Developing Tools for Multimessenger Gravitational Wave Astronomy

We present work in progress to craft open-sourced numerical tools that will enable the calculation of electromagnetic counterparts to gravitational waveforms: the {\tt GiRaFFE} (General Relativistic Force-Free Electrodynamics) code. {\tt GiRaFFE} numerically solves the general relativistic magnetohydrodynamics system of equations in the force-free limit, to model the magnetospheres surrounding compact binaries, in order (1) to characterize the nonlinear interaction between the source and its surrounding magnetosphere, and (2) to evaluate the electromagnetic counterparts of gravitational waves, including the production of collimated jets. We apply this code to various configurations of spinning black holes immersed in external magnetic field, in order to both test our implementation, and to explore the effect of strong gravitational field, high spins and of misalignment between the magnetic field lines an black hole spin, on the electromagnetic output and the collimation of Poynting jets. We will extend our work to collisions of black holes immersed in external magnetic field, which are prime candidates for coincident detection in both gravitational and electromagnetic spectra.Comment: 6 pages, 6 figures, MG15 proceeding

Merging Black Holes: Assessing the Performance of Two Analytic Gravitational Waves Models

Merging black holes produce the loudest signal in the detectors. However, this is the most difficult signal to accurately predict with analytical techniques. Only computer simulations account for the nonlinear physics during the collision, but they are inherently complex, costly, and affected by numerical errors. In order to bypass this problem, two analytical models for the merger have been developed: the Implicit Rotating Source and the newer Backwards one Body. In this work, we assess the performance of the latter model by comparing it with the former model and with the numerical data, identifying its strengths and weaknesses. We start with a comparison of the analytical approximations for estimating the final black hole mass, spin, ringdown frequency and quality factor. We continue by implementing the models in a set of new Mathematica notebooks and calculating the waveforms. Our main finding reveals discrepancies in amplitude, but overall excellent accord in frequency. The newer model is comparable with the older and with the simulations, having the added advantage that it depends only indirectly on numerical data, it accounts for spin, and it offers a seamless fit with the analytical formalisms for the inspiral. By independently evaluating and testing those models, we not only bring evidence of their reproducibility, thus upholding high scientific standards, but we enable readers to evaluate our results themselves.Comment: 28 pages, 7 figure

Eccentric Pairs: Analytic Gravitational Waves from Binary Black Holes in Elliptic Orbits

Gravitational waves (GW) from eccentric binaries have intricate signals encoding important features about the location, creation and evolution of the sources. Eccentricity shortens the merger time, making the emitted GW statistically predominant in the observed data once detectors will reach the required sensitivity. We present a novel implementation of fully analytical GW templates from eccentric binary black hole (BBH) mergers within the \texttt{Wolfram Mathematica} software. We increase the accuracy by identifying and minimizing the possible source of errors. We start with an overview of the physics involved in eccentric mergers, then assemble the strain for the inspiral by employing up to six post-Newtonian (PN) corrections. We complete the eccentric inspiral with the quasi-circular Backwards one Body (BoB) merger model in frequency, amplitude and phase, then we build the hybrid GW strain for the whole evolution of the binary. For low eccentricity we reach coincidence in the overlap, with no ambiguity in the time interval, a remarkable improvement from the usual matching techniques. For high-eccentricity we compensate for the implicit quasi-circular assumption of the BOB approach, by introducing a small rescaling in amplitude. Our streamlined implementation is relevant for the new field of GW astronomy and is straightforward to understand, use and extend, offering researchers in the field a valuable open resource tool.Comment: 27 pages, 10 figures, accepted for publication in the International Journal of Modern Physics

