  The recent advanced LIGO/Virgo detections of gravitational waves (GWs) from
stellar binary black hole (BBH) mergers, in particular GW190521, which is
potentially associated with a quasar, have stimulated renewed interest in
active galactic nuclei (AGNs) as factories of merging BBHs. Compact objects
evolving from massive stars are unavoidably enshrouded by a massive envelope to
form accretion-modified stars (AMSs) in the dense gaseous environment of a
supermassive black hole (SMBH) accretion disk. We show that most AMSs form
binaries due to gravitational interaction with each other during radial
migration in the SMBH disk, forming BBHs inside the AMS. When a BBH is born,
its orbit is initially governed by the tidal torque of the SMBH. Bondi
accretion onto BBH at a hyper-Eddington rate naturally develops and then
controls the evolution of its orbits. We find that Bondi accretion leads to
efficient removal of orbital angular momentum of the binary, whose final merger
produces a GW burst. Meanwhile, the Blandford-Znajek mechanism pumps the spin
energy of the merged BH to produce an electromagnetic counterpart (EMC).
Moreover, hyper-Eddington accretion onto the BBH develops powerful outflows and
triggers a Bondi explosion, which manifests itself as a EMC of the GW burst,
depending on the viscosity of the accretion flow. Thermal emission from Bondi
sphere appears as one of EMCs. BBHs radiate GWs with frequencies $\sim
10^{2}\,$Hz, which are accessible to LIGO.
