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

Cauchy-Characteristic Patching with Improved Accuracy

Cauchy-characteristic extractions (CCE) avoids the errors due to extraction at finite worldtube. The Cauchy and the characteristic approaches have complementary strengths and weaknesses. Unification of the two methods is a promising way of combining the strengths of both formalisms

Steps Towards a Nonlinear Cauchy-characteristic Code Patching

Cauchy-characteristic extractions (CCE) avoids the errors due to extraction at finite worldtube. The Cauchy and the characteristic approaches have complementary strengths and weaknesses. Unification of the two methods is a promising way of combining the strengths of both formalisms

Realistic Binary Neutron Stars Collisions Simulations: Challenges and Opportunities

Since 2015, when the LIGO-Virgo collaboration announced the first simultaneous detections of gravitational waves (GW150914) until now, more than 66 gravitational wave detections were reported, but only two signals came from a binary neutron star collision, the GW170817 and GW190425 events. GW170817 was accompanied by an electromagnetic outburst manifested as a kilonova and an off-axis jet. However, no conclusive electromagnetic signature was found to come from the GW190425. Indeed, nature proves again more complicated than our models, and it is still a big question how to model kilonovae, or to understand the mechanisms driving astrophysical jets and gamma ray bursts. Not only the electromagnetic counterpart of such collisions, but also the nature of the remnant is also mostly unknown, and its investigation can give glimpses of the internal structure of neutron stars. This talk will focus on the challenges encountered endeavoring to perform numerical relativity simulations of realistic binary neutron star collisions, using open-source General-Relativistic Magnetohydrodynamic codes such as GRHydro, Spritz and IllinoisGRMHD, that run within the Einstein Toolkit software platform. We construct initial data for 6 parametrized hybrid equations of state, and add both interior magnetic field, and exterior magnetosphere, then perform simulations of BNS mergers within the mass range of the two detections. Besides addressing the problem of the reproducibility of the results, the aim of this talk is to forge a path through this complicated procedure, and make it approachable to graduate students, or researchers that just enter the field

Simulating Magnetospheres with Numerical Relativity: The GiRaFFE code

Numerical Relativity is successful in the simulation of black holes and gravitational waves. In recent years, teams have tackled the problem of the interaction of gravitational and electromagnetic waves. We developed a new code for the numerical simulation of neutron and black hole magnetospheres, using the FFE formalism. We tested the performance of the new code named GiRaFFE, in 1D and 3D test suits. We will study magnetospheres, focusing on jets by the Blandford -Znajek mechanism

Adding Light to the Gravitational Waves on the Null Cone

Recent interesting astrophysical observations point towards a multi-messenger, multi-wavelength approach to understanding strong gravitational sources, like compact stars or black hole collisions, supernovae explosions, or even the big bang. Gravitational radiation is properly defined only at future null infinity, but usually is estimated at a finite radius, and then extrapolated. Our group developed a characteristic waveform extraction tool, implemented in an open source code, which computes the gravitational waves infinitely far from their source, in terms of compactified null cones, by numerically solving Einstein equation in Bondi space-time coordinates. The goal is extend the capabilities of the code, by solving Einstein-Maxwell\u27s equations together with the Maxwell\u27s equations, to obtain the energy radiated asymptotically at infinity, both in gravitational and electromagnetic waves. The purpose is to analytically derive and numerically calculate both the gravitational waves and the electromagnetic counterparts at infinity, in this characteristic framework. The method is to treat the source of gravitational and electromagnetic radiation as a black box, and therefore the code will be very flexible, with potentially large applicability

Towards Improved Accuracy of Gravitational Waves Extraction

Results in developing two new methods to improve the accuracy of waveform extraction using characteristic evolution. Numerical method: circular boundaries, with angular dissipation in the characteristic code. Geometric method: computation of Weyl tensor component Y4 at null infinity, in a conformally compactified treatment. Comparison and calibration in tests problems based upon linearized waves

Gravitational-waveform extraction by the characteristic method

When a pair of black holes spiral into each other and collide, the very fabric of space-time shakes, and gravitational waves are created. Gravitational waves carry information about their source, and will increase our understanding of relativistic systems in astrophysics. Gravitational wave observatories like LIGO and Virgo are tuned to detect the emission of these waves from the inspiral and merger of binary black holes, neutron stars, supernovae, etc… Problem: any small vibration is detected, so templates are essential to discern the real signal. It is hard to compute the waveforms obtained from numerical simulations accurately – gravitational radiation is properly defined only at null infinity, but is estimated at a finite radius. Cauchy-Characteristic Extraction (CCE) is the most precise and refined “extraction” method available. The CCE technique connects the strong-field “Cauchy” evolution of the space-time near the merger to the “characteristic” evolution far from the merger – at null infinity, where the waveform is extracted and detectors measure it. We present a stand-alone “characteristic” waveform extraction tool that has demonstrated accuracy and convergence of the numerical error and is used by the numerical relativity groups for the unambiguous extraction of waveforms. We prove that the numerical error of CCE satisfies the standards of the detection criteria required for Advanced LIGO data analysis. The tool provides a means for accurate calculation of waveforms generated by evolution codes based upon different analytic and numerical formulations of the Einstein equations

Characteristics of gravitational and electromagnetic radiation

Gravitational waves from the early universe are detectable, but detection is difficult. The strain is extremely small of magnitude 10-3 the width of a proton. There are detection and computational challenges