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

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 to guide the accretion flow outside the NS surface, keeping the NS in a slow rotation state during the whole accretion process.

Mass-gap extreme mass ratio inspirals

“…Compared with the β-disk, more sBHs aggregate around r ∼ 10 2 M • in the fiducial α-disk because of slower migration speed. A similar result (with slightly higher surface number densities) was obtained in our previous work [75] though the disk capture rate was not calculated from first principles.…”

“…In term of gas accretion onto stars, MGOs and sBHs within the AGN disk, the star size makes a difference. Compact objects of relatively small sizes only grow mildly within the AGN disk, while the stellar evolution is expected to be impacted by gas accretion onto stars of much larger sizes [75,76]. In this work, we are not intended to model the star evolution in detail, and we simply assume no mass change of all the star species during the evolution period.…”

“…As a result, a major part of the inflowing gas may escape as outflow and only the remaining part is accreted by the sBH (see Ref. [75] and references therein for detailed modeling). Because of the circularization process, it is reasonable to expect that the outflow carries minimal net momentum with respect to the sBH, and the momentum carried by the inflow eventually transfers to the sBH.…”

Top-heavy stellar mass distribution in galactic nuclei inferred from the universally high abundance ratio of [Fe/Mg]

“…However, the progenitors of those WDs would grow further in mass and become heavy enough to end up as NSs or BHs instead of WDs (e.g., Cantiello et al 2021;Dittmann et al 2021). Moreover, an analytical study by Pan & Yang (2021) claimed that accreting WDs spin up and thus prevent mass accretion before reaching the critical mass for the onset of thermonuclear explosions. Therefore, type Ia SNe induced by accreting WDs in dense AGN disks would not make a significant contribution to iron-enrichment of BLRs.…”

High-energy Neutrinos from Stellar Explosions in Active Galactic Nuclei Accretion Disks

“…These AGN stars will end up with supernovae (SNe) to pollute the disk with heavy elements, which can offer a possible explanation for the observational features of high-metallicity environment in AGN disks (e.g., Hamann & Ferland 1999;Warner et al 2003), and hence leave behind some stellar remnants inside the disks. AGN disks provide a natural environment for embedded stars and compact objects to grow, to accrete materials, and to migrate within it (e.g., Bellovary et al 2016;Cantiello et al 2021;Dittmann et al 2021;Jermyn et al 2021;Wang et al 2021a;Kaaz et al 2021;Kimura et al 2021;Peng & Chen 2021;Pan & Yang 2021). There could be abundant stars and compact objects gathering in the inner part of the AGN disks, so that some stellar explosions can frequently occur there.…”