Population Properties of Compact Objects from the Second LIGO-Virgo Gravitational-Wave Transient Catalog (2010.14533v2)

Abstract: We report on the population of the 47 compact binary mergers detected with a false-alarm rate 1/yr in the second LIGO--Virgo Gravitational-Wave Transient Catalog, GWTC-2. We observe several characteristics of the merging binary black hole (BBH) population not discernible until now. First, we find that the primary mass spectrum contains structure beyond a power-law with a sharp high-mass cut-off; it is more consistent with a broken power law with a break at $39.7^{+20.3}{-9.1}\,M\odot$, or a power law with a Gaussian feature peaking at $33.1^{+4.0}{-5.6}\,M\odot$ (90\% credible interval). While the primary mass distribution must extend to $\sim65\,M_\odot$ or beyond, only $2.9^{+3.5}_{1.7}\%$ of systems have primary masses greater than $45\,M_\odot$. Second, we find that a fraction of BBH systems have component spins misaligned with the orbital angular momentum, giving rise to precession of the orbital plane. Moreover, 12% to 44% of BBH systems have spins tilted by more than $90^\circ$, giving rise to a negative effective inspiral spin parameter $\chi_\mathrm{eff}$. Under the assumption that such systems can only be formed by dynamical interactions, we infer that between 25% and 93% of BBH with non-vanishing $|\chi_\mathrm{eff}| > 0.01$ are dynamically assembled. Third, we estimate merger rates, finding $\mathcal{R}\text{BBH} = 23.9^{+14.3}{8.6}$ Gpc$^{-3}$ yr$^{-1}$ for BBH and $\mathcal{R}\text{BNS}= 320^{+490}{-240}$ Gpc$^{-3}$ yr$^{-1}$ for binary neutron stars. We find that the BBH rate likely increases with redshift ($85\%$ credibility), but not faster than the star-formation rate ($86\%$ credibility). Additionally, we examine recent exceptional events in the context of our population models, finding that the asymmetric masses of GW190412 and the high component masses of GW190521 are consistent with our models, but the low secondary mass of GW190814 makes it an outlier.

• The paper presents an in-depth analysis of compact object populations using LIGO-Virgo GWTC-2 data to investigate mass and spin properties.
• It establishes that the binary black hole mass distribution deviates from a single power law, featuring a break near 40 M⊙ likely linked to pulsational pair-instability supernovae.
• It refines merger rate estimates for BBHs and BNS while demonstrating significant spin misalignments that hint at dynamic assembly in dense stellar environments.

Insightful Overview of the Population Properties of Compact Objects from the Second LIGO-Virgo Gravitational-Wave Transient Catalog

The paper "Population properties of compact objects from the second LIGO–Virgo Gravitational-Wave Transient Catalog" presents an in-depth analysis of the latest data from gravitational-wave (GW) observations, emphasizing the population properties of binary black holes (BBHs), binary neutron stars (BNS), and potential neutron star-black hole (NSBH) candidates. The data analyzed stems from the LIGO-Virgo Gravitational-Wave Transient Catalog 2 (GWTC-2), encompassing the first half of the third observing run combined with earlier runs, thereby significantly expanding the detected sample size and allowing for novel insights into the characteristics and formation channels of compact binary systems.

Mass Distribution Insights

One of the key findings is the complexity beyond a power-law distribution in the primary mass spectrum of BBH systems. The analysis indicates that a broken power law or a power law with a Gaussian feature offers a better fit to the data. Specifically, the broken power law infers a break at approximately 39.7 M⊙, with a steep drop-off beyond this point, suggesting that such a break could be an imprint of pulsational pair-instability supernovae (PPSN). Additionally, there is evidence for a substantial number of systems with masses extending beyond the traditional observational boundaries, with a significant yet small population of BBHs exceeding 45 M⊙.

Spin Characteristics

Gravitational-wave data also shed light on spin characteristics, revealing that a fraction of BBH systems exhibit misaligned component spins relative to the orbital angular momentum. This results in orbital plane precession, with some spins tilted by over 90°, implying a potential dynamic assembly origin. The paper utilizes two different spin distribution models, which largely agree in detecting spin-induced precession, providing evidence for dynamically assembled binaries.

Merger Rate and Evolution

The paper provides refined estimates of merger rates, with values for BBH and BNS systems standing at 23.9 and 320 Gpc^-3 yr^-1, respectively. Notably, the inferred BBH merger rate shows consistency with dynamic assembly in dense populations, like stellar clusters. The work also explores the redshift evolution of the BBH merger rate, suggesting a likely increase with redshift, although at a rate slower than that of the star-formation rate.

Implications and Future Directions

These findings have profound implications on our understanding of compact object formation and evolution. The break in the mass spectrum aligns with expectations from stellar evolution models, potentially marking the influence of PPSN events. The detection of systems with negative effective inspiral spin parameters strengthens the case for dynamical interactions in dense environments. Consequently, future observational campaigns can refine these interpretations by increasing the detected event sample, particularly at higher redshifts, allowing further probing into the mass, spin, and merger rate distributions across cosmic time.

Conclusion

This paper elegantly navigates the complexities of gravitational-wave observations to elucidate the evolutionary pathways of compact binary systems. The evolutionary insights refined through the GWTC-2 analysis set a robust foundation for ongoing gravitational-wave astronomy and underline the necessity for continued observational and theoretical efforts to unravel the mysteries surrounding massive stellar remnants in the universe. The evolution of population models remains a dynamic interplay between data acquisition and theoretical advancements, promising further exciting revelations in the field of high-energy astrophysical phenomena.