Lensing and dynamics of ultra-compact bosonic stars

TL;DR

This paper explores ultra-compact bosonic stars, highlighting their unique lensing signatures and demonstrating through simulations that they tend to decay into black holes, thus challenging their ability to mimic black hole phenomenology.

Contribution

It provides a detailed analysis of the lensing patterns and dynamical stability of ultra-compact bosonic stars, distinguishing them from black holes both observationally and dynamically.

Findings

Distinct lensing patterns with infinite Einstein rings near light rings

Ultra-compact bosonic stars decay into Schwarzschild black holes within a short time

Strong lensing region smaller than black hole shadows for similar masses

Abstract

Spherically symmetric bosonic stars are one of the few examples of gravitating solitons that are known to form dynamically, via a classical process of (incomplete) gravitational collapse. As stationary solutions of the Einstein--Klein-Gordon or the Einstein--Proca theory, bosonic stars may also become sufficiently compact to develop light rings and hence mimic, in principle, gravitational-wave observational signatures of black holes (BHs). In this paper, we discuss how these horizonless ultra-compact objects (UCOs) are actually distinct from BHs, both phenomenologically and dynamically. In the electromagnetic channel, the light ring associated phenomenology reveals remarkable lensing patterns, quite distinct from a standard BH shadow, with an infinite number of Einstein rings accumulating in the vicinity of the light ring, both inside and outside the latter. The strong lensing region, moreover, can be considerably smaller than the shadow of a BH with a comparable mass. Dynamically, we investigate the fate of such UCOs under perturbations, via fully non-linear numerical simulations and observe that, in all cases, they decay into a Schwarzschild BH within a time scale of $\mathcal{O}(M)$, where $M$ is the mass of the bosonic star. Both these studies reinforce how difficult it is for horizonless UCOs to mimic BH phenomenology and dynamics, in all its aspects.