Shadow images of regular black hole with finite boundary

TL;DR

This paper introduces a new regular black hole model with a well-defined boundary, analyzes its photon trajectories, and simulates its shadow images, revealing differences from classical black holes and other horizonless models.

Contribution

The authors propose a novel regular black hole model with a finite boundary, extending the Hayward black hole, and analyze its shadow features and photon dynamics.

Findings

Horizonful shadow slightly differs from Schwarzschild black hole for R > 3M.

Horizonless configuration shows unique inner light ring structures.

Shadow images vary between horizonful and horizonless configurations.

Abstract

Regular black hole is one of the bottom-up solutions designed to eliminate the singularity at the center of black holes. Its horizonless solution has gained interest recently to model ultracompact star. Despite interesting, this proposal is problematic due to the absence of a well-defined boundary. In this work, we introduce a novel regular black hole model inspired by the Hayward black hole, incorporating additional terms to define a clear and well-defined `surface' radius $R$. We analyze the null geodesics around the object, both horizonful and horizonless configurations, by studying the photon effective potential. We further simulate the shadow images of the object surrounded by a thin accretion disk. Our results indicate that for $R > 3M$ the horizonfull shadow differs slightly from that of a Schwarzschild black hole. In the horizonless configuration, we identify distinct inner light ring structures near the central region of the shadow image, which differ from those observed in horizonless Hayward black holes.