GW190814: Gravitational Waves from the Coalescence of a 23 M$_\odot$ Black Hole with a 2.6 M$_\odot$ Compact Object (2006.12611v1)

Abstract: We report the observation of a compact binary coalescence involving a 22.2 - 24.3 $M_{\odot}$ black hole and a compact object with a mass of 2.50 - 2.67 $M_{\odot}$ (all measurements quoted at the 90$\%$ credible level). The gravitational-wave signal, GW190814, was observed during LIGO's and Virgo's third observing run on August 14, 2019 at 21:10:39 UTC and has a signal-to-noise ratio of 25 in the three-detector network. The source was localized to 18.5 deg$^2$ at a distance of $241^{+41}_{-45}$ Mpc; no electromagnetic counterpart has been confirmed to date. The source has the most unequal mass ratio yet measured with gravitational waves, $0.112^{+0.008}_{-0.009}$, and its secondary component is either the lightest black hole or the heaviest neutron star ever discovered in a double compact-object system. The dimensionless spin of the primary black hole is tightly constrained to $\leq 0.07$. Tests of general relativity reveal no measurable deviations from the theory, and its prediction of higher-multipole emission is confirmed at high confidence. We estimate a merger rate density of 1-23 Gpc$^{-3}$ yr$^{-1}$ for the new class of binary coalescence sources that GW190814 represents. Astrophysical models predict that binaries with mass ratios similar to this event can form through several channels, but are unlikely to have formed in globular clusters. However, the combination of mass ratio, component masses, and the inferred merger rate for this event challenges all current models for the formation and mass distribution of compact-object binaries.

• The paper presents the first observation of an extremely asymmetric merger featuring a 23 M☉ black hole paired with a 2.6 M☉ compact object.
• It uses high-precision gravitational-wave detection with a signal-to-noise ratio of 25 to constrain component masses and spin characteristics.
• Findings challenge existing astrophysical models and neutron star equation of state, signaling the need for revised formation scenarios.

GW190814: Gravitational Waves from the Coalescence of a 23 M$_\odot$ Black Hole with a 2.6 M$_\odot$ Compact Object

The paper presents the observation and analysis of the gravitational-wave event GW190814, detected by the LIGO and Virgo observatories. This event marks the coalescence of a 23 M$_\odot$ black hole with a 2.6 M$_\odot$ compact object. Notably, GW190814 exhibits the most unequal mass ratio ever observed in gravitational-wave astronomy, with a ratio of approximately 0.112. The secondary component's classification as either the lightest black hole or the heaviest neutron star ever detected within a double compact-object system is a significant focus of this research.

Key Findings and Analysis

1. Gravitational-Wave Detection: GW190814 was identified during the third observing run of LIGO and Virgo, and it was observed to have a signal-to-noise ratio of 25 within the three-detector network. The event was localized with high precision, albeit without an electromagnetic counterpart.
2. Component Masses and Spin Characteristics: The primary component, a black hole, has its spin tightly constrained, showing minimal rotation. The secondary component's mass places it within the hypothesized lower mass gap between known neutron stars and black holes. The detection and analysis suggest that being the lightest black hole or the heaviest neutron star both remain feasible interpretations for the secondary.
3. Merger Rate Density: The analysis of GW190814 contributes to understanding the merger rate density for sources like it, with an estimated merger rate density of around 7^{+16} events per year per cubic gigaparsec.
4. Implications for Neutron Star Equation of State: The secondary component's mass exceeds typical neutron star masses, suggesting significant constraints on the equation of state if it is indeed a neutron star. This challenges previously held constraints on the maximum mass neutron stars can achieve.
5. Astrophysical Formation Models: The formation scenario of GW190814 is not easily explained by existing models, which struggle to predict the formation of such highly asymmetric mass systems. Both isolated binary evolution and dynamic assembly in dense star environments present challenges, suggesting the necessity for an expanded understanding or revised models in these areas.
6. Tests of General Relativity: The paper reports no significant deviations from general relativity, even with the additional data from higher-multipole emissions that GW190814 provided.

Conclusion and Future Directions

GW190814 challenges several aspects of compact binary formation and compact object classification. It provides insights into the limits of neutron star masses and the physics governing these enigmatic cosmic phenomena. This event exemplifies the significance of multi-messenger astrophysics and gravitational-wave astronomy's potential to refine theoretical models. Additionally, continued observations and analyses of similar events are crucial for further testing general relativity under unprecedented conditions and improving our understanding of the universe's most extreme structures, potentially informing new physics in the regime of strong gravitational fields. The findings underscore the need for more comprehensive models and observational campaigns to explain novel systems such as GW190814.