Mass-ratio and Magnetic Flux-Dependence of Modulated Accretion from Circumbinary Disks

TL;DR

This study investigates how the mass ratio and magnetic flux influence the formation of density lumps in circumbinary disks around supermassive black hole binaries, affecting their electromagnetic signatures.

Contribution

It introduces a model linking internal stress to lump formation and shows how mass ratio and magnetic flux affect EM modulation signals in such systems.

Findings

Lump amplitude decreases with lower mass ratios, disappearing between 1:2 and 1:5.

Higher magnetic flux weakens the density lump and EM modulation.

A threshold internal stress level predicts lump formation, consistent with simulations.

Abstract

Accreting supermassive binary black holes (SMBBHs) are potential multi-messenger sources because they emit both gravitational wave and electromagnetic (EM) radiation. Past work has shown that their EM output may be periodically modulated by an asymmetric density distribution in the circumbinary disk, often called an "overdensity" or "lump;" this modulation could possibly be used to identify a source as a binary. We explore the sensitivity of the overdensity to SMBBH mass ratio and magnetic flux through the accretion disk. We find that the relative amplitude of the overdensity and its associated EM periodic signal both degrade with diminishing mass ratio, vanishing altogether somewhere between 1:2 and 1:5. Greater magnetization also weakens the lump and any modulation of the light output. We develop a model to describe how lump formation results from internal stress degrading faster in the lump region than it can be rejuvenated through accretion inflow, and predicts a threshold value in specific internal stress below which lump formation should occur and which all our lump-forming simulations satisfy. Thus, detection of such a modulation would provide a constraint on both mass-ratio and magnetic flux piercing the accretion flow.