On characteristic initial data for a star orbiting a black hole

TL;DR

This paper advances the characteristic approach to simulate a neutron star orbiting a black hole, demonstrating that initial data quickly relaxes to a quasi-equilibrium state with minimal spurious radiation.

Contribution

It develops a characteristic gravity-matter code with conservation form hydrodynamics, enabling more realistic simulations of compact objects in orbit around black holes.

Findings

System rapidly reduces spurious gravitational radiation.

System relaxes to a quasi-equilibrium state with approximate helical symmetry.

Initial data details have limited impact on long-term evolution.

Abstract

We take further steps in the development of the characteristic approach to enable handling the physical problem of a compact self-gravitating object, such as a neutron star, in close orbit around a black hole. We examine different options for setting the initial data for this problem and, in order to shed light on their physical relevance, we carry out short time evolution of this data. To this end we express the matter part of the characteristic gravity code so that the hydrodynamics are in conservation form. The resulting gravity plus matter relativity code provides a starting point for more refined future efforts at longer term evolution. In the present work we find that, independently of the details of the initial gravitational data, the system quickly flushes out spurious gravitational radiation and relaxes to a quasi-equilibrium state with an approximate helical symmetry corresponding to the circular orbit of the star.