Axisymmetric core collapse simulations using characteristic numerical relativity

TL;DR

This paper presents axisymmetric core collapse simulations in general relativity using a characteristic foliation approach, enabling direct gravitational wave extraction at null infinity and revealing discrepancies with traditional quadrupole methods.

Contribution

The study introduces a characteristic numerical relativity framework for core collapse simulations, allowing unambiguous gravitational wave extraction at null infinity and comparing it with quadrupole estimates.

Findings

Gravitational wave signals show oscillation frequencies around 0.5 kHz.

Quadrupole formula may not produce physical gravitational waves in this setup.

Characteristic approach enables direct wave extraction at null infinity.

Abstract

We present results from axisymmetric stellar core collapse simulations in general relativity. Our hydrodynamics code has proved robust and accurate enough to allow for a detailed analysis of the global dynamics of the collapse. Contrary to traditional approaches based on the 3+1 formulation of the gravitational field equations, our framework uses a foliation based on a family of outgoing light cones, emanating from a regular center, and terminating at future null infinity. Such a coordinate system is well adapted to the study of interesting dynamical spacetimes in relativistic astrophysics such as stellar core collapse and neutron star formation. Perhaps most importantly this procedure allows for the unambiguous extraction of gravitational waves at future null infinity without any approximation, along with the commonly used quadrupole formalism for the gravitational wave extraction. Our results concerning the gravitational wave signals show noticeable disagreement when those are extracted by computing the Bondi news at future null infinity on the one hand and by using the quadrupole formula on the other hand. We have strong indication that for our setup the quadrupole formula on the null cone does not lead to physical gravitational wave signals. The Bondi gravitational wave signals extracted at infinity show typical oscillation frequencies of about 0.5 kHz.