How does a Secular Instability Grow in a Hyperaccretion Flow?

TL;DR

This paper investigates how secular instability in hyperaccretion flows with an N-shaped equilibrium curve influences flow structure and observable signatures, revealing intermittent luminosity variations relevant to gamma-ray bursts.

Contribution

It provides a detailed analysis of the time evolution of secular instability in hyperaccretion flows and its impact on luminosity variability, connecting theoretical instability to observable phenomena.

Findings

Surface density changes induce non-steady mass flows.

Non-steady flows propagate over the disk but don't cause global transitions without variable mass injection.

Luminosity varies intermittently with step-function-like light curves.

Abstract

Hyperaccretion flows with mass accretion rates far above the Eddington rate have an N-shaped equilibrium curve on the $\Sigma$-$\dot{M}$ plane (with $\Sigma$ and $\dot{M}$ being surface density and mass accretion rate, respectively). The accretion flow on the lower $\Sigma$ branch of the N-shape is optically thick, advection-dominated accretion flow (ADAF) while that on the upper one is neutrino-dominated accretion flow (NDAF). The middle branch has a negative slope on the $\Sigma$-$\dot{M}$ plane, meaning that the flow on this branch is secularly unstable. To investigate how the instability affects the flow structure and what observable signatures are produced, we study the time evolution of the unstable hyperaccretion flow in response to variable mass injection rates by solving the height-averaged equations for viscous accretion flows. When a transition occurs from the lower branch to the upper branch (or from the upper branch to the lower branch), the surface density rapidly increases (decreases) around that transition region, which induces locally enhanced mass flow (referred to as non-steady mass flow) into (out of) that region. We confirm that the non-steady flow can create a kind of disturbance and that it propagates over the whole disk. However, the non-steady mass flow is not strong enough to induce coherent transition over the whole disk, unless the mass injection rate varies with time. When the injection rate continuously changes, the neutrino luminosity varies intermittently, thus producing step-function-like light curves, as the radiation efficiency discontinuously changes every time the local transition occurs. The effects of changing the N-shape and possible observational consequences on the gamma-ray burst emission are briefly discussed in relation to gamma-ray bursts.