Mass flow in the circumbinary disk with gap around supermassive binary
  black holes
  In this paper, we study the interaction between the supermassive binary black
holes in elliptical orbit and their surrounding disk with a gap.
  The gap in the disk is a low density region formed due to the tidal effects
of the less massive black hole. The binary we have investigated has a
sub-parsec separation and is coplanar with the disk. We find that the maximum
variation of the surface density in the gap reaches 50% during an orbital
period. However, in other regions of the disk, the density variation is much
less than 1%. Furthermore, we calculate the corresponding variation of spectral
energy distribution within a period, but little variation is found. The reason
for these results is that the viscosity timescale of the disk at the binary
radius is much longer than the orbital period of the binary.
