Abstract
We study the optical appearance of a thin accretion disk around the strongly naked static Janis–Newman–Winicour singularity. The solution does not possess a photon sphere, which results in the formation of a complex structure of bright rings in the central region of the disk image. Such structure is absent in the case of the Schwarzschild black hole with a thin accretion disk, where instead of the image we observe the black hole shadow. Some of the rings emit with the maximal observable radiation flux from the accretion disk, and should be experimentally detectable. Thus, this qualitatively new feature can be used to distinguish observationally black holes from naked singularities. We elucidate the appearance of the ring structure by revealing the physical mechanism of its formation, and explaining the nature of each of the ring images. We make the conjecture that a similar structure would also appear for other solutions without a photon sphere and it can serve as a general observational signature for distinguishing compact objects possessing no photon sphere from black holes.
Highlights
Exotic compact objects do not possess an event horizon
We study the optical appearance of a thin accretion disk around the strongly naked static Janis–Newman– Winicour singularity
We make the conjecture that a similar structure would appear for other solutions without a photon sphere and it can serve as a general observational signature for distinguishing compact objects possessing no photon sphere from black holes
Summary
Exotic compact objects do not possess an event horizon. other characteristic surfaces and distances, which are typical for black holes, can be present in their spacetimes. The weakly naked singularity possesses a photon sphere, and its observable properties in the electromagnetic spectrum are expected to resemble those of the Schwarzschild black hole It was shown in [27] that if the compact object is surrounded by a geometrically thick and optically thin medium, its shadow closely approaches the image of the Schwarzschild black hole, while in our previous work [45] we investigated the optical appearance of a thin accretion disk in its vicinity. We should note that this is not the only mechanism, which can lead to the appearance of ring images They can be observed for other exotic compact objects such as boson stars and wormholes possessing a photon region [47,48], where they are formed by different physical reasons.
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