Geodesic motion in the space-time of a non-compact boson star

TL;DR

This paper investigates the trajectories of particles around non-compact boson stars, revealing unique orbital behaviors near the star's center and implications for astrophysical observations like those of Sagittarius A*.

Contribution

It provides a numerical analysis of geodesic motion in boson star space-times, highlighting differences from black holes and Schwarzschild solutions.

Findings

Discovery of additional bound orbits near the boson star center

Deviation from Schwarzschild space-time close to the star

Potential implications for interpreting EMRI observations

Abstract

We study the geodesic motion of test particles in the space-time of non-compact boson stars. These objects are made of a self-interacting scalar field and -- depending on the scalar field's mass -- can be as dense as neutron stars or even black holes. In contrast to the former these objects do not contain a well-defined surface, while in contrast to the latter the space-time of boson stars is globally regular, can -- however -- only be given numerically. Hence, the geodesic equation also has to be studied numerically. We discuss the possible orbits for massive and massless test particles and classify them according to the particle's energy and angular momentum. The space-time of a boson star approaches the Schwarzschild space-time asymptotically, however deviates strongly from it close to the center of the star. As a consequence, we find additional bound orbits of massive test particles close to the center of the star that are not present in the Schwarzschild case. Our results can be used to make predictions about extreme-mass-ratio inspirals (EMRIs) and we hence compare our results to recent observational data of the stars orbiting Sagittarius A* - the radiosource at the center of our own galaxy.