Secular evolution of compact binaries near massive black holes: gravitational wave sources and other exotica (1203.2938v2)

Abstract: The environment near super massive black holes (SMBHs) in galactic nuclei contain a large number of stars and compact objects. A fraction of these are likely to be members of binaries. Here we discuss the binary population of stellar black holes and neutron stars near SMBHs and focus on the secular evolution of such binaries, due to the perturbation by the SMBH. Binaries with highly inclined orbits in respect to their orbit around the SMBH are strongly affected by secular Kozai processes, which periodically change their eccentricities and inclinations (Kozai-cycles). During periapsis approach, at the highest eccentricities during the Kozai-cycles, gravitational wave emission becomes highly efficient. Some binaries in this environment can inspiral and coalesce at timescales much shorter than a Hubble time and much shorter than similar binaries which do not reside near a SMBH. The close environment of SMBHs could therefore serve as catalyst for the inspiral and coalescence of binaries, and strongly affect their orbital properties. Such compact binaries would be detectable as gravitational wave (GW) sources by the next generation of GW detectors (e.g. advanced- LIGO). About 0.5% of such nuclear merging binaries will enter the LIGO observational window while on orbit that are still very eccentric (e>~0.5). The efficient gravitational wave analysis for such systems would therefore require the use of eccentric templates. We also find that binaries very close to the MBH could evolve through a complex dynamical (non-secular) evolution leading to emission of several GW pulses during only a few yrs (though these are likely to be rare). Finally, we note that the formation of close stellar binaries, X-ray binaries and their merger products could be induced by similar secular processes, combined with tidal friction rather than GW emission as in the case of compact object binaries.

• The paper shows how the Kozai-Lidov mechanism near SMBHs induces significant eccentricity oscillations in compact binaries, boosting GW emission efficiency.
• It finds that secular perturbations drastically shorten merger times, with roughly 0.5% of binaries entering LIGO’s range featuring high eccentricities.
• The study integrates relativistic and tidal precession effects to offer a comprehensive framework for modeling binary evolution in dense galactic nuclei.

Secular Evolution of Compact Binaries Near Massive Black Holes: A Theoretical Investigation

The dynamics of compact binaries, comprising stellar mass black holes (BHs) and neutron stars (NSs), in proximity to supermassive black holes (SMBHs) is an intriguing topic of paper due to its implications for gravitational wave (GW) astronomy. Antonini and Perets explore these dynamics by modeling the secular evolution of such binaries in galactic nuclei, a region characterized by dense stellar populations and dominated by a central SMBH. The authors provide a detailed analysis of secular processes, particularly the Kozai-Lidov mechanism, which can induce significant eccentricity oscillations in binaries with highly inclined orbits relative to their trajectory around the SMBH. This eccentricity enhancement is crucial as it amplifies the emission of gravitational waves, potentially expediting the merger process.

Key Findings and Numerical Results:

• Kozai-Lidov Mechanism: Binaries near SMBHs, particularly those on highly inclined orbits, are subject to periodic eccentricity enhancement, known as Kozai cycles. This process can significantly reduce the binary's separation at closest approach to its companion, dramatically increasing GW radiation efficiency and shortening inspiral timescales to well below a Hubble time.
• Gravitational Wave Emission: The secular perturbations enable some binaries to merge on timescales much shorter than those predicted for similar binaries isolated from the influence of a SMBH. They estimate that around 0.5% of these merging binaries will enter the LIGO observatory's sensitivity range with substantial eccentricities (e.g., $e \gtrsim 0.5$). This aspect necessitates the development of eccentric GW templates for accurate detection and analysis.
• Complex Dynamics and Rare Events: Binaries very close to SMBHs can undergo complex dynamical evolution, potentially emitting multiple GW bursts over a short period. Although these events are expected to be rare, they offer a fascinating avenue for observational campaigns.

The inclusion of relativistic and tidal precession effects is paramount in the analysis, as these forces can either mitigate or enhance Kozai-induced eccentricity cycles. This treatment highlights regions of phase space where the combined effect of these processes can facilitate binary mergers.

Implications:

• Astrophysical Insight: The work presents a framework to understand how the dynamical environment near SMBHs can act as a catalyst for compact binary coalescences, enriching our understanding of stellar dynamics in such extreme environments.
• Gravitational Wave Astronomy: Binaries with high eccentricities are of particular interest, as they represent potential sources of unique GW signals, different from those anticipated by quasi-circular inspirals currently dominating LIGO/Virgo observations. Eccentric mergers could also provide constraints on dynamical processes in galactic centers and support efforts to develop enhanced GW data analysis strategies.
• Future Directions: The identification of these dynamical mechanisms lays the groundwork for subsequent studies using more detailed N-body simulations and the observation of gravitational waves emanating from eccentric compact binaries.

This paper contributes significantly to the theoretical landscape of GW source modeling and the structure of dense star clusters around SMBHs, suggesting that these environments provide a rich arena for forming and evolving GW sources. Future investigations might explore further the coupling between dynamical processes and binary evolution in disks or apply these findings to the broader range of galactic nuclei beyond our own Milky Way.