Lidov-Kozai Cycles with Gravitational Radiation: Merging Black Holes in Isolated Triple Systems (1608.07642v2)

Abstract: We show that a black-hole binary with an external companion can undergo Lidov-Kozai cycles that cause a close pericenter passage, leading to a rapid merger due to gravitational-wave emission. This scenario occurs most often for systems in which the companion has mass comparable to the reduced mass of the binary and the companion orbit has semi-major axis within a factor of $\sim 10$ of the binary semi-major axis. Using a simple population-synthesis model and 3-body simulations, we estimate the rate of mergers in triple black hole systems in the field to be about six per Gpc$^3$ per year in the absence of natal kicks during black hole formation. This value is within the low end of the 90\% credible interval for the total black-hole black-hole merger rate inferred from the current LIGO results. There are many uncertainties in these calculations, the largest of which is the unknown distribution of natal kicks. Even modest natal kicks of $40\mbox{km s}^{-1}$ will reduce the merger rate by a factor of 40. A few percent of these systems will have eccentricity greater than 0.999 when they first enter the frequency band detectable by aLIGO (above 10 Hz).

Analysis of Lidov-Kozai Cycles in Black Hole Mergers within Triple Systems

The paper "Lidov-Kozai Cycles with Gravitational Radiation: Merging Black Holes in Isolated Triple Systems" explores a compelling mechanism for the merger of black holes within isolated triple systems. Instead of focusing on black holes formed in globular clusters or through binary evolution alone, this paper introduces a scenario where a black-hole binary with an external companion undergoes Lidov-Kozai cycles. These cycles, generally triggered by a third massive body, can induce high eccentricity oscillations in the inner binary's orbit, driving the system towards a close pericenter passage and eventually a rapid merger through gravitational-wave emission.

Key Findings

• Merger Rate Estimation: Using a population synthesis model alongside three-body simulations, the authors estimate approximately six merger events per cubic gigaparsec per year in a scenario absent of natal kicks during black hole formation. This figure lies within the low end of currently inferred merger rates from LIGO observations, indicating a significant contribution from isolated triple systems.
• Impact of Natal Kicks: The paper highlights the profound effect of unknown natal kick distributions on merger rates. Even a modest kick velocity of 40 km/s reduces the estimated merger rate by a factor of 40, underscoring the sensitivity of these systems to initial conditions post-black-hole formation.
• Eccentricity Characteristics: Approximately a few percent of systems enter the LIGO frequency detection range with eccentricities greater than 0.999, predicting detectable non-zero eccentricity in gravitational waves from these mergers, which could allow for the identification of such formation channels.

Implications and Future Directions

The implications of these findings are multifold:

• Detection Prospects: The predicted eccentricities could provide distinct signatures in gravitational-wave detections, potentially distinguishing these mergers from those formed in globular clusters or through alternate channels.
• Theoretical Advances: The paper contributes to theoretical models by introducing the role of Lidov-Kozai cycles in black hole dynamics, challenging more classical binary evolution models and expanding on the diversity of pathways leading to gravitational wave events.
• Observational Strategy Adjustments: Future observation strategies for gravitational-wave detectors might aim to refine the sensitivity to high-eccentricity signals, thus enhancing the ability to detect these unique merger events.

While the paper provides insightful quantitative estimates, uncertainties remain, particularly regarding natal kick distributions and their stochastic nature. Further work could seek to refine these models by incorporating more detailed stellar evolution parameters, analyzing varied mass distributions, and employing enhanced computational techniques to simulate these complex interactions more accurately.

Moreover, studies exploring environments conducive to such triple systems (e.g., field populations versus dense stellar environments) could integrate observational data to offer a clearer picture of the prevalence and characteristics of these systems within the broader cosmic landscape. The continued advancement of gravitational-wave detector technology and enhanced sensitivity will likely propel this area of research, providing more precise constraints on these merger rates and improving the understanding of black hole formation channels.