Eccentric Disks from Gaseous Rings around Equal-Mass, Circular Binaries

TL;DR

This study uses high-resolution hydrodynamics simulations to explore how gaseous rings around equal-mass, circular binaries evolve into eccentric disks, affecting accretion and potential observational signatures.

Contribution

It reveals the conditions under which circumbinary rings develop high eccentricity and the mechanisms driving this growth, with implications for black hole binaries and quasar observations.

Findings

Cold, small rings reach eccentricities up to 0.7.

Eccentricity growth is driven by stream impacts at pericenter.

Eccentric disks can influence quasar light curves and emission lines.

Abstract

We perform high-resolution, grid-based hydrodynamics simulations of gaseous rings viscously spreading into disks around equal-mass, circular binaries. We find that all systems suppress accretion onto the binary when the gas is relatively cold. Circumbinary rings (CBRs) display weak variability above the binary orbital frequency $\Omega_b$ and a dominant spectral peak at $\sim0.1\Omega_b$ (half the fiducial lump frequency of $\sim0.2\Omega_b$). The evolution of CBR eccentricity depends strongly on both the initial ring radius and gas temperature, with smaller, colder rings exhibiting higher eccentricity up to $e \simeq 0.7$. Cold, compact rings develop nearly radius-independent eccentricity profiles, maintaining large $e$ out to several times the initial gas semimajor axis. We find that eccentricity growth favors a stream impact mechanism, in which gas torqued by the binary at pericenter passage exerts a perturbative force on the cavity wall. We consider inefficiently-accreting, intermediate-mass ($\sim10^4 M_\odot$) black hole binaries as sources of quasi-periodic eruptions when rejected streams shock the cavity wall and radiate in the UV or soft X-ray. We discuss the implications of eccentric disks evolved from CBRs for quasar light curves and asymmetric, time-variable double-peaked line emission from disks in galactic nuclei. If binaries drive asymmetry in accretion disk line profiles, our study suggests that the progenitor CBR must have been very compact.