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

Abstract

Binary black holes have been in limelight off late due to the detection of a gravitational waves from coalescing compact binaries in the events GW150914 and GW151226. In this paper we study gravitational lensing by the binary black holes modelled as an equal mass Majumdar-Papapetrou di-hole metric and show that this system displays features that are quite unprecedented and absent in any other lensing configuration investigated so far in the literature. 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 various techniques developed for the equatorial lensing in the spherically symmetric spacetimes. If distance between the two black holes is below a certain threshold value, then 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 after admitting minimum turns upwards and blows up for the smaller values of radii 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 is increased, two photon spheres approach one another, merge and eventually disappear. In the absence of the 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 the binary black hole lensing.

Black hole binaries: Ergoregions, photon surfaces, wave scattering, and quasinormal modes

“…Substituting E and L and setting δ = 0 in the normalization condition, the equation for null geodesics becomeṡ Provided that M 2 > 27/8, the effective potential V eff has two critical points, which correspond to a stable (inner) and an unstable (outer) closed orbit. 3 The analytic expressions for these two radii are [31] ρ out = 4M 2 9 1 + 2 cos…”

Effect of a second compact object on stable circular orbits

“…We can extract the specific features of phenomena around a binary black hole system by using these dihole spacetimes. Indeed, the eyebrows structure of the binary black hole shadow is reproduced in the (quasi)static dihole spacetime [20][21][22][23][24].The aim of the present paper is to reveal how the marginally stable circular orbit (MSCO) or ISCO of the dihole spacetime varies compared to those of the single black hole spacetime. To achieve this, we adopt the Majumdar-Papapetrou (MP) dihole spacetime, which contains two extremal Reissner-Nordström black holes.…”

Gravitational lensing by a quantum deformed Schwarzschild black hole

“…This will allow photons to loop around such an object by several times and to produce relativistic images [70]. The direct image of M87* by EHT [2][3][4][5][6][7] demonstrates the possibility of observing the strong deflection gravitational lensing by black holes, which would be helpful for understanding the physical properties of the black holes [71][72][73][74][75][76][77][78], for probing and distinguishing various kinds of black holes [79][80][81][82][83][84] and horizonless compact objects [85][86][87][88][89][90][91][92][93][94][95][96][97][98][99][100][101].…”