The dynamics and electromagnetic signatures of accretion in unequal mass binary black hole inspirals

TL;DR

This study models the electromagnetic signatures of accreting binary black holes during inspiral, revealing observable UV and X-ray variations years before merger, with implications for LISA follow-up.

Contribution

It provides the first detailed theoretical predictions of electromagnetic signals from unequal mass binary black hole inspirals, combining analytic estimates and hydrodynamics simulations.

Findings

Electromagnetic signals show UV brightening and X-ray dimming years before merger.

X-ray disappearance occurs hours to days before the gravitational wave burst.

Spectral peaks are in UV and X-ray, with inner disk emission suppressed.

Abstract

We present a theoretical study of the gravitational wave (GW) driven inspirals of accreting black hole binaries with mass $M = 10^7 M_\odot$ and mass ratios between $10^{-3}$ and $10^{-1}$. Our results are based on analytic estimates, and grid-based hydrodynamics simulations run for many thousands of binary orbits before the merger. We show that the GW inspiral is evident in the light curves and color evolution of a binary-hosting quasar, over years to decades before a merger. The long-term electromagnetic (EM) signature is characterized by a gradual UV brightening, and X-ray dimming, followed by an X-ray disappearance hours to days before the GW burst, and finally a years-like re-brightening as the disk relaxes and refuels the remnant black hole. These timescales are surprisingly insensitive to the amplitude of viscous stress in the disk. The spectrum of quasi-thermal disk emission shows two peaks: one in the UV, and another in the X-ray, associated with the outer and circum-secondary disks respectively; emission from the inner disk is suppressed because the secondary consumes most of the inflowing gas. We discuss implications for real-time and archival EM followup of GW bursts detected by LISA.