arXiv subjects

Nami Nishimura

Publications and source records attributed to Nami Nishimura.

3 recordsLinked to original sources

arXiv2026

Self-force calculations with numerical relativity methods

To model gravitational waveforms from extreme mass-ratio inspirals (EMRIs) for the upcoming LISA space mission, gravitational self-force calculations are needed to second order in perturbation theory. However, to date these calculations have only been attempted for the simplest case of circular orbits in Schwarzschild spacetime. In this work, we present a new computational method aimed at performing generic second-order self-force calculations in Kerr spacetime using methods from the adjacent field of numerical relativity. We perform an $m$-mode separation of variables, add null ("$vtu$") slicing in horizon-penetrating coordinates, and solve the resulting elliptic PDEs using high-order discontinuous Galerkin discretization, adaptive mesh-refinement, and an iterative Krylov-type linear solver with parallelizable multigrid-Schwarz preconditioning. We find that our method achieves exponential convergence for the self-force on a scalar point charge in Kerr spacetime up to spins of $a=0.998$ (Thorne limit) on circular equatorial orbits as close as the ISCO (prograde and retrograde), despite the non-smooth puncture on the grid. We solve for 20 $m$-modes in parallel in a few seconds and retain the flexibility to extend the method to gravitational self-force and more generic orbits in the future. The code to perform these calculations is publicly available in the open-source numerical relativity code SpECTRE.

gr-qc↗

arXiv2026

Advancing the Effective-One-Body Framework in the Test-Mass Limit

We present SEOB-TML, an enhanced effective-one-body (EOB) framework for the test-mass limit, optimized for quasi-circular, spin-aligned binary black holes. On the dynamical side, we introduce a quadrupole-factorized (Q-factorized) prescription that maps the total energy flux-including horizon absorption-onto a single (2,2) mode baseline. This approach effectively captures higher-order multipole contributions without explicit mode summation, while simultaneously leading to a dramatic reduction in fractional flux errors. To ensure a smooth transition to the post-merger stage, we replace traditional next-to-quasicircular corrections with a phenomenological ansatz, enabling a flexible, mode-dependent attachment prescription. For the merger-ringdown stage, we utilize quasi-normal mode coefficients extracted from numerical waveforms via qnmfinder to explicitly model mode-mixing effects. These enhancements lead to a substantial reduction in residuals, capturing the complex physical modulations prominent in retrograde configurations. Additionally, we implement the (2,0) mode across the full waveform, further extending the model's physical coverage and accuracy. Overall, our framework generates highly accurate late inspiral-merger-ringdown waveforms for extreme-mass-ratio systems, significantly reducing dephasing and improving the near-merger reconstruction. We demonstrate the performance of SEOB-TML against the current state-of-the-art SEOBNRv5HM model, highlighting how our specialized developments extend the reliability of the EOB framework into the test-mass limit.

gr-qc↗

arXiv2022

New self-force method via elliptic partial differential equations for Kerr inspiral models

We present a new method designed to avoid numerical challenges that have impeded calculation of the Lorenz gauge self-force acting on a compact object inspiraling into a Kerr black hole. This type of calculation is valuable in creating waveform templates for extreme mass-ratio inspirals, which are an important source of gravitational waves for the upcoming Laser Interferometer Space Antenna mission. Prior hyperbolic partial differential equation (PDE) formulations encountered numerical instabilities involving unchecked growth in time; our new method is based on elliptic PDEs, which do not exhibit instabilities of that kind. For proof of concept, we calculate the self-force acting on a scalar charge in a circular orbit around a Kerr black hole. We anticipate this method will subsequently facilitate calculation of first-order Lorenz gauge Kerr metric perturbations and self-force, which could serve as a foundation for second-order Kerr self-force investigations.

gr-qc↗