Curious case of gravitational lensing by binary black holes: a tale of two photon spheres, new relativistic images and caustics

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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.