Supercritical accretion of stellar-mass compact objects in active galactic nuclei

TL;DR

This paper develops a relativistic supercritical inflow-outflow model for stellar-mass compact objects in AGN disks, revealing that most accreted gas escapes as outflows, leading to mildly super-Eddington accretion and impacting the evolution of black holes, neutron stars, and white dwarfs.

Contribution

It introduces a new supercritical accretion model accounting for outflows, providing insights into mass and spin evolution of compact objects in AGN disks.

Findings

Supercritical accretion efficiency is too low to explain GW190521's electromagnetic counterpart.

Most inflowing gas escapes as outflows, resulting in mildly super-Eddington accretion.

White dwarfs are unlikely to reach Chandrasekhar limit via accretion due to spin-up effects.

Abstract

Accretion disks of active galactic nuclei (AGN) have been proposed as promising sites for producing both (stellar-mass) compact object mergers and extreme mass ratio inspirals. Along with the disk-assisted migration/evolution process, ambient gas materials inevitably accrete onto the compact objects. The description of this process is subject to significant theoretical uncertainties in previous studies. It was commonly assumed that either an Eddington accretion rate or a Bondi accretion rate (or any rate in between) takes place, although these two rates can differ from each other by several orders of magnitude. As a result, the mass and spin evolution of compact objects within AGN disks are essentially unknown. In this work, we construct a relativistic supercritical inflow-outflow model for black hole (BH) accretion. We show that the radiation efficiency of the supercritical accretion of a stellar-mass BH (sBH) is generally too low to explain the proposed electromagnetic counterpart of GW190521. Applying this model to sBHs embedded in AGN disks, we find that, although the gas inflow rates at Bondi radii of these sBHs are in general highly super-Eddington, a large fraction of inflowing gas eventually escapes as outflows so that only a small fraction accretes onto the sBH, resulting in mildly super-Eddington BH absorption in most cases. We also implement this inflow-outflow model to study the evolution of neutron stars (NS) and white dwarfs (WD) in AGN disks, taking into account corrections from star sizes and star magnetic fields. It turns out to be difficult for WDs to grow to the Chandrasekhar limit via accretion because WDs are spun up more efficiently to reach the shedding limit before the Chandrasekhar limit. For NSs the accretion-induced collapse is possible if NS magnetic fields are sufficiently strong, keeping the NS in a slow rotation state during accretion.