Horizonless star based on regular black hole with finite radius and its observational signatures

Abstract The horizonless configuration of regular black holes has recently attracted attention as a model for ultracompact stars. In this paper, we propose a new class of regular black hole models sourced by a de Sitter vacuum with a finite radius. We focus on studying its horizonless configuration, which is modified into an anisotropic gravastar by proposing an ansatz of equation of states. We confirm that an anisotropic gravastar approaching horizon formation must violate the dominant energy condition. We also found that the proposed object has an effectively similar structure as a frozen star on the time geometry at the extremal configuration. From the proposed model, we investigate the photon geodesics inside the object and predict the optical appearance of the object surrounded by a thin accretion disk. Our imaging results indicate that, assuming light does not interact with the object’s interior, its optical appearance differs from that of a thin-shell gravastar. “Chaotic” photon ring merges for $$x&gt;x_{m}$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mrow> <mml:mi>x</mml:mi> <mml:mo>&gt;</mml:mo> <mml:msub> <mml:mi>x</mml:mi> <mml:mi>m</mml:mi> </mml:msub> </mml:mrow> </mml:math> , where $$x_{m}$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:msub> <mml:mi>x</mml:mi> <mml:mi>m</mml:mi> </mml:msub> </mml:math> represents the minimum value required for the photon sphere to exist. In addition to its optical appearance, we investigate the axial gravitational perturbations emitted by this horizonless star. Notably, echo trains are found to exist for $$x&gt;x_{m}$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mrow> <mml:mi>x</mml:mi> <mml:mo>&gt;</mml:mo> <mml:msub> <mml:mi>x</mml:mi> <mml:mi>m</mml:mi> </mml:msub> </mml:mrow> </mml:math> , as determined by numerically solving the time-dependent Regge–Wheeler equation. By comparing the echo time with the GW170817 observation, we find that a frequency of 72 Hz can be achieved, albeit at the cost of requiring a relatively high value of $$\ell $$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mi>ℓ</mml:mi> </mml:math> .