We present a new algorithm for evolving orbiting black-hole binaries that does not require excision or a corotating shift. Our algorithm is based on a novel technique to handle the singular puncture conformal factor. This system, based on the BSSN formulation of Einstein's equations, when used with a 'pre-collapsed' initial lapse, is non-singular at the start of the evolution, and remains nonsingular and stable provided that a good choice is made for the gauge. As a test case, we use this technique to fully evolve orbiting black-hole binaries from near the Innermost Stable Circular Orbit (ISCO) regime. We show fourth order convergence of waveforms and compute the radiated gravitational energy and angular momentum from the plunge. These results are in good agreement with those predicted by the Lazarus approach.PACS numbers: 04.25. Dm, 04.25.Nx, 04.30.Db, 04.70.Bw One of the most significant goals of numerical relativity is to compute accurate gravitational waveforms from astrophysically realistic simulations of merging black-hole binaries. The expectation of very strong gravitational wave emission from the merger of two black holes, and some of the newest astrophysical observations, from supermassive galactic nuclei just about to merge [1] to stellar size black-hole binaries, make these systems one of the most extraordinary astrophysical objects under study today. Binary black hole mergers are expected not only to provide information about the history and formation of the binary system but also to provide important precise tests of strong-field, highly dynamical relativity.Motivated by the forthcoming observations of groundbased gravitational wave detectors, such as LIGO [2], and by the next generation of space-based detectors, such as LISA [3], the numerical relativity community has dedicated a great deal of effort to solving the binary-blackhole problem over the past few decades. After the 'Binary Black Hole Grand Challenge' [4] several new approaches have been pursued in the attempt to produce stable three-dimensional (3D) numerical codes capable of evolving the full Einstein field equations in the absence of any symmetry. This includes the introduction of new formulations of these equations and the development of numerical techniques for accurate evolutions of black-hole binaries, such as higher order finite differencing, spectral methods, and adaptive mesh refinement (AMR) (see Ref.[5] and references therein).The calculation of the gravitational radiation emitted from plunging black-hole binaries was pioneered through the use of the Lazarus approach, which bridges numerical relativity and perturbative techniques to extract approximate gravitational waveforms [6,7,8]. More recently important progress has been made toward evolving orbiting binary-black-hole spacetimes with the use of stable full 3D numerical relativity codes using corotating gauge conditions and singularity excision [9,10,11].Here we present a novel technique for evolving orbiting black holes based on puncture data. This technique does not ...

The LIGO detection of GW150914 provides an unprecedented opportunity to study the two-body motion of a compact-object binary in the large-velocity, highly nonlinear regime, and to witness the final merger of the binary and the excitation of uniquely relativistic modes of the gravitational field. We carry out several investigations to determine whether GW150914 is consistent with a binary black-hole merger in general relativity. We find that the final remnant's mass and spin, as determined from the low-frequency (inspiral) and high-frequency (postinspiral) phases of the signal, are mutually consistent with the binary black-hole solution in general relativity. Furthermore, the data following the peak of GW150914 are consistent with the least-damped quasinormal mode inferred from the mass and spin of the remnant black hole. By using waveform models that allow for parametrized general-relativity violations during the inspiral and merger phases, we perform quantitative tests on the gravitational-wave phase in the dynamical regime and we determine the first empirical bounds on several high-order post-Newtonian coefficients. We constrain the graviton Compton wavelength, assuming that gravitons are dispersed in vacuum in the same way as particles with mass, obtaining a 90%-confidence lower bound of 10 13 km. In conclusion, within our statistical uncertainties, we find no evidence for violations of general relativity in the genuinely strong-field regime of gravity.

On September 14, 2015, the Laser Interferometer Gravitational-Wave Observatory (LIGO) detected a gravitational-wave transient (GW150914); we characterize the properties of the source and its parameters. The data around the time of the event were analyzed coherently across the LIGO network using a suite of accurate waveform models that describe gravitational waves from a compact binary system in general relativity. GW150914 was produced by a nearly equal mass binary black hole of masses 36 þ5 −4 M ⊙ and 29 þ4 −4 M ⊙ ; for each parameter we report the median value and the range of the 90% credible interval. The dimensionless spin magnitude of the more massive black hole is bound to be < 0.7 (at 90% probability). The luminosity distance to the source is 410 −0.07 . This black hole is significantly more massive than any other inferred from electromagnetic observations in the stellar-mass regime.

Recent calculations of gravitational radiation recoil generated during black-hole binary mergers have reopened the possibility that a merged binary can be ejected even from the nucleus of a massive host galaxy. Here we report the first systematic study of gravitational recoil of equal-mass binaries with equal, but counter-aligned, spins parallel to the orbital plane. Such an orientation of the spins is expected to maximize the recoil. We find that recoil velocity (which is perpendicular to the orbital plane) varies sinusoidally with the angle that the initial spin directions make with the initial linear momenta of each hole and scales up to a maximum of ∼ 4000 km s −1 for maximally-rotating holes. Our results show that the amplitude of the recoil velocity can depend sensitively on spin orientations of the black holes prior to merger. Introduction: Generic black-hole-binary mergers will display a rich spectrum of gravitational effects in the last few orbits prior to the formation of the single rotating remnant hole. These effects include spin and orbital plane precession, radiation of mass, linear and angular momentum, as well as spin-flips of the remnant horizon. Thanks to recent breakthroughs in the full non-linear numerical evolution of black-hole-binary spacetimes [1,2,3], it is now possible to accurately simulate the merger process and examine these effects in this highly non-linear regime [4,5,6,7,8,9,10,11,12,13,14,15,16,17,18]. Black-hole binaries will radiate between 2% and 8% of their total mass and up to 40% of their angular momenta, depending on the magnitude and direction of the spin components, during the merger [6,7,8]. In addition, the radiation of net linear momentum by a black-hole binary leads to the recoil of the final remnant hole [19,20,21,22,23,24,25,26,27,28]. This phenomenon can lead to astrophysically important effects [29,30].A non-spinning black-hole binary will emit net linear momentum parallel to its orbital plane if the individual holes have unequal masses. However, the maximum recoil in this case (which occurs when the mass ratio is q ≈ 0.36) is a relatively small ∼ 175 km s −1 [22].The first generic simulation of black-hole binaries with unequal masses and spins was reported in [24]. These black holes displayed spin precession and spin flips, and for the first time, recoil velocities over 400km s −1 , mostly along the orbital angular momentum direction. It was thus found that the unequal spin components to the recoil velocity can be much larger than those due to unequal masses, and that comparable mass, maximally spinning holes with spins in the orbital plane and counter-aligned

We report the first results from the evolution of generic black hole binaries, i.e., binaries containing unequalmass black holes with misaligned spins. Our configuration, which has a mass ratio of , consists of an initially 2 : 1 nonspinning hole orbiting a larger, rapidly spinning hole (specific spin ), with the spin direction a/m p 0.885 oriented Ϫ45Њ with respect to the orbital plane. We track the inspiral and merger for ∼2 orbits and find that the remnant receives a substantial kick of 454 km s Ϫ1 , more than twice as large as the maximum kick from nonspinning binaries. The remnant spin direction is flipped by 103Њ with respect to the initial spin direction of the larger hole. We performed a second run with antialigned spins, lying in the orbital plane that produces a kick a/m p ‫5.0ע‬ of ∼1830 km s Ϫ1 off the orbital plane. This value scales to nearly 4000 km s Ϫ1 for maximally spinning holes. Such a large recoil velocity opens up the possibility that a merged binary can be ejected even from the nucleus of a massive host galaxy.

We describe the Einstein Toolkit, a community-driven, freely accessible computational infrastructure intended for use in numerical relativity, relativistic astrophysics, and other applications. The Toolkit, developed by a collaboration involving researchers from multiple institutions around the world, combines a core set of components needed to simulate astrophysical objects such as black holes, compact objects, and collapsing stars, as well as a full suite of analysis tools. The Einstein Toolkit is currently based on the Cactus Framework for high-performance computing and the Carpet adaptive mesh refinement driver. It implements spacetime evolution via the BSSN evolution system and general-relativistic hydrodynamics in a finite-volume discretization. The toolkit is under continuous development and contains many new code components that have been publicly released for the first time and are described in this article. We discuss the motivation behind the release of the toolkit, the philosophy underlying its development, and the goals of the project. A summary of the implemented numerical techniques is included, as are results of numerical test covering a variety of sample astrophysical problems.

As two black holes bound to each other in a close binary approach merger their inspiral time eventually becomes shorter than the characteristic inflow time of surrounding orbiting matter. Using an innovative technique in which we represent the changing spacetime in the region occupied by the orbiting matter with a 2.5PN approximation and the binary orbital evolution with 3.5PN, we have simulated the MHD evolution of a circumbinary disk surrounding an equal-mass non-spinning binary. Prior to the beginning of the inspiral, the structure of the circumbinary disk is predicted well by extrapolation from Newtonian results. The binary opens a low-density gap whose radius is roughly two binary separations, and matter piles up at the outer edge of this gap as inflow is retarded by torques exerted by the binary; nonetheless, the accretion rate is diminished relative to its value at larger radius by only about a factor of 2. During inspiral, the inner edge of the disk at first moves inward in coordination with the shrinking binary, but as the orbital evolution accelerates, the rate at which the inner edge moves toward smaller radii falls behind the rate of binary compression. In this stage, the rate of angular momentum transfer from the binary to the disk slows substantially, but the net accretion rate decreases by only 10-20%. When the binary separation is tens of gravitational radii, the rest-mass efficiency of disk radiation is a few percent, suggesting that supermassive binary black holes in galactic nuclei could be very luminous at this stage of their evolution. If the luminosity were optically thin, it would be modulated at a frequency that is a beat between the orbital frequency of the disk's surface density maximum and the binary orbital frequency. However, a disk with sufficient surface density to be luminous should also be optically thick; as a result, the periodic modulation may be suppressed.Subject headings: Black hole physics -magnetohydrodynamics -accretion, accretion disks -Galaxies: nuclei by using a similar technique to obtain Equation (C1).

We use the ''moving puncture'' approach to perform fully nonlinear evolutions of spinning quasicircular black-hole binaries with individual spins unaligned with the orbital angular momentum. We evolve configurations with the individual spins (parallel and equal in magnitude) pointing in the orbital plane and 45 above the orbital plane. We introduce a technique to measure the spin direction and track the precession of the spin during the merger, as well as measure the spin flip in the remnant horizon. The former configuration completes 1.75 orbits before merging, with the spin precessing by 98 and the final remnant horizon spin flipped by 72 with respect to the component spins. The latter configuration completes 2.25 orbits, with the spins precessing by 151 and the final remnant horizon spin flipped 34 with respect to the component spins. These simulations show for the first time how the spins are reoriented during the final stage of black-hole-binary mergers verifying the hypothesis of the spin-flip phenomenon.We also compute the track of the holes before merger and observe a precession of the orbital plane with frequency similar to the orbital frequency and amplitude increasing with time.