Photon Rings around Kerr and Kerr-like Black Holes

Abstract

Very-long baseline interferometric observations have resolved structure on scales of only a few Schwarzschild radii around the supermassive black holes at the centers of our Galaxy and M87. In the near future, such observations are expected to image the shadows of these black holes together with a bright and narrow ring surrounding their shadows. For a Kerr black hole, the shape of this photon ring is nearly circular unless the black hole spins very rapidly. Whether or not, however, astrophysical black holes are truly described by the Kerr metric as encapsulated in the no-hair theorem still remains an untested assumption. For black holes that differ from Kerr black holes, photon rings have been shown numerically to be asymmetric for small to intermediate spins. In this paper, I calculate semi-analytic expressions of the shapes of photon rings around black holes described by a new Kerr-like metric which is valid for all spins. I show that photon rings in this spacetime are affected by two types of deviations from the Kerr metric which can cause the ring shape to be highly asymmetric. I argue that the ring asymmetry is a direct measure of a potential violation of the no-hair theorem and that both types of deviations can be detected independently if the mass and distance of the black hole are known. In addition, I obtain approximate expressions of the diameters, displacements, and asymmetries of photon rings around Kerr and Kerr-like black holes.

13 pages, 6 figures

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• Shadow casted by a Konoplya-Zhidenko rotating non-Kerr black hole
• Understanding photon sphere and black hole shadow in dynamically evolving spacetimes
• Testing Rotating Regular Metrics as Candidates for Astrophysical Black Holes
• Post-Kerr black hole spectroscopy
• Ergosphere and shadow of a rotating regular black hole
• Rotating black holes without ${\mathbb{Z}}_2$ symmetry and their shadow images
• Charged particle motion around a quasi-Kerr compact object immersed in an external magnetic field
• Does the shape of the shadow of a black hole depend on motional status of an observer?
• X-ray Probes of Black Hole Accretion Disks for Testing the No-Hair Theorem
• Using Simulations of Black Holes to Study General Relativity and the Properties of Inner Accretion Flow