由二元黑洞引起的引力透镜的奇特事例：双光子球的故事，新的相对论图象和焦散线
Curious case of gravitational lensing by binary black holes: a tale of two photon spheres, new relativistic images and caustics
摘要：
由于在事件GW 150914和GW 151226中探测到来自合并的致密双星的引力波，双星黑洞最近一直处于聚光灯下。在本文中，我们研究的引力透镜的二元黑洞建模为等质量的Majumdar-Papapetrou双洞度规，并表明，该系统显示的功能是相当前所未有的，在任何其他透镜的配置研究到目前为止。我们把注意力限制在两个完全相同的黑洞之间的平面上运动的光线上，这使我们能够采用为球对称时空中的赤道透镜而发展的技术。如果两个黑洞之间的距离低于某个阈值，系统就允许两个光子球。在单个黑洞的情况下，由于从外部（不稳定）光子球之外的区域返回的光线，形成了无限多的相对论图像，所有这些都位于相对于透镜的临界角半径之外。然而，在内部（稳定）光子球的存在下，有效势再次增加，进入外部光子球的光线可以返回，导致形成一组新的无限多个相对论图像，所有这些图像都位于上述透镜的临界半径以下。随着两个黑洞之间距离的增加，两个光子球相互接近，合并并最终消失。在没有光子球的情况下，除了形成有限数量的离散相对论图像外，该系统还显著地允许径向焦散，这在以前的相对论透镜中从未观察到。因此，双黑洞系统在光子球存在和不存在的情况下都具有新的特征。我们讨论了可能的观测签名和影响的二元黑洞透镜。
英文摘要：
Binary black holes have been in limelight off late due to the detection of gravitational waves from coalescing compact binaries in the events GW150914 and GW151226. In this paper we study gravitational lensing by the binary black holes modeled as equal mass Majumdar-Papapetrou dihole metric and show that this system displays features that are quite unprecedented and absent in any other lensing configuration investigated so far. We restrict our attention to the light rays which move on the plane midway between the two identical black holes, which allows us to employ techniques developed for the equatorial lensing in spherically symmetric spacetimes. If distance between the two black holes is below a certain threshold value, the system admits two photon spheres. As in the case of single black hole, infinitely many relativistic images are formed due to the light rays which turn back from the region outside the outer (unstable) photon sphere, all of which lie beyond a critical angular radius with respect to the lens. However in the presence of the inner (stable) photon sphere, the effective potential now increases again and the light rays that enter the outer photon sphere can turn back, leading to the formation of a new set of infinitely many relativistic images, all of which lie below the critical radius from the lens mentioned above. As the distance between the two black hole increases, two photon spheres approach each other, merge and eventually disappear. In the absence of photon sphere, apart from the formation of a finite number of discrete relativistic images, the system remarkably admits a radial caustic, which has never been observed in the context of relativistic lensing before. Thus the system of binary black hole admits novel features both in the presence and absence of photon spheres. We discuss possible observational signatures and implications of binary black hole lensing.
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