Nuclear burning in an accretion flow around a stellar-mass black hole embedded within an AGN disk

TL;DR

This paper investigates nuclear burning in the accretion flow around stellar-mass black holes within AGN disks, revealing stable burning processes that influence metal production and observable spectral line ratios.

Contribution

It introduces a detailed numerical analysis of nuclear burning in black hole accretion disks within AGN, highlighting stability and nucleosynthesis effects distinct from neutron star or white dwarf accretion.

Findings

Nuclear burning occurs mainly in the inner disk region.

The disk remains stable against thermal and secular instabilities.

Outflows can alter observed spectral line ratios, indicating super-solar metallicity.

Abstract

A stellar-mass black hole, embedded within the accretion disk of an active galactic nuclei (AGN), has the potential to accrete gas at a rate that can reach approximately $\sim 10^9$ times the Eddington limit. This study explores the potential for nuclear burning in the rapidly accreting flow towards this black hole and studies how nucleosynthesis affects metal production. Using numerical methods, we have obtained the disk structure while considering nuclear burning and assessed the stability of the disk. In contrast to gas accretion onto the surface of a neutron star or white dwarf, the disk remains stable against the thermal and secular instabilities because advection cooling offsets the nuclear heating effects. The absence of a solid surface for a black hole prevents excessive mass accumulation in the inner disk region. Notably, nuclear fusion predominantly takes place in the inner disk region, resulting in substantial burning of $\rm ^{12}C$ and $\rm ^{3}He$, particularly for black holes around $M = 10\, M_\odot$ with accretion rates exceeding approximately $\sim 10^7$ times the Eddington rate. The ejection of carbon-depleted gas through outflows can lead to an increase in the mass ratio of oxygen or nitrogen to carbon, which may be reflected in observed line ratios such as $\rm N\, V/C\, IV$ and $\rm O\, IV/C\, IV$. Consequently, these elevated spectral line ratios could be interpreted as indications of super-solar metallicity in the broad line region.