# Formal stability analysis for the recent $\gamma=5/3$ power-law spherical accretion solution

## TL;DR

This paper performs a formal stability analysis of a recently proposed $oldsymbol{
ho \, \propto \, R^{-3/2}}$ spherical accretion solution with constant Mach number, demonstrating its global stability across all parameters, enhancing its astrophysical relevance.

## Contribution

It provides the first formal stability analysis of the $oldsymbol{
ho \, \propto \, R^{-3/2}}$ power-law accretion solution, confirming its global stability for all parameter values.

## Key findings

- The solution is globally stable for all parameter values.
- Stability holds for both accretion and outflow modes.
- Supports the solution's relevance to observed AGN density profiles.

## Abstract

Recently, an exact spherically symmetric analytic accretion solution was presented having simple $\rho \propto R^{-3/2}$ and $V \propto R^{-1/2}$ scalings in Hernandez et al. (2023). In dimensionless variables that solution forms a one-parameter family of solutions ranging from an empty free-fall solution to a hydrostatic equilibrium configuration. This power-law solution is characterised by a constant Mach number for the flow, which can vary from zero to infinity as a function of the one parameter of the solution, and has an accretion density profile which naturally goes to zero at large radii. This accretion density profile was shown in Hernandez et al. (2023) to be an accurate representation of the accretion density profiles of a sample of AGN galaxies, over hundreds of Bondi radii. The observed density profiles fall by many orders of magnitude in density beyond their Bondi radii, something which is inconsistent with classical Bondi models where the accretion density profiles rapidly converge to a constant outside of the Bondi radius. While the good agreement with observations is suggestive of a global stability for the solution mentioned, no formal stability analysis for it has previously been presented. Here we perform such stability analysis and show the solution mentioned to be globally stable for all values of the parameters governing it, both for its accretion and outflow modes. This result makes the $\gamma=5/3$ power-law spherical accretion model an interesting analytical addition to the study and description of accretion problems in astrophysics.