Developing Tools for Multimessenger Gravitational Wave Astronomy

The Marcel Grossmann triennial meetings are focused on reviewing developments in gravitation and general relativity, aimed at understanding and testing Einstein’s theory of gravitation. The 15th meeting (Rome, 2018) celebrated the 50th anniversary of the first neutron star discovery (1967), and the birth of relativistic astrophysics. Another discovery of the same caliber is the detection of the binary neutron star GW170817 in 2017 – almost as if to celebrate the same jubilee – marking the beginning of multi-messenger gravitational wave astronomy. We present work in progress to craft open-sourced numerical tools that will enable the calculation of electromagnetic counterparts to gravitational waveforms: the GiRaFFE (General Relativistic Force-Free Electrodynamics) code. GiRaFFE numerically solves the general relativistic magnetohydrodynamics system of equations in the force-free limit, to model the magnetospheres surrounding compact binaries, in order (1) to characterize the nonlinear interaction between the source and its surrounding magnetosphere, and (2) to evaluate the electromagnetic counterparts of gravitational waves, including the production of collimated jets. We apply this code to various configurations of spinning black holes immersed in an external magnetic field, in order both to test our implementation and to explore the effects of (1) strong gravitational field, (2) high spins, and (3) tilt between the magnetic field lines and black hole spin, all on the amplification and collimation of Poynting jets. We will extend our work to collisions of black holes immersed in external magnetic field, which are prime candidates for coincident detection in both gravitational and electromagnetic spectra

Electromagnetic Signature of Magnetized Binary Neutron Star Mergers

The detection of a binary neutron star merger back in August 2017, in both gravitational and electromagnetic waves, received worldwide attention from astrophysicists. Since then, the LIGO/Virgo scientific collaboration reports detection of such mergers at a rate of one per month. To catch up with this pace, work needs to be done in order to understand the coupling between electromagnetic fields and gravity, and to decipher the signature of the remnant physics in the detected radiation. Here we present numerical simulations of magnetized binary neutron star mergers, and our analysis of the gravitational waves electromagnetic counterparts generated by such events. We prescribe initial data for the neutron stars in the range constrained by the GW170817 event, using the LORENE code. We choose masses between 1.6 and 1.2 solar masses, and two soft equations of state, a strange quark matter equation, and a hyperonic one. We endow the neutron stars with dipolar magnetic fields, and simulate the magnetospheres with our open source general relativistic force-free electrodynamics code GiRaFFE. We monitor the electromagnetic Poynting luminosity in order to quantify the relationship between the electromagnetic signature and the lifetime of the remnant, or the chosen equation of state. By the end of the merger, when the blackhole forms, and the Blandford-Znajek mechanism is activated, we analyze the collimation of the Poynting luminosity, which indicates the incipience of a jet. Our work helps with understanding highly energetic gravitational wave sources accompanied by electromagnetic counterparts

Testing numerical evolution with the shifted gauge wave

Computational methods are essential to provide waveforms from coalescing black holes, which are expected to produce strong signals for the gravitational wave observatories being developed. Although partial simulations of the coalescence have been reported, scientifically useful waveforms have so far not been delivered. The goal of the AppleswithApples (AwA) Alliance is to design, coordinate and document standardized code tests for comparing numerical relativity codes. The first round of AwA tests has now been completed and the results are being analyzed. These initial tests are based upon the periodic boundary conditions designed to isolate performance of the main evolution code. Here we describe and carry out an additional test with periodic boundary conditions which deals with an essential feature of the black hole excision problem, namely a non-vanishing shift. The test is a shifted version of the existing AwA gauge wave test. We show how a shift introduces an exponentially growing instability which violates the constraints of a standard harmonic formulation of Einstein\u27s equations. We analyze the Cauchy problem in a harmonic gauge and discuss particular options for suppressing instabilities in the gauge wave tests. We implement these techniques in a finite difference evolution algorithm and present test results. Although our application here is limited to a model problem, the techniques should benefit the simulation of black holes using harmonic evolution codes

Harmonic Initial-Boundary Evolution in General Relativity

Computational techniques which establish the stability of an evolution-boundary algorithm for a model wave equation with shift are incorporated into a well-posed version of the initial-boundary value problem for gravitational theory in harmonic coordinates. The resulting algorithm is implemented as a 3-dimensional numerical code which we demonstrate to provide stable, convergent Cauchy evolution in gauge wave and shifted gauge wave testbeds. Code performance is compared for Dirichlet, Neumann, and Sommerfeld boundary conditions and for boundary conditions which explicitly incorporate constraint preservation. The results are used to assess strategies for obtaining physically realistic boundary data by means of Cauchy-characteristic matching

The Einstein Toolkit

The Einstein Toolkit is a&nbsp;community-driven software platform of core computational tools to advance and support research in relativistic astrophysics and gravitational physics. </span