Gravastar Shadows

TL;DR

This paper explores the optical signatures of gravastars, a super-compact alternative to black holes, demonstrating that their unique photon orbit features produce distinguishable images that could be observed with future high-resolution telescopes.

Contribution

It introduces a detailed model of gravastar optical images, showing how their photon orbit structure creates observable differences from black holes.

Findings

Unstable photon orbits can exist around gravastars.

Distinct optical images, such as bright disks and dark rings, differentiate gravastars from black holes.

Future VLBI observations could identify gravastars based on their unique images.

Abstract

Direct observation of black holes is one of the grand challenges in astronomy. If there are super-compact objects which possess unstable circular orbits of photons, however, it may be difficult to distinguish them from black holes by observing photons. As a model of super-compact objects, we consider a gravastar (gravitational-vacuum-star) which was originally proposed by Mazur and Mottola. For definiteness, we adopt a spherical thin-shell model of a gravastar developed by Visser and Wiltshire, which connects interior de-Sitter geometry and exterior Schwarzschild geometry. We find that unstable circular orbits of photons can appear around the gravastar. Then, we investigate the optical images of the gravastar possessing unstable circular orbits, with assuming the optically transparent surface of it and two types of optical sources behind the gravastar: (i) an infinite optical plane and (ii) a companion star. The main feature of the image of (i) is that a bright disk and a dark thick ring surrounding the disk appear in the center of the region which would be completely dark if the compact object was not the gravastar but Schwarzschild black hole. Also in the case (ii), a small disk and arcs around the disk appear in the region which would be completely dark for the lensing image by Schwarzschild black hole. Because characteristic images appear inside the gravastar in both cases, we could tell the difference between a black hole and a gravastar with high-resolution VLBI observations near future.