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.

Figure 1

Black hole shadows for the two black hole system plotted in $\alpha \mathrm{\text{−}}\beta$ space normalized by $ϵM$ with each physical time $t/|H{|}^{-1}=0$, 1.6, 3.7, 5.3, 14.5, 1 6. Here we have used the following parameters: $\theta =\pi /2$, $m_1=m_2=M/2$, $H=-0.1/M$, and $ϵ=0.01$.