  The main idea of our research is to estimate the physical coalescence time of
the double supermassive black hole (SMBH) system in the centre of NGC 6240
based on the X-ray observations from the Chandra space observatory. The spectra
of the Northern and Southern nuclei were fitted by spectral models from Sherpa
and both presented the narrow component of the Fe K$\alpha$ emission line. It
enabled us to apply the spectral model to these lines and to find relative
offset $\approx0.02$ keV. The enclosed dynamical mass of the central region of
NGC 6240 with radius 1 kpc was estimated $\approx 2.04\times 10^{11} \rm\;
M_{\odot}$. These data allowed us to carry on the high resolution direct N-body
simulations with Newtonian and post-Newtonian (up to $2.5\mathcal{PN}$
correction) dynamics for this particular double SMBH system. As a result, from
our numerical models we approximated the central SMBH binary merging time for
the different binary eccentricities. In our numerical parameters range the
upper limit for the merging time, even for the very small eccentricities, is
still below $\approx70$ Myr. Gravitational waveforms and amplitude-frequency
pictures from such events can be detected using Pulsar Timing Array (PTA)
projects at the last merging phase.
