The imitation game: Proca stars that can mimic the Schwarzschild shadow

Abstract

Can a dynamically robust bosonic star (BS) produce an (effective) shadow that mimics that of a black hole (BH)? The BH shadow is linked to the existence of light rings (LRs). For free bosonic fields, yielding mini-BSs, it is known that these stars can become ultra-compact - i.e., possess LRs - but only for perturbatively unstable solutions. We show this remains the case even when different self-interactions are considered. However, an effective shadow can arise in a different way: if BSs reproduce the existence of an innermost stable circular orbit (ISCO) for timelike geodesics (located at $r_{\mathrm{ISCO}}=6M$ for a Schwarzschild BH of mass M), the accretion flow morphology around BHs is mimicked and an effective shadow arises in an astrophysical environment. Even though spherical BSs may accommodate stable timelike circular orbits all the way down to their centre, we show the angular velocity along such orbits may have a maximum away from the origin, at $R_{\Omega }$; this scale was recently observed to mimic the BH's ISCO in some scenarios of accretion flow. Then: (i) for free scalar fields or with quartic self-interactions, $R_{\Omega }\ne 0$ only for perturbatively unstable BSs; (ii) for higher scalar self-interactions, e.g. axionic, $R_{\Omega }\ne 0$ is possible for perturbatively stable BSs, but no solution with $R_{\Omega }=6M$ was found in the parameter space explored; (iii) but for free vector fields, yielding Proca stars (PSs), perturbatively stable solutions with $R_{\Omega }\ne 0$ exist, and indeed $R_{\Omega }=6M$ for a particular solution. Thus, dynamically robust spherical PSs can mimic the shadow of a (near-)equilibrium Schwarzschild BH with the same M, in an astrophysical environment, despite the absence of a LR, at least under some observation conditions, as we confirm by comparing the lensing of such PSs and Schwarzschild BHs.

Abstract abridged due to arXiv length limit; 22 pages, 9 figures

References in corpus (20)

• First M87 Event Horizon Telescope Results. I. The Shadow of the Supermassive Black Hole
• Ultralight scalars as cosmological dark matter
• GW190521: A Binary Black Hole Merger with a Total Mass of $150M_{\odot }$
• Light ring stability in ultra-compact objects
• Light rings as observational evidence for event horizons: long-lived modes, ergoregions and nonlinear instabilities of ultracompact objects
• Chaotic lensing around boson stars and Kerr black holes with scalar hair
• Fundamental photon orbits: black hole shadows and spacetime instabilities
• Gravitational Wave Signatures of Highly Compact Boson Star Binaries
• Orbital Dynamics of Binary Boson Star Systems
• Asymptotically flat scalar, Dirac and Proca stars: discrete vs. continuous families of solutions
• Gravitational Lensing by Spinning Black Holes in Astrophysics, and in the Movie Interstellar
• Lensing and dynamics of ultra-compact bosonic stars
• Models of rotating boson stars and geodesics around them: new type of orbits
• Stability of rotating scalar boson stars with nonlinear interactions
• Dynamical formation of a Reissner-Nordström black hole with scalar hair in a cavity
• Rotating Axion Boson Stars
• Comparing timelike geodesics around a Kerr black hole and a boson star
• Iron K$\alpha$ line of boson stars
• Scalar Field Dark Matter: behavior around black holes
• Evolution of a mass-less test scalar field on Boson Stars space-times

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• Symplectic quantization of multi-field Generalized Proca electrodynamics
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