Eccentric Disks from Circumbinary Rings

TL;DR

The study shows that finite circumbinary rings around equal-mass circular binaries can robustly suppress accretion in cold gas, while relaxing into eccentric disks whose eccentricity and observational signatures depend on the initial ring radius. Rings exhibit a distinct accretion periodicity near $\sim 0.1\Omega_b$, different from the $\sim 0.2\Omega_b$ peak in infinite disks, and smaller rings can develop very high disk eccentricities ($e$ up to ~0.3). These dynamics have implications for EM counterparts of SMBHB and IMBHB systems, including QPEs, LINER-like LLAGN signatures, and asymmetric double-peaked line emission; the framework also provides LISA-based population estimates for IMBHBs with QPE-like periods, suggesting a substantial reservoir of candidate sources. Overall, the work links circumbinary gas morphology to observable variability and line profiles, informing strategies to search for and interpret binary black holes across astrophysical environments.

Abstract

We perform high-resolution, grid-based hydrodynamics simulations of finite gaseous circumbinary rings (CBRs) viscously spreading into disks around binaries. We find that all systems suppress accretion onto the binary when the gas is relatively cold. CBRs display weak variability above the binary orbital frequency $Ω_b$ and a dominant, robust spectral peak at $\sim0.1Ω_b$ (half the fiducial lump frequency of $\sim0.2Ω_b$). Smaller rings relax into disks with enhanced gas eccentricity up to $e\simeq 0.3$. We consider the possibility that inefficiently-accreting, intermediate-mass ($\sim10^4 M_\odot$) black hole binaries may be 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.

Figures (5)

• Figure 1: (Top) Accretion rate timeseries of the infinite disk in quasi-steady-state, normalized by the large-scale inflow rate, $\dot{M}_\infty=3\pi\Sigma_0\bar{\nu}a^2\Omega_b$. (Bottom) Accretion rate timeseries of the $R_0=4a$ ring well after $t_{\rm visc}$. In both plots, the dashed lines represent the time-averaged normalized accretion rates.
• Figure 2: Accretion (top), gravity (middle), and total (bottom) contributions to the time-derivative of the binary semimajor axis $a$.
• Figure 3: 100-orbit windows of the accretion rate (top) and the corresponding Lomb-Scargle periodogram (bottom) computed over 100 orbits in quasi-steady-state. Left and right panels compare ring and infinite disk initial conditions (ICs) respectively.
• Figure 4: Eccentricity $e=\left( \tilde{e}_1^2 + \tilde{e}_2^2 \right)^{1/2}$ of the circumbinary gas binned by semi-major axis of the gas orbit, $a_{\rm gas}$. The distributions are time-averaged at around $t \approx 300$ orbits.
• Figure 5: (Top) Density snapshots of ring $R_0=2a$ and $\mathcal{M}=\{10,\ 60\}$ at similar phases of precession. (Bottom) Corresponding velocity line profiles along the line of sight (LOS) chosen along the positive $x$-direction.