Modification of photon trapping orbits as a diagnostic of non-Kerr spacetimes
| Autor: | Kostas Glampedakis, George Pappas |
|---|---|
| Rok vydání: | 2019 |
| Předmět: |
Physics
Photon Spacetime 010308 nuclear & particles physics Gravitational wave General relativity Kerr metric FOS: Physical sciences Physics::Optics General Relativity and Quantum Cosmology (gr-qc) 01 natural sciences General Relativity and Quantum Cosmology Separable space Classical mechanics Deflection (physics) 0103 physical sciences Astrophysics::Earth and Planetary Astrophysics Circular orbit 010306 general physics |
| Zdroj: | Physical Review D |
| ISSN: | 2470-0029 2470-0010 |
| DOI: | 10.1103/physrevd.99.124041 |
| Popis: | Photon circular orbits, an extreme case of light deflection, are among the hallmarks of black holes and are known to play a central role in a variety of phenomena related to these extreme objects. The very existence of such orbits when motion is not confined in the equatorial plane, i.e. spherical orbits, is indeed a special property of the separable Kerr metric and may not occur, for instance, in the spacetime of other more speculative ultracompact objects. In this paper we consider a general stationary-axisymmetric spacetime and examine under what circumstances spherical or more general, variable-radius, `spheroidal' non-equatorial photon orbits may exist with the ultimate goal of using the modifications -- or even loss -- of photon trapping orbits as a telltale of non-Kerr physics. In addressing this issue, we first derive a general necessary condition for the existence of spherical/spheroidal orbits and then go on to study photon trapping orbits in a variety of known non-Kerr metrics (Johannsen, Johanssen-Psaltis, and Hartle-Thorne). The first of these is an example of a separable spacetime which supports Kerr-like spherical photon orbits. A more detailed analysis reveals a deeper connection between the presence of spherical orbits and the separability of a metric (that is, the existence of a third integral of motion). Specifically, a spacetime that does not admit spherical orbits in any coordinates is necessarily non-separable. The other two spacetimes considered here exhibit a clear non-Kerr behaviour by having spherical photon orbits replaced by spheroidal ones. More importantly, subject to the degree of deviation from Kerr, equatorial photon rings give place to non-equatorial ones with an accompanying loss of low-inclination spheroidal orbits. The implications of these results for the electromagnetic and gravitational wave signature of non-Kerr objects are briefly discussed. 27 pages, 19 figures, changes and expanded discussion on several topics, such as the off-equatorial photon rings and the loss of low inclination photon trapping orbits, after the feedback from the referees |
| Databáze: | OpenAIRE |
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