GW190425: Observation of a Compact Binary Coalescence with Total Mass $\sim 3.4 M_{\odot}$

Abstract: On 2019 April 25, the LIGO Livingston detector observed a compact binary coalescence with signal-to-noise ratio 12.9. The Virgo detector was also taking data that did not contribute to detection due to a low signal-to-noise ratio, but were used for subsequent parameter estimation. The 90% credible intervals for the component masses range from 1.12 to 2.52 $M_{\odot}$ (1.45 to 1.88 $M_{\odot}$ if we restrict the dimensionless component spin magnitudes to be smaller than 0.05). These mass parameters are consistent with the individual binary components being neutron stars. However, both the source-frame chirp mass $1.44^{+0.02}_{-0.02} M_{\odot}$ and the total mass $3.4^{+0.3}{-0.1}\,M{\odot}$ of this system are significantly larger than those of any other known binary neutron star system. The possibility that one or both binary components of the system are black holes cannot be ruled out from gravitational-wave data. We discuss possible origins of the system based on its inconsistency with the known Galactic binary neutron star population. Under the assumption that the signal was produced by a binary neutron star coalescence, the local rate of neutron star mergers is updated to $250-2810 \text{Gpc}^{-3}\text{yr}^{-1}$.

• The paper reports GW190425, a gravitational-wave detection of a compact binary with a total mass of ~3.4 M☉, exceeding known binary neutron star systems.
• It employs single-detector observation with a signal-to-noise ratio of 12.9 and utilizes Virgo data for parameter estimation.
• The findings challenge traditional formation scenarios and suggest revised merger rate estimates and a reconsideration of the mass gap in compact objects.

Analysis of GW190425: A Compact Binary Coalescence with Atypical Mass Distribution

The paper reports on the gravitational wave (GW) event GW190425, observed by the LIGO Livingston