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arXiv 0806.4102gr-qcastro-ph
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Kerr时空的光焦散:完整结构

Optical caustics of Kerr spacetime: the full structure

V. Bozza

摘要:

本文对旋转黑洞对无穷远处观测者的引力透镜的光学焦散进行了详尽的数值研究。除了主焦散,我们研究高阶焦散,形成光子执行一个或几个循环周围的黑洞。我们的研究涵盖了整个参数空间,包括黑洞自旋,它相对于视线的倾斜度,源距离和焦散阶。通过比较我们的结果与可用的分析近似,我们发现在各自的有效域完美的协议。然后,我们证明,所有焦散保持其形状(一个管的星形截面)在整个参数空间中,而不遭受任何过渡到不同的焦散形状。然而,对于接近极端的自旋,高阶焦散会变得非常大,以至于它们在固定半径处的横截面绕着黑洞缠绕了好几圈。因此,对于每个焦散阶,图像的数量范围从2到2(n+1),其中n是焦散所跨越的循环的数量。至于临界曲线,我们注意到,对于高自旋值,它们在对应于反轨道的一侧形成一个小的倾角。

英文摘要:

We present an exhaustive numerical investigation of the optical caustics in gravitational lensing by a spinning black hole for an observer at infinity. Besides the primary caustic, we examine higher order caustics, formed by photons performing one or several loops around the black hole. Our investigation covers the whole parameter space, including the black hole spin, its inclination with respect to the line of sight, the source distance, and the caustic order. By comparing our results with the available analytical approximations, we find perfect agreement in their respective domains of validity. We then prove that all caustics maintain their shape (a tube with astroidal cross-section) in the entire parameter space without suffering any transitions to different caustic shapes. For nearly extremal spin, however, higher order caustics grow so large that their cross-sections at fixed radii wind several times around the black hole. As a consequence, for each caustic order, the number of images ranges from 2 to 2(n+1), where n is the number of loops spanned by the caustic. As for the critical curves, we note that for high values of the spin they develop a small dip on the side corresponding to prograde orbits.

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