The imitation game (r)evolutions: $Q$-star effective shadow from GRMHD analysis

TL;DR

This paper investigates the potential of stable bosonic Q-star configurations to mimic black hole shadows through general relativistic magnetohydrodynamic simulations, revealing that such stars can produce observable shadows similar to black holes.

Contribution

It demonstrates, for the first time, that stable Q-star models can generate effective shadows comparable to black holes without requiring ultracompactness or specific accretion disk assumptions.

Findings

Stable Q-stars can form persistent low-density central regions.

Simulated shadows of Q-stars are comparable in size to Schwarzschild black holes.

The shadow formation does not depend on ultracompactness or special accretion disks.

Abstract

$Q$-stars are a class of boson stars arising in scalar-field theories with interacting potentials, minimally coupled to gravity. We show that, in certain regions of parameter space, the angular velocity of stable timelike circular geodesics around $Q$-stars can attain a maximum at a nonzero radius. Notably, this behaviour may occur for stable configurations. This feature has been argued to produce effective shadows, but so far it has only been investigated for unstable solutions. We test this possibility by performing general relativistic magnetohydrodynamic evolutions for a representative stable $Q$-star model. A low-density, low-luminosity central region is indeed observed to form and persist -- at least until the evolution becomes affected by numerical viscosity. As a proof of principle, this suggests that families of stable bosonic stars can act as black hole mimickers. Moreover, for the model at hand, a heuristic analysis shows that the effective shadow has a comparable size to that of a Schwarzschild black hole with the same mass. Importantly, this mechanism for generating an effective shadow does not rely on the object being ultracompact, or an ad hoc chosen accretion disk.