Accretion-modified Stars in Accretion Disks of Active Galactic Nuclei: Gravitational-wave Bursts and Electromagnetic Counterparts from Merging Stellar Black Hole Binaries

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 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 a 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 the 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 an EMC of the GW burst, depending on the viscosity of the accretion flow. Thermal emission from the Bondi sphere appears as one of the EMCs. The BBHs radiate GWs with frequencies of ∼102 Hz, which are accessible to LIGO.