Multi-ring images of thin accretion disk of a regular naked compact object

TL;DR

This paper explores the optical appearance of a horizonless, regular naked compact object with a multi-ring structure caused by light winding around it, highlighting potential observational signatures distinguishable from black holes.

Contribution

It introduces a detailed analysis of multi-ring images in horizonless compact objects with Minkowski cores, extending previous models and discussing their detectability.

Findings

Multi-ring images can be significantly luminous compared to Schwarzschild black holes.

The shape of the potential influences the brightness and structure of the rings.

Multi-ring features may be observable with future very-long baseline interferometry projects.

Abstract

We discuss the importance of multi-ring images in the optical appearance of a horizonless spherically symmetric compact object, when illuminated by an optically thin accretion disk. Such an object corresponds to a sub-case of an analytically tractable extension of the Kerr solution dubbed as the {\it eye of the storm} by Simpson and Visser in [JCAP \textbf{03} (2022) 011], which merits in removing curvature singularities via an asymptotically Minkowski core, while harbouring both a critical curve and an infinite potential barrier at the center for null geodesics. This multi-ring structure is induced by light rays winding several times around the object, and whose luminosity is significantly boosted as compared to the Schwarzschild solution by the modified shape of the potential. Using three toy-profiles for the emission of an infinitely thin disk, truncated at its inner edge (taking its maximum value there) and having different decays with the distance, we discuss the image created by up to eight rings superimposed on top of the direct emission of the disk as its edge is moved closer to the center of the object. Our results point out to the existence of multi-ring images with a non-negligible luminosity in shadow observations when one allows for the existence of other compact objects in the cosmic zoo beyond the Schwarzschild solution. Such multi-ring images could be detectable within the future projects on very-long baseline interferometry.