Merging of Unequal Mass Binary Black Holes in Non-Axisymmetric Galactic
  Nuclei
  In this work we study the stellar-dynamical hardening of unequal mass
supermassive black hole (SMBH) binaries in the central regions of galactic
nuclei. We present a comprehensive set of direct N-body simulations of the
problem, varying both the total mass and the mass ratio of the SMBH binary
(SMBHB). Simulations are carried out with the phi-GPU N-body code, which
enabled us to fully exploit supercomputers equipped with graphic processing
units (GPUs). We adopt initial axisymmetric, rotating models, aimed at
reproducing the properties of a galactic nucleus emerging from a galaxy merger
event, containing two SMBHs initially unbound. We find no "final-pc problem",
as our SMBH's tend to pair and shrink without showing significant signs of
stalling. This confirms earlier results and extends them to large particle
numbers and for rotating systems. We find that the SMBHB hardening depends on
the binary reduced mass ratio via a single parameter function. Our results
suggest that, at fixed value of the SMBHB primary mass, the merger time of
highly asymmetric binaries is up to four order of magnitudes smaller than
equal-mass binaries. This can significantly affect the population of SMBHs
potentially detectable as gravitational wave sources.
