Binary Black Hole Accretion During Inspiral and Merger

TL;DR

This study uses advanced hydrodynamical simulations to explore how accretion onto merging supermassive black hole binaries evolves through different inspiral phases, revealing a gradual decline in accretion and a peak in X-ray luminosity before merger.

Contribution

It provides the first detailed simulation-based analysis of accretion and luminosity changes during SMBH binary inspiral and merger, including viscous, shock heating, and cooling effects.

Findings

Accretion declines gradually without abrupt halts at decoupling.

X-ray luminosity peaks before the merger due to shock heating.

Post-merger, accretion and luminosity relax to steady-state values.

Abstract

We present the results of 2D, moving mesh, viscous hydrodynamical simulations of accretion onto merging supermassive black hole (SMBH) binaries. We include viscous heating, shock heating, and radiative cooling, and simulate the transition from the "pre-decoupling" epoch, where the inspiral timescale is longer than the viscous timescale, to the "post-decoupling" epoch, where the inspiral timescale is shorter than the viscous timescale. We find that there is no abrupt halt to the accretion at decoupling, but rather the accretion shows a slow decay, with significant accretion well after the expected decoupling. Moreover, we find that the luminosity in X-rays is significantly higher prior to the merger, as orbital energy from the SMBH binary is converted to heat via strong shocks inside the cavity, and radiated away. Following the merger, the cavity refills viscously and the accretion rate relaxes to the Shakura-Sunyaev value, while the X-ray luminosity drops as the shocks quickly dissipate.