Images and photon regions of continuous photon sphere spacetime

TL;DR

This paper investigates the images and photon regions of spacetimes with continuous photon spheres, revealing universal image features and potential observational signatures that differentiate these from black holes and Kerr spacetimes.

Contribution

It introduces the concept of continuous photon spheres, analyzes their image profiles, and explores their photon regions, highlighting universal features and distinctions from Kerr black holes.

Findings

Images of CPS spacetimes resemble Schwarzschild black holes under certain conditions.

Universal image profiles are found for static, spherically symmetric CPS spacetimes.

Distinct photon region structures in rotating CPS spacetimes may produce observable signatures.

Abstract

We study images of spacetimes containing continuous photon spheres (CPS). For a self-gravitating, isotropic, spherically symmetric spacetime with CPS, we find that a thin accretion disk produces images that closely resemble those of a Schwarzschild black hole, despite significant differences in photon dynamics. More generally, for any static, pherically symmetric spacetime with a luminous CPS core, the image profile is universal: members of this class produce identical image shapes, differing only by an overall normalization factor. This universality is, however, sensitive to the nature of the accretion flow and breaks down for spherically symmetric infalling accretion, where Doppler shifts and non-static emission introduce image features that depend on the flow dynamics and the metric. Finally, we investigate photon regions in a rotating CPS spacetime and find that unlike in Kerr spacetime, the photon region appears as one or two angular sectors in a constant-$\phi$ cross section. These distinctive photon region properties could produce observable signatures that distinguish rotating CPS spacetimes from the Kerr one.