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

TL;DR

This paper explores how accretion-modified stars in AGN disks can form binary black holes that merge, producing detectable gravitational waves and electromagnetic signals, with new insights into their formation and observational signatures.

Contribution

It introduces a model where BBHs form inside accretion-modified stars in AGN disks, highlighting the role of hyper-Eddington accretion and outflows in their evolution and electromagnetic counterparts.

Findings

BBHs form inside AMSs due to gravitational interactions.

Hyper-Eddington accretion efficiently removes orbital angular momentum.

Merged BBHs produce GW bursts detectable by LIGO.

Abstract

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.