Is a photon ring invariably a closed structure?

TL;DR

This paper explores how the photon ring around a rotating compact object can become discontinuous or vanish, depending on the observer's angle and the object's surface fluctuations, revealing new phenomena in black hole imaging.

Contribution

It introduces a novel analysis of photon ring discontinuities and disappearance phenomena, dependent on observational angle and surface fluctuations, with a comprehensive physical explanation.

Findings

Photon rings can be discontinuous or absent depending on viewing angle.

Disruption of photon rings is a new phenomenon in black hole imaging.

Analysis of characteristic functions explains the physical mechanisms behind these effects.

Abstract

In this study, we investigate the image of a rotating compact object (CO) illuminated by a geometrically thin, optically thin disk on the equatorial plane. As the radius of the CO's surface fluctuates, the CO may partially or entirely obscure the photon region. We observe that the perceived photon ring may exhibit discontinuities, deviating from a closed structure, and may even disappear entirely. We find that the disruption and disappearance of the photon ring are dependent on the observational angle$-$a novel phenomenon not previously observed in black hole imaging studies. Our study reveals that while the factors influencing this unique photon ring phenomenon are diverse and the outcomes complex, we can provide a clear and comprehensive explanation of the physical essence and variation trends of this phenomenon. We do this by introducing and analyzing the properties and interrelationships of three characteristic functions, $\tilde{\eta}$, $\eta_o$, and $\eta_s$ related to the photon impact parameters. Additionally, our analysis of the intensity cuts and inner shadows of the images uncovers patterns that differ significantly from the shadow curve.