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

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 Ω b and a dominant spectral peak at ∼0.1Ω b (half the fiducial lump frequency of ∼0.2Ω 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 ≃ 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 (∼10 4 M ⊙ ) black hole binaries as sources of quasiperiodic 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.