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Register a new userDetecting Gravitational Waves With Disparate Detector Responses: Two New Binary Black Hole Mergers
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We introduce a new technique to search for gravitational wave events from compact binary mergers that produce a clear signal only in a single gravitational wave detector, and marginal signals in other detectors. Such a situation can arise when the detectors in a network have different sensitivities, or when sources have unfavorable sky locations or orientations. We start with a short list of loud single-detector triggers from regions of parameter space that are empirically unaffected by glitches (after applying signal-quality vetoes). For each of these triggers, we compute evidence for astrophysical origin from the rest of the detector network by coherently combining the likelihoods from all detectors and marginalizing over extrinsic geometric parameters. We report the discovery of two new binary black hole (BBH) mergers in the second observing run of Advanced LIGO and Virgo (O2), in addition to the ones that were reported in Abbott et al. (2018) and Venumadhav et al. (2019). We estimate that the two events have false alarm rates of one in 19 years (60 O2) and one in 11 years (36 O2). One of the events, GW170817A, has primary and secondary masses $m_1^{rm src} = 56_{-10}^{+16} , M_odot$ and $m_2^{rm src} = 40_{-11}^{+10} , M_odot$ in the source frame. The existence of GW170817A should be very informative about the theoretically predicted upper mass gap for stellar mass black holes. Its effective spin parameter is measured to be $chi_{rm eff} = 0.5 pm 0.2$, which is consistent with the tendency of the heavier detected BBH systems to have large and positive effective spin parameters. The other event, GWC170402, will be discussed thoroughly in future work.
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We review the main physical processes that lead to the formation of stellar binary black holes (BBHs) and to their merger. BBHs can form from the isolated evolution of massive binary stars. The physics of core-collapse supernovae and the process of common envelope are two of the main sources of uncertainty about this formation channel. Alternatively, two black holes can form a binary by dynamical encounters in a dense star cluster. The dynamical formation channel leaves several imprints on the mass, spin and orbital properties of BBHs.
We study whether binary black hole template banks can be used to search for the gravitational waves emitted by general binary coalescences. To recover binary signals from noisy data, matched-filtering techniques are typically required. This is especially true for low-mass systems, with total mass $M lesssim 10 , M_odot$, which can inspiral in the LIGO and Virgo frequency bands for thousands of cycles. In this paper, we focus on the detectability of low-mass binary systems whose individual components can have large spin-induced quadrupole moments and small compactness. The quadrupole contributes to the phase evolution of the waveform whereas the compactness affects the merger frequency of the binary. We find that binary black hole templates (with dimensionless quadrupole $kappa=1$) cannot be reliably used to search for objects with large quadrupoles ($kappagtrsim 20$) over a wide range of parameter space. This is especially true if the general object is highly spinning and has a larger mass than its binary companion. A binary that consists of objects with small compactness could merge in the LIGO and Virgo frequency bands, thereby reducing its accumulated signal-to-noise ratio during the inspiraling regime. Template banks which include these more general waveforms must therefore be constructed. These extended banks would allow us to realistically search for the existence of new astrophysical and beyond the Standard Model compact objects.
We present results from a controlled numerical experiment investigating the effect of stellar density gas on the coalescence of binary black holes (BBHs) and the resulting gravitational waves (GWs). This investigation is motivated by the proposed stellar core fragmentation scenario for BBH formation and the associated possibility of an electromagnetic counterpart to a BBH GW event. We employ full numerical relativity coupled with general-relativistic hydrodynamics and set up a $30 + 30 M_odot$ BBH (motivated by GW150914) inside gas with realistic stellar densities. Our results show that at densities $rho gtrsim 10^6 - 10^7 , mathrm{g , cm}^{-3}$ dynamical friction between the BHs and gas changes the coalescence dynamics and the GW signal in an unmistakable way. We show that for GW150914, LIGO observations conclusively rule out BBH coalescence inside stellar gas of $rho gtrsim 10^7 , mathrm{g,cm}^{-3}$. Typical densities in the collapsing cores of massive stars are in excess of this density. This excludes the fragmentation scenario for the formation of GW150914.
In dense stellar environments, the merger products of binary black hole mergers may undergo additional mergers. These hierarchical mergers are predicted to have higher masses than the first generation of black holes made from stars. The components of hierarchical mergers are expected to have significant characteristic spins $chisim 0.7$. However, since the population properties of first-generation black holes are uncertain, it is difficult to know if any given merger is first-generation or hierarchical. We use observations of gravitational waves to reconstruct the binary black hole mass and spin spectrum of a population containing hierarchical merger events. We employ a phenomenological model that captures the properties of merging binary black holes from simulations of dense stellar environments. Inspired by recent work on the isolated formation of low-spin black holes, we include a zero-spin subpopulation. We analyze binary black holes from LIGO and Virgos first two observing runs, and find that this catalog is consistent with having no hierarchical mergers. We find that the most massive system in this catalog, GW170729, is mostly likely a first-generation merger, having a $4%$ probability of being a hierarchical merger assuming a $5 times 10^5 M_{odot}$ globular cluster mass. Using our model, we find that $99%$ of first-generation black holes in coalescing binaries have masses below 44 $M_{odot}$, and the fraction of binaries with near-zero spin is $0.051^{+0.156}_{-0.048}$ ($90%$ credible interval). Upcoming observations will determine if hierarchical mergers are a common source of gravitational waves.
In 2016, LIGO and Virgo announced the first observation of gravitational waves from a binary black hole merger, known as GW150914. To establish the confidence of this detection, large-scale scientific workflows were used to measure the events statistical significance. They used code written by the LIGO/Virgo and were executed on the LIGO Data Grid. The codes are publicly available, but there has not yet been an attempt to directly reproduce the results, although several analyses have replicated the analysis, confirming the detection. We attempt to reproduce the result presented in the GW150914 discovery paper using publicly available code on the Open Science Grid. We show that we can reproduce the main result but we cannot exactly reproduce the LIGO analysis as the original data set used is not public. We discuss the challenges we encountered and make recommendations for scientists who wish to make their work reproducible.
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