Twisting shadows: Light rings, lensing and shadows of black holes in swirling universes

Abstract

Using the Ernst formalism, a novel solution of vacuum general relativity (GR) was recently obtained [1], describing a Schwarzschild black hole (BH) immersed in a nonasymptotically flat rotating background, dubbed swirling universe, with the peculiar property that north and south hemispheres spin in opposite directions. We investigate the null geodesic flow and, in particular, the existence of light rings in this vacuum geometry. By evaluating the total topological charge $w$, we show that there exists one unstable light ring ($w=-1$) for each rotation sense of the background. We observe that the swirling background drives the Schwarzschild BH light rings outside the equatorial plane, displaying counterrotating motion with respect to each other, while (both) corotating with respect to the swirling universe. Using backwards ray tracing, we obtain the shadow and gravitational lensing effects, revealing a novel feature for observers on the equatorial plane: the BH shadow displays an odd ${\mathbb{Z}}_2$ (north-south) symmetry, inherited from the same type of symmetry of the spacetime itself: a twisted shadow.

22 pages, 14 figures. Minor corrections to Figs. 11 and 12, along with small post-publication updates. Conclusions unchanged

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Cited by in corpus (7)

• Kerr-Newman black hole in a Melvin-swirling universe
• Can light-rings self-gravitate?
• Rotating and swirling binary black hole system balanced by its gravitational spin-spin interaction
• Accretion Structures around Kerr Black Holes in a Swirling Background
• Characterisation of circular light rays in a plasma
• Chaotic motion of particles around a dyonic Kerr-Newman black hole immersed in the Melvin-swirling universe
• Gravitational waves from equatorially eccentric extreme mass ratio inspirals around swirling Kerr black holes