Recent detailed observations of the radio-loud quasar

3C 186 indicate the possibility that a supermassive recoiling black

hole is moving away from the host galaxy at a speed of nearly

2100km/s. If this is the case, we can model the mass ratio and spins

of the progenitor binary black hole using the results of numerical

relativity simulations. We find that the black holes in the

progenitor must have comparable masses with a mass ratio

$q=m_1/m_2>1/4$ and the spin of the primary black hole must be

$\alpha_2=S_2/m_2^2>0.4$. The final remnant of the merger is bounded

by $\alpha_f>0.45$ and at least $4\%$ of the total mass of the

binary system is radiated into gravitational waves. We consider

four different pre-merger scenarios that further narrow those

values. Assuming, for instance, a cold accretion driven merger

model, we find that the binary had comparable masses with

$q=0.70^{+0.29}_{-0.21}$ and the normalized spins of the larger and

smaller black holes were $\alpha_2=0.94^{+0.06}_{-0.22}$ and

$\alpha_1=0.95^{+0.05}_{-0.09}$. We can also estimate the final

recoiling black hole spin $\alpha_f=0.93^{+0.02}_{-0.03}$ and that

the system radiated $9.6^{+0.8}_{-1.4}\%$ of its total mass, making

the merger of those black holes the most energetic event ever

observed.