Shadows of Colliding Black Holes
  We compute the shadows of colliding black holes using the Kastor-Traschen cosmological multiblack hole solution that is an exact solution describing the collision of maximally charged black holes with a positive cosmological constant. We find that in addition to the shadow of each black hole, an eyebrowlike structure appears as the black holes come close to each other. These features can be used as probes to find the multiblack hole system at the final stage of its merger process. 
Daisuke Nitta, Takeshi Chiba, Naoshi Sugiyama
gr-qc astro-ph.CO astro-ph.HE