An axion-like scalar field environment effect on binary black hole
  merger
  Environment, such as the accretion disk, could modify the signal of the
gravitational wave from the astrophysical black hole binaries. In this article,
we model the matter field around the intermediate-mass binary black holes by
means of an axion-like scalar field and investigate their joint evolution. In
details, we consider the equal mass binary black holes surrounded by a shell of
axion-like scalar field both in spherical symmetric and non-spherical symmetric
cases, and with different strength of the scalar field. Our result shows that
the environmental scalar field could essentially modify the dynamics. Firstly,
in the spherical symmetric case, with increasing of the scalar field strength,
the number of circular orbit of the binary black hole is reduced. It means that
the scalar field could significantly accelerate the merger process. Secondly,
once the scalar field strength exceeds certain critical value, the scalar field
could collapse into a third black hole with its mass being larger than the
binary. Consequently, the new black hole collapsed from the environmental
scalar field could accrete the binary promptly and the binary collides head-on
between each other. In this process, there is almost no any quadrupole signal
produced, namely the gravitational wave is greatly suppressed. Thirdly, when
the scalar field strength is relatively smaller than the critical value, the
black hole orbit could develop eccentricity through the accretion of the scalar
field. Fourthly, during the initial stage of the inspire, the gravitational
attractive force from the axion-like scalar field could induce a sudden turn in
the binary orbits, hence result in a transient wiggle in the gravitational
waveform. Finally, in the non-spherical case, the scalar field could
gravitationally attract the binary moving toward the mass center of the scalar
field and slow down the merger process.
