GW190521: orbital eccentricity and signatures of dynamical formation in a binary black hole merger signal (2009.04771v3)

Abstract: Pair instability supernovae are thought to restrict the formation of black holes in the mass range ~50 - 135 solar masses. However, black holes with masses within this "high mass gap" are expected to form as the remnants of binary black hole mergers. These remnants can merge again dynamically in densely populated environments such as globular clusters. The hypothesis that the binary black hole merger GW190521 formed dynamically is supported by its high mass. Orbital eccentricity can also be a signature of dynamical formation, since a binary that merges quickly after becoming bound may not circularize before merger. In this work, we measure the orbital eccentricity of GW190521. We find that the data prefer a signal with eccentricity $e \geq 0.1$ at 10 Hz to a non-precessing, quasi-circular signal, with a log Bayes factor $\ln{\cal B}=5.0$. When compared to precessing, quasi-circular analyses, the data prefer a non-precessing, $e \geq 0.1$ signal, with log Bayes factors $\ln{\cal B}\approx2$. Using injection studies, we find that a non-spinning, moderately eccentric ($e = 0.13$) GW190521-like binary can be mistaken for a quasi-circular, precessing binary. Conversely, a quasi-circular binary with spin-induced precession may be mistaken for an eccentric binary. We therefore cannot confidently determine whether GW190521 was precessing or eccentric. Nevertheless, since both of these properties support the dynamical formation hypothesis, our findings support the hypothesis that GW190521 formed dynamically.

Overview of "GW190521: Orbital Eccentricity and Signatures of Dynamical Formation in a Binary Black Hole Merger Signal"

The paper examines GW190521, a binary black hole merger signal of particular interest due to the proposed high mass of its constituent black holes and the potential implications for their formation pathway. This paper investigates whether the mass and dynamics of the merger favor a hypothesis of dynamical formation, using gravitational wave data analysis to assess the orbital eccentricity of the system.

Key Results and Analysis

The authors measure the orbital eccentricity of GW190521 using detector data from LIGO and Virgo. The presence of eccentricity in binary systems can suggest a dynamical formation, as such systems may not achieve circularized orbits before merging. The authors find a significant preference for an eccentric signal $e \geq 0.1$ over a non-precessing, quasi-circular signal, with a log Bayes factor of $\ln{\cal B}=5.0$. This finding complements gravitational wave characteristics indicating that components of previous mergers and remnants may undergo further mergers in densely populated environments like globular clusters, which support dynamical formation scenarios.

Injection studies conducted in the paper reveal that a non-spinning, moderately eccentric binary could be misidentified as a quasi-circular, precessing binary. Conversely, a quasi-circular binary exhibiting spin-induced precession can be confused with an eccentric binary. These findings suggest a need for careful interpretation of precession and eccentricity due to inherent degeneracies.

Implications for Dynamical Formation and Astrophysical Context

The results fortify the dynamical formation hypothesis, positing that GW190521 may have formed in an active and densely populated astrophysical environment. This formation mechanism allows for potential hierarchical mergers, where remnants from prior black hole collisions subsequently partake in new mergers. Such a path is consistent with GW190521's position in the so-called "high mass gap," a region where black holes formed via direct stellar collapse are not expected due to pair-instability supernovae.

The dynamical formation scenario could interpret observed spin precession and non-zero orbital eccentricity as natural outcomes of frequent gravitational interactions within dense environments. These are conditions present in globular clusters or perhaps active galactic nuclei disks.

Future Directions and Conclusions

The authors highlight the need for waveform models incorporating both precession and eccentricity to more accurately interpret GW190521-like signals and refine model selection. Such models would reduce parameter degeneracy issues and improve our understanding of the astrophysical processes governing black hole mergers.

If the findings on GW190521's eccentricity and potential dynamical formation are corroborated by further observations, they will impact our comprehension of black hole mass distribution and evolution. This could enhance the characterization of intermediate-mass black holes and offer new insights into the conditions fostering recurring black hole mergers.

In summary, the paper presents a compelling case for dynamical formation processes in the context of GW190521, advocating for advanced detection strategies and theoretical models to verify and extend these observations to future gravitational wave discoveries.