⦿ Oasuya and Kobayashi in their recent paper considered throat effects on shadows of Kerr-like wormholes and showed that existence of the throat alters the shape of the shadow of Kerr wormholes significantly.
⦿ They also showed that the radius of the unstable circular photon orbits is smaller for the Kerr-like wormhole than that of the Kerr black hole with the same mass.
⦿ Atlast they manifested that even if you don’t know the mass of the object, you can determine whether the object is Kerr wormhole or black hole just by throat effects.
A wormhole is a spacetime bridge connecting two separate points in spacetime. Astrophysically, it is plausible to regard that wormholes are rotating. One of the metric for the rotating wormhole is the Kerr-like wormhole proposed. It is important to distinguish this wormhole from the Kerr black hole, and one such way is to study shadows of the Kerr-like wormholes. Remarkable attention to shadows cast by compact objects grows recently, since the Event Horizon Telescope observed the black hole shadow at the center of the galaxy M87.
Now, Oasuya and Kobayashi have revisited to investigate the shadow cast by the Kerr-like wormhole. They determined the boundary of the shadow by unstable circular photon orbits and have found that, in certain parameter regions (like larger wormhole spin ‘a’ and larger deviation parameter λ), the orbit is located at the throat of the Kerr-like wormhole (as shown in Fig 1), which was not considered in the literatures.
In these cases, the existence of the throat alters the shape of the shadow considerably, and it will be much easier to differentiate it from that of the Kerr black hole, compared with those cases without the throat effects taken into account, where the shapes of the Kerr-like wormhole and the Kerr black hole are found to be similar figures.
“We point out that the effects of the wormhole throat become prominent when the unstable circular photon orbits are located at the throat in certain parameter space. In these cases, the shape of the shadow is altered considerably, and it will be much easier for us to figure out the differences.”— told Kobayashi, second author of the study
Also, the radius of the unstable circular photon orbits is smaller for the Kerr-like wormhole than that of the Kerr black hole with the same mass.
On the other hand, they make us aware of another perspectives. Like, if there are throat effects on the shadow shape, one can figure out whether the observed shadow is cast by the Kerr-like wormhole or the Kerr black hole “even in the case” when the object mass is unknown, not determined by other observations such as by motions of objects around the wormhole/black hole.
Reference: Shinta Kasuya, Masataka Kobayashi, “Throat effects on shadows of Kerr-like wormholes”, pp. 1-6, ArXiv, 2021. https://arxiv.org/abs/2103.13086
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