# The Dynamics of Truncated Black Hole Accretion Disks I: Viscous, Hydrodynamic Case

## TL;DR

This paper presents the first detailed hydrodynamic simulation of a truncated black hole accretion disk, revealing how thermal instability and convective motions influence disk dynamics and outflows.

## Contribution

It introduces a well-resolved viscous hydrodynamic model of truncated disks, highlighting the role of thermal instability and convective cells in their evolution.

## Key findings

- Thermal instability causes quasi-periodic transitions in disk cooling states.
- Outflows are launched from hot/cold gas interfaces, affecting disk structure.
- Convective cells driven by thermal instability influence angular momentum transport.

## Abstract

Truncated accretion disks are commonly invoked to explain the spectro-temporal variability from accreting black holes in both small systems, i.e. state transitions in galactic black hole binaries (GBHBs), and large systems, i.e. low-luminosity active galactic nuclei (LLAGNs). In the canonical truncated disk model of moderately low accretion rate systems, gas in the inner region of the accretion disk occupies a hot, radiatively inefficient phase, which leads to a geometrically thick disk, while the gas in the outer region occupies a cooler, radiatively efficient phase that resides in the standard geometrically thin disk. Observationally, there is strong empirical evidence to support this phenomenological model, but a detailed understanding of the dynamics of truncated disks is lacking. We present a well-resolved viscous, hydrodynamic simulation that uses an ad hoc cooling prescription to drive a thermal instability and, hence, produce the first sustained truncated accretion disk. With this simulation, we perform a study of the dynamics, angular momentum transport, and energetics of a truncated disk. We find that time variability introduced by the quasi-periodic transition of gas from efficient cooling to inefficient cooling impacts the evolution of the simulated disk. A consequence of the thermal instability is that an outflow is launched from the hot/cold gas interface which drives large, sub-Keplerian convective cells in the disk atmosphere. The convective cells introduce a viscous $\theta-\phi$ stress that is less than the generic $r-\phi$ viscous stress component, but greatly influences the evolution of the disk. In the truncated disk, we find that the bulk of the accreted gas is in the hot phase.