Astronomy and Astrophysics – Astrophysics
Scientific paper
2007-10-31
Phys.Rev.D78:023008,2008
Astronomy and Astrophysics
Astrophysics
7 pages, 5 figures; v2: extended discussion, 2 figures added, results unchanged; in press on PRD
Scientific paper
10.1103/PhysRevD.78.023008
The primary optical caustic surface behind a Kerr black hole is a four-cusped tube displaced from the line of sight. We derive the caustic surface in the nearly asymptotic region far from the black hole through a Taylor expansion of the lightlike geodesics up to and including fourth-order terms in m/b and a/b, where $m$ is the black hole mass, a the spin and b the impact parameter. The corresponding critical locus in the observer's sky is elliptical and a point-like source inside the caustics will be imaged as an Einstein cross. With regard to lensing near critical points, a Kerr lens is analogous to a circular lens perturbed by a dipole and a quadrupole potential. The caustic structure of the supermassive black hole in the Galactic center could be probed by lensing of low mass X-ray binaries in the Galactic inner regions or by hot spots in the accretion disk.
de Luca Fabiana
Sereno Mauro
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