Bound orbits near black holes with scalar hair

TL;DR

This paper investigates the properties of bound orbits around scalar hair black holes, revealing distinctive orbital precession features that could help distinguish these objects from traditional black holes in astronomical observations.

Contribution

It introduces a model of scalar field black holes with horizons smaller than Schwarzschild black holes and analyzes their bound orbit characteristics numerically.

Findings

Event horizon radius can be arbitrarily close to zero.

Bound orbits exhibit unique precession angles less than Schwarzschild black holes.

Numerical simulations of orbit shapes around scalar hair black holes.

Abstract

We consider spherically symmetric black holes with minimally coupled scalar fields and concentrate our attention on asymptotically flat self-gravitating configurations having the event horizons located at radii much smaller than $2m$. We think of such configurations as rigorous mathematical models of the gravitating objects, surrounded by dark matter, in the centres of normal galaxies. It turns out that the radius of the event horizon of a scalar field black hole always less than the Schwarzschild radius of vacuum black hole of the same mass and can be arbitrary close to zero. In astronomical observations, a key role in distinguishing between black holes, wormholes, and naked singularities plays measuring parameters of bound quasiperiodic orbits, in particular, the shape of an orbit and the angle of precession of its pericentre. We consider a typical two-parameter family of compact scalar field black holes and compute numerically the shapes of some bound orbits. We find that a key feature of bound orbits around a compact black hole is that the angle between closest pericentre points is either negative or, at least, less than that for the Schwarzschild black hole of the same mass.