  The astrophysical origin of binary black hole (BBH) mergers remains uncertain
though many events have been observed by the LIGO-Virgo-KAGRA network. Such
mergers are predicted to originate in the vicinity of massive black holes
(MBHs). Especially, GW190814, due to its secondary mass and mass ratio being
beyond the expectations of isolated stellar evolution theories, is a promising
event that has happened in an active galactic nucleus(AGN) disk. In this model,
a compact object resides in the vicinity of a merging BBH. Here we report
multiple pieces of evidence pointing to the fact that GW190814 is a BBH merging
near a compact object. The orbital motion of BBHs around the third body
produces a line-of-sight acceleration (LSA) and induces a varying Doppler
shift. Using a waveform template that considers LSA, we perform Bayesian
inference on a few BBH events with a high signal-to-noise ratio in the
gravitational-wave transient catalog (GWTC). Compared to the model for isolated
BBH mergers, we obtain significantly higher network signal-to-noise ratios for
GW190814 by that with the LSA and constrain the LSA to $a =
0.0014^{+0.0014}_{-0.0022} ~c~\mathrm{s}^{-1}$. In addition, the logarithmic
Bayes factor for the LSA case over the isolated case is $16.6$, which means the
LSA model is significantly preferred by the GW data. We conclude that this is
the first indication showing merging BBHs are located near a compact object.
