Accretion into the Central Cavity of a Circumbinary Disk (1210.0536v2)

Abstract: A near-equal mass binary black hole can clear a central cavity in a circumbinary accretion disk; however, previous works have revealed accretion streams entering this cavity. Here we use 2D hydrodynamical simulations to study the accretion streams and their periodic behavior. In particular, we perform a suite of simulations, covering different binary mass ratios $q=M_2/M_1$ in the range $0.01 \leq q \leq 1$. In each case, we follow the system for several thousand binary orbits, until it relaxes to a stable accretion pattern. We find the following results: (i) while the binary is efficient in maintaining a low-density cavity, the time-averaged mass accretion rate into the cavity, through narrow coherent accretion streams, is suppressed by at most a factor of $\sim 5$, compared to a disk with a single BH with the same mass; (ii) the largest suppression occurs for $q\approx 0.05$; binaries whose mass ratios are either lower or higher both suppress accretion less significantly; (iii) for $q \gtrsim 0.05$, the accretion rate is strongly modulated by the binary, and depending on the precise value of $q$, the power spectrum of the accretion rate shows either one, two, or three distinct periods; and (v) for $q \lesssim 0.05$, the accretion rate becomes steady, with no time-variations. Most binaries produced in galactic mergers are expected to have $q\gtrsim 0.05$. If the luminosity of these binaries tracks their accretion rate, then a periodogram of their light-curve could help in their identification, and to constrain their mass ratio and disk properties.

• The paper demonstrates that 2D hydrodynamic simulations capture how BBHs maintain low-density cavities with only modest reduction in mass accretion compared to single black holes.
• The study finds that for mass ratios q > 0.05, accretion exhibits distinct periodic modulations, while lower ratios lead to steadier, continuous accretion.
• The paper suggests that observable luminosity variability in merging galaxies can serve as a diagnostic tool for identifying MBHBs, enhancing gravitational wave detection strategies.

Accretion into the Central Cavity of a Circumbinary Disc

The paper "Accretion into the Central Cavity of a Circumbinary Disc" by D'Orazio, Haiman, and MacFadyen provides a compelling examination of matter accretion streams within circumbinary discs containing nearly equal-mass binary black holes (BBHs). Utilizing two-dimensional hydrodynamical simulations, the authors explore accretion dynamics across different binary mass ratios, offering insights relevant to astrophysical models of galaxy evolution and gravitational wave astronomy.

Simulations and Methodology

The paper employs 2D hydrodynamic models to analyze accretion patterns within circumbinary discs. The simulations range over a variety of mass ratios $q$ from 0.003 to 1, maintaining conditions for several thousand binary orbits to ensure stability and discern periodic behavior. These simulations refine previous work by revealing how effectively a binary maintains a low-density cavity and the factors influencing mass accretion rates into these cavities.

Key Findings

1. Cavity Maintenance: The simulations confirm the efficacy of BBHs in creating and maintaining low-density cavities within circumbinary discs. The degree of suppression of mass accretion into the cavity varies with the relative mass ratio, achieving only a modest reduction compared to scenarios involving single black holes of equivalent mass.
2. Mass Ratio Impact on Accretion:
   • For mass ratios $q > 0.05$, the accretion rates reveal distinct periodic modulations. The power spectra exhibit one, two, or three distinct periods depending on the mass ratio, underscoring the influence of binary dynamics on the accretion process.
   • For $q \leq 0.05$, accretion stabilizes, with streams becoming less periodic and indicative of systems dominated by slow, steady accretion processes.
3. Observational Signatures: These simulations suggest that binaries resulting from galaxy mergers, which often have mass ratios $q > 0.05$, could be identified via variability in their luminosity patterns. Specifically, analyzing the periodogram of the light-curve modulations may offer a diagnostic tool for constraining binary characteristics and elucidating disc properties.

Implications

Practical Implications: Since massive black hole binaries (MBHBs) serve as potential targets for gravitational wave detection, understanding their accretion dynamics is crucial. This paper lays foundational knowledge for anticipatory models correlating electromagnetic and gravitational-wave observations, thereby enhancing detection strategies for MBHBs.

Theoretical Implications: The findings underscore the need for detailed 3D simulations to refine interpretations on disc-binary interactions. The dynamics of accretion streams and modulated mass transfer rates are vital for theories surrounding BBH evolution post-galactic mergers. These insights may lead to new developments in our understanding of accretion physics under extreme gravitation influences.

Future Directions

The research could be extended to include detailed investigations into 3D disc structures and magnetohydrodynamic effects, potentially incorporating additional physics into the simulations. Examining models with varying disc temperatures and analyzing the impact on accretion stream behaviors could provide further precision in predicting observational outcomes. Moreover, exploring alternative initial conditions such as eccentric binary configurations might yield diverse evolutionary pathways and accretion profiles, broadening the scope of application for these theoretical models.

In conclusion, the paper delivers pertinent contributions to astrophysical research on BBHs, adding quantitative and qualitative depth to our existing compendium on circumbinary accretion processes. Combining theoretical predictions with observational data from future surveys and gravitational wave detections promises significant advancements in mapping the lifecycle of MBHBs in galactic nuclei.