Accurate models of gravitational waves from merging black holes are necessary for detectors to observe as many events as possible while extracting the maximum science. Near the time of merger, the gravitational waves from merging black holes can be computed only using numerical relativity. In this paper, we present a major update of the Simulating eXtreme Spacetimes (SXS) Collaboration catalog of numerical simulations for merging black holes. The catalog contains 2018 distinct configurations (a factor of 11 increase compared to the 2013 SXS catalog), including 1426 spin-precessing configurations, with mass ratios between 1 and 10, and spin magnitudes up to 0.998. The median length of a waveform in the catalog is 39 cycles of the dominant = m = 2 gravitational-wave mode, with the shortest waveform containing 7.0 cycles and the longest 351.3 cycles. We discuss improvements such as correcting for moving centers of mass and extended coverage of the parameter space. We also present a thorough analysis of numerical errors, finding typical truncation errors corresponding to a waveform mismatch of ∼ 10 −4 . The simulations provide remnant masses and spins with uncertainties of 0.03% and 0.1% (90 th percentile), about an order of magnitude better than analytical models for remnant properties. The full catalog is publicly available at https://www.black-holes.org/waveforms . black holes and of the surrounding spacetime [31,32]. Simulations have also been used for visualizations of curved spacetime [33][34][35][36][37][38][39][40], investigations of spin quantities [41], and the relaxation of spacetime to the Kerr solution following merger [42][43][44]. The motion of the black hole horizons and horizon curvature quantities have been used to explore eccentric dynamics [45][46][47][48], spin precession [49][50][51][52], and the first law of binary black hole mechanics [53][54][55][56][57]. These in turn have been compared to analytic post-Newtonian and self-force approximations (see also [58][59][60]), mapping out the bounds of validity of these approximations.A key application of BBH simulations is the accurate modeling of gravitational waves emitted by these systems during their late inspiral, merger, and final ringdown. Waveforms extracted from BBH simulations are essential for analyzing observed gravitational-wave signals from black hole binaries. Indeed, all BBH observations by LIGO and Virgo were analyzed using waveform families that rely on numerical relativity for their construction, most notably effective-one-body waveform models [61-65] and phenomenological waveform models [66][67][68]. Numerical simulations are also central in validating such waveform models [69][70][71][72][73][74][75][76], and were used to validate GW searches [77][78][79]. Waveforms from numerical relativity are also used directly in parameter estimation [80,81], to construct template banks [82], and to construct waveform families without intermediate analytical models, using methods such as reduced order modeling [83][84][85][86]. Today's simula...

We present accurate fits for the remnant properties of generically precessing binary black holes, trained on large banks of numerical-relativity simulations. We use Gaussian process regression to interpolate the remnant mass, spin, and recoil velocity in the 7-dimensional parameter space of precessing black-hole binaries with mass ratios q ≤ 2, and spin magnitudes χ1, χ2 ≤ 0.8. For precessing systems, our errors in estimating the remnant mass, spin magnitude, and kick magnitude are lower than those of existing fitting formulae by at least an order of magnitude (improvement is also reported in the extrapolated region at high mass ratios and spins). In addition, we also model the remnant spin and kick directions. Being trained directly on precessing simulations, our fits are free from ambiguities regarding the initial frequency at which precessing quantities are defined. We also construct a model for remnant properties of aligned-spin systems with mass ratios q ≤ 8, and spin magnitudes χ1, χ2 ≤ 0.8. As a byproduct, we also provide error estimates for all fitted quantities, which can be consistently incorporated into current and future gravitationalwave parameter-estimation analyses. Our model(s) are made publicly available through a fast and easy-to-use Python module called surfinBH. arXiv:1809.09125v2 [gr-qc]

We present a method of calculating the strong-field gravitational lensing caused by many analytic and numerical spacetimes. We use this procedure to calculate the distortion caused by isolated black holes and by numerically evolved black hole binaries. We produce both demonstrative images illustrating details of the spatial distortion and realistic images of collections of stars taking both lensing amplification and redshift into account. On large scales the lensing from inspiraling binaries resembles that of single black holes, but on small scales the resulting images show complex and in some cases self-similar structure across different angular scales.

We introduce a new relativistic astrophysics code, SpECTRE, that combines a discontinuous Galerkin method with a task-based parallelism model. SpECTRE's goal is to achieve more accurate solutions for challenging relativistic astrophysics problems such as core-collapse supernovae and binary neutron star mergers. The robustness of the discontinuous Galerkin method allows for the use of high-resolution shock capturing methods in regions where (relativistic) shocks are found, while exploiting high-order accuracy in smooth regions. A taskbased parallelism model allows efficient use of the largest supercomputers for problems with a heterogeneous workload over disparate spatial and temporal scales. We argue that the locality and algorithmic structure of discontinuous Galerkin methods will exhibit good scalability within a task-based parallelism framework. We demonstrate the code on a wide variety of challenging benchmark problems in (non)-relativistic (magneto)hydrodynamics. We demonstrate the code's scalability including its strong scaling on the NCSA Blue Waters supercomputer up to the machine's full capacity of 22, 380 nodes using 671, 400 threads. variety of astrophysics codes (e.g., Refs. [6,[9][10][11][12][13][14]) have been designed based on these fundamental building blocks.These strategies work well when the computations are reasonably homogeneous or when one seeks good parallelization to only a few thousand cores. As the number of MPI processes increases, so does the cost of communication which, together with non-uniform workload typical of astrophysics problems, limits the maximum number of useful cores that codes can run on. Efficient core utilization becomes non-trivial, often requiring careful optimization by hand to achieve good scalability [15]. Standard finite-volume and finitedifference methods achieve higher order accuracy with increasingly large (overlapping) stencil sizes, and may require additional effort to achieve scalability on massively parallel machines.As one looks ahead to the arrival of exascale computing, it will become increasingly important to focus on developing algorithms that can take full advantage of these very large machines.Discontinuous Galerkin (DG) methods [16-21], together with a task-based parallelization strategy, have the potential to tackle many of these problems. DG methods offer high-order accuracy in smooth regions (although, for stability, increasing the scheme's order requires decreasing the timestep, which restricts the largest usable order in practice), robustness for shocks and other discontinuities, and grid flexibility including a formulation that allows for comparatively straightforward hp-adaptivity and local timestepping. DG methods can be combined with positivity preserving strategies [22][23][24] or "atmosphere treatments" [25] which seek to maintain non-negative values of the pressure and density in challenging regions such as those containing high-speed astrophysical flow. DG methods are also well suited for parallelization: Their formulation in terms of l...

Numerical simulations of neutron star-neutron star and neutron starblack hole binaries play an important role in our ability to model gravitational wave and electromagnetic signals powered by these systems. These simulations have to take into account a wide range of physical processes including general relativity, magnetohydrodynamics, and neutrino radiation transport. The latter is particularly important in order to understand the properties of the matter ejected by many mergers, the optical/infrared signals powered by nuclear reactions in the ejecta, and the contribution of that ejecta to astrophysical nucleosynthesis. However, accurate evolutions of the neutrino transport equations that include all relevant physical processes remain beyond our current reach. In this review, I will discuss the current state of neutrino modeling in general relativistic simulations of neutron star mergers and of their post-merger remnants, focusing in particular on the three main types of algorithms used in simulations so far: leakage, moments, and Monte-Carlo scheme. I will discuss the advantages and limitations of each scheme, as well as the various neutrino-matter interactions that should be included in simulations. We will see that the quality of the treatment of neutrinos in merger simulations has greatly increased over the last decade, but also that many potentially important interactions remain difficult to take into account in simulations (pair annihilation, oscillations, inelastic scattering).