A Very Close Binary Black Hole in a Giant Elliptical Galaxy 3C 66B and
  its Black Hole Merger
  Recent observational results provide possible evidence that binary black
holes (BBHs) exist in the center of giant galaxies and may merge to form a
supermassive black hole in the process of their evolution. We first detected a
periodic flux variation on a cycle of $93\pm1$ days from the 3-mm monitor
observations of a giant elliptical galaxy \object{3C 66B} for which an orbital
motion with a period of $1.05\pm0.03$ years had been already observed. The
detected signal period being shorter than the orbital period can be explained
by taking into consideration the Doppler-shifted modulation due to the orbital
motion of a BBH. Assuming that the BBH has a circular orbit and that the jet
axis is parallel to the binary angular momentum, our observational results
demonstrate the presence of a very close BBH that has the binary orbit with an
orbital period of $1.05\pm0.03$ years, an orbital radius of $(3.9\pm1.0) \times
10^{-3}$ pc, an orbital separation of $(6.1^{+1.0}_{-0.9}) \times 10^{-3}$ pc,
the larger black hole mass of $(1.2^{+0.5}_{-0.2}) \times 10^9$ $M_{\sun}$, and
the smaller black hole mass of $(7.0^{+4.7}_{-6.4}) \times 10^8$ $M_{\sun}$.
The BBH decay time of $(5.1^{+60.5}_{-2.5})\times 10^2$ years provides evidence
for the occurrence of black hole mergers. This Letter will demonstrate the
interesting possibility of black hole collisions to form a supermassive black
hole in the process of evolution, one of the most spectacular natural phenomena
in the universe.
