Gravitational-wave captures by intermediate-mass black holes in galactic
  nuclei
  Intermediate-mass black holes (IMBHs) have not been detected beyond any
reasonable doubt, despite their potential role as massive seeds for quasars and
sources of tidal disruption events, ultra-luminous X-ray sources, dwarf galaxy
feedback, and hypervelocity stars. Gravitational wave (GW) observations can
help to find and confirm the existence of IMBHs. Current and upcoming
detectors, such as LIGO, Virgo, KAGRA, LISA, ET, and DECIGO promise to identify
the full range from stellar-mass to supermassive black holes (SMBHs). In this
paper, we address the question of whether IMBHs can produce GWs in galactic
nuclei. We consider the possibility that stellar black holes (SBHs) form bound
systems and later coalesce with an IMBH through gravitational captures in the
dense nucleus. We show that this mechanism is efficient for IMBH masses in the
range $\sim 3\times 10^3$ M$_\odot$--$2\times 10^4$ M$_\odot$. We find that the
typical distributions of peak frequencies and merger timescales depend mainly
on the IMBH mass. In particular, the typical peak frequency is about $0.2\,$Hz,
$0.1\,$Hz, $0.09\,$Hz, and $0.05\,$Hz for $M_{\rm IMBH}=5\times 10^3$
M$_\odot$, $8\times 10^3$ M$_\odot$, $1\times 10^4$ M$_\odot$, and $2\times
10^4$ M$_\odot$, respectively. Our results show that, at design sensitivity,
both DECIGO and ET should be able to detect these IMBH--SBH mergers.
Furthermore, most of the mergers will appear eccentric ($e \gtrsim 0.1$),
providing an indication of their dynamical origin.
