https://doi.org/10.1140/epjc/s10052-025-14645-5
Regular Article - Theoretical Physics
Horizonless star based on regular black hole with finite radius and its observational signatures
1
Departemen Fisika, FMIPA, Universitas Indonesia, 16424, Depok, Indonesia
2
High Energy Physics Theory Group, Department of Physics, Faculty of Science, Chulalongkorn University, 10330, Bangkok, Thailand
3
Department of Physics, University of Virginia, 382 McCormick Road, 22904-4714, Charlottesville, VA, USA
Received:
1
May
2025
Accepted:
14
August
2025
Published online:
25
August
2025
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 
, where 
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 , 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 
.
© The Author(s) 2025
Open Access This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons licence, and indicate if changes were made. The images or other third party material in this article are included in the article’s Creative Commons licence, unless indicated otherwise in a credit line to the material. If material is not included in the article’s Creative Commons licence and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this licence, visit http://creativecommons.org/licenses/by/4.0/.
Funded by SCOAP3.
