Accretion Disks Around Binary Black Holes: A Simple GR-Hybrid Evolution Model

TL;DR

This paper presents a simplified general relativistic model for the evolution of accretion disks around binary black holes, capturing key GR effects while using Newtonian approximations for tidal interactions, and explores observable electromagnetic signatures of mergers.

Contribution

It introduces a GR-hybrid evolution equation for binary black hole accretion disks, combining full GR viscous torque treatment with Newtonian tidal torque approximation, applicable across different merger stages.

Findings

Identifies electromagnetic signatures associated with black hole mergers.

Demonstrates differences between GR-hybrid and Newtonian models in disk evolution.

Provides predictions for post-merger electromagnetic radiation.

Abstract

We consider a geometrically thin, Keplerian disk in the orbital plane of a binary black hole (BHBH) consisting of a spinning primary and low-mass secondary (mass ratio q < 1). To account for the principle effects of general relativity (GR), we propose a modification of the standard Newtonian evolution equation for the (orbit-averaged) time-varying disk surface density. In our modified equation the viscous torque in the disk is treated in full GR, while the tidal torque is handled in the Newtonian limit. Our GR-hybrid treatment is reasonable because the tidal torque is concentrated near the orbital radius of the secondary and is most important prior to binary-disk decoupling, when the orbital separation is large and resides in the weak-field regime. The tidal torque on the disk diminishes during late merger and vanishes altogether following merger. By contrast, the viscous torque drives the flow into the strong-field region and onto the primary during all epochs. Following binary coalescence, the viscous torque alone governs the time-dependent accretion onto the remnant, as well as the temporal behavior, strength and spectrum of the aftermath electromagnetic radiation from the disk. We solve our GR-hybrid equation for a representative BHBH-disk system, identify several observable EM signatures of the merger, and compare results obtained for the gas and EM radiation with those found with the Newtonian prescription.