Chaotic photon orbits and shadows of a non-Kerr object described by the Hartle-Thorne spacetime

Abstract

The data from the event horizon telescope have provided a novel view of the vicinity of the horizon of a black hole (BH), by imaging the region around the light-ring. They have also raised hopes for measuring in the near future, features of the image (or the shadow) related to higher order effects of photons traveling in these regions, such as the appearance of higher order bright rings. While the prospect of measuring these fine features of Kerr BHs is very interesting in itself, there are some even more intriguing prospects for observing novel features of possible non-Kerr objects, in the case that the subjects of our images are not the BH solutions of GR. In the hope of sufficient resolution being available in the future, we explore in this work the structure and properties of null geodesics around a Hartle-Thorne spacetime that includes a deformation from the Kerr spacetime characterised by the quadrupole deformation $\delta q$. These spacetimes have been found to exhibit a bifurcation of the equatorial light-ring to two off-equatorial light-rings in a range of $\delta q$s and spin parameters. In addition to this, there is a range of parameters where both the equatorial and the off-equatorial light-rings are present. This results in the formation of a pocket that can trap photons. We investigate the properties of these trapped orbits and find that chaotic behaviour emerges. Some of these chaotic orbits are additionally found to be "sticky" and get trapped close to periodic orbits for long times. We also explore how these novel features affect the shadow and find that the off-equatorial light-rings produce distinctive features that deform its circular shape, while the chaotic behaviour associated to the pocket creates features with fractal structure. These results are shown to be quite general, extending to higher order Hartle-Thorne spacetimes.

A little restructuring of the paper. Results unchanged and extended to 3rd order Hartle-Thorne. 38 pages, 21 figures, some references fixed, reduced figure sizes, notebooks with calculations available on github

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Cited by in corpus (10)

• Black Hole Images as Tests of General Relativity: Effects of Spacetime Geometry
• Structure-preserving numerical simulations of test particle dynamics around slowly rotating neutron stars within Hartle-Thorne approach
• Is a black hole shadow a reliable test of the no-hair theorem?
• Slowly-rotating compact objects: the nonintegrability of Hartle-Thorne particle geodesics
• Spacetime Measurements with the Photon Ring
• Observational features of reflection asymmetric black holes
• Fractal signatures of non-Kerr spacetimes in the shadow of light-ring bifurcations
• Third type of spacetime with the coexistence of integrability and non-integrability
• Chaotic Behavior of Geodesics in Kerr-like Spacetime
• Time-reversed Shannon entropy as a chaos indicator for non-integrable systems