Stochastic Gravitational Wave Background from PBH-ABH Mergers
  The measurement of gravitational waves produced by binary black-hole mergers
at the Advanced LIGO has encouraged extensive studies on the stochastic
gravitational wave background. Recent studies have focused on gravitational
wave sources made of the same species, such as mergers from binary primordial
black holes or those from binary astrophysical black holes. In this paper, we
study a new possibility -- the stochastic gravitational wave background
produced by mergers of one primordial black hole and one astrophysical black
hole. Such systems are necessarily present if primordial black holes exist. We
study the isotropic gravitational wave background produced through the history
of the Universe. We find it is very challenging to detect such a signal. We
also demonstrate that it is improper to treat the gravitational waves produced
by such binaries in the Milky Way as a directional stochastic background, due
to a very low binary formation rate.
