Shadows of Bonnor black dihole by chaotic lensing

We numerically study the shadows of a Bonnor black dihole through the technique of backward ray tracing. The presence of a magnetic dipole yields nonintegrable photon motion, which sharply affects the shadow of the compact object. Our results show that there exists a critical value for the shadow. When the magnetic dipole parameter is less than the critical value the shadow is a black disk, but when the magnetic dipole parameter is larger than the critical value the shadow becomes a concave disk with eyebrows possessing a self-similar fractal structure. These behaviors are very similar to those of the equal-mass and nonspinning Majumdar-Papapetrou binary black holes. However, we find that the two larger shadows and the smaller eyebrow-like shadows are joined together by the middle black zone for the Bonnor black dihole, which is different from that in the Majumdar-Papapetrou binary black hole spacetime where they are disconnected. With the increase of the magnetic dipole parameter, the middle black zone connecting the main shadows and the eyebrow-like shadows becomes narrow. Our results show that the spacetime properties arising from the magnetic dipole yield interesting patterns for the shadow cast by a Bonnor black dihole.