colliding winds from two member stars, both classified as Wolf-Rayet type, are responsible for the observed X-ray, infrared and radio emission from IRS13E. We have conducted an in-depth study of the X-ray spatial, temporal and spectral properties of IRS13E, based on 5.6 Ms of ultra-deep Chandra observations obtained over 20 years. These X-ray observations show no significant evidence for source variability. We have also explored the kinematics of the cluster members, using Keck near-infrared imaging and spectroscopic data on a 14-yr baseline that considerably improve the accuracy of stars proper motions. The observations are interpreted using 3-dimensional hydrodynamical simulations of colliding winds tailored to match the physical conditions of IRS13E, leading us to conclude that the observed X-ray spectrum and morphology can be well explained by the colliding wind scenario, in the meantime offering no support for the presence of a putative IMBH. An IMBH more massive than a few $10^3{rm~M_odot}$ is also strongly disfavored by the stellar kinematics.
We evolve high-mass disks of mass $15$-$50M_odot$ orbiting a $50M_odot$ spinning black hole in the framework of numerical relativity. Such high-mass systems could be an outcome during the collapse of rapidly-rotating very-massive stars. The massive d
isks are dynamically unstable to the so-called one-armed spiral-shape deformation with the maximum fractional density-perturbation of $delta rho/rho gtrsim 0.1$, and hence, high-amplitude gravitational waves are emitted. The waveforms are characterized by an initial high-amplitude burst with the frequency of $sim 40$-$50$ Hz and the maximum amplitude of $(1$-$10)times 10^{-22}$ at the hypothetical distance of 100 Mpc and by a subsequent low-amplitude quasi-periodic oscillation. We illustrate that the waveforms in our models with a wide range of the disk mass resemble that of GW190521. We also point out that gravitational waves from rapidly-rotating very-massive stars can be the source for 3rd-generation gravitational-wave detectors for exploring the formation process of rapidly-spinning high-mass black holes of mass $sim 50$-$100M_odot$ in an early universe.
With the black hole mass function (BHMF; assuming an exponential cutoff at a mass of $sim 40,M_odot$) of coalescing binary black hole systems constructed with the events detected in the O1 run of the advanced LIGO/Virgo network, Liang et al.(2017) pr
edicted that the birth of the lightest intermediate mass black holes (LIMBHs; with a final mass of $gtrsim 100,M_odot$) is very likely to be caught by the advanced LIGO/Virgo detectors in their O3 run. The O1 and O2 observation run data, however, strongly favor a cutoff of the BHMF much sharper than the exponential one. In this work we show that a power-law function followed by a sudden drop at $sim 40,M_odot$ by a factor of $sim $a few tens and then a new power-law component extending to $geq 100M_odot$ are consistent with the O1 and O2 observation run data. With this new BHMF, quite a few LIMBH events can be detected in the O3 observation run of advanced LIGO/Virgo. The first LIMBH born in GW190521, an event detected in the early stage of the O3 run of advanced LIGO/Virgo network, provides additional motivation for our hypothesis.
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