The Population of Viscosity- and Gravitational Wave-Driven Supermassive Black Hole Binaries Among Luminous AGN

TL;DR

This paper models the orbital decay of supermassive black hole binaries in galactic nuclei, exploring their observable signatures in electromagnetic and gravitational wave signals, and discusses how future surveys can detect and characterize these populations.

Contribution

It introduces a simplified disk interaction model to predict SMBHB populations and their observable signatures, linking orbital decay regimes to potential detections in electromagnetic and gravitational wave observations.

Findings

SMBHBs spend significant time with orbital periods between a day and a year.

Detection of periodic quasars can reveal SMBHB populations and their decay physics.

Low-mass SMBHBs are detectable by LISA, with viscous effects influencing the gravitational wave background.

Abstract

Supermassive black hole binaries (SMBHBs) in galactic nuclei are thought to be a common by-product of major galaxy mergers. We use simple disk models for the circumbinary gas and for the binary-disk interaction to follow the orbital decay of SMBHBs with a range of total masses (M) and mass ratios (q), through physically distinct regions of the disk, until gravitational waves (GWs) take over their evolution. Prior to the GW-driven phase, the viscous decay is in the stalled "secondary-dominated" regime. SMBHBs spend a non-negligible fraction of 10^7 years at orbital periods t_var between a day and a year. A dedicated optical or X-ray survey could identify coalescing SMBHBs statistically, as a population of periodically variable quasars, whose abundance N_var is proportional to t_var^alpha, in a range of periods t_var around tens of weeks. SMBHBs with M < 10^7 M_sun, with 0.5 < alpha < 1.5, would probe the physics of viscous orbital decay, whereas the detection of a population of higher-mass binaries, with alpha=8/3, would confirm that their decay is driven by GWs. The lowest mass SMBHBs (M < 10^{5-6} M_sun) enter the GW-driven regime at short orbital periods, in the frequency band of the Laser Interferometric Space Antenna (LISA). While viscous processes are strongly sub-dominant in the last few years of coalescence, they could reduce the amplitude of any unresolved background of near-stationary LISA sources. We discuss constraints on the SMBHB population available from existing data, and the sensitivity and sky coverage requirements for a detection in future surveys. SMBHBs may also be identified from velocity shifts in their spectra; we discuss the expected abundance of SMBHBs as a function of their orbital velocity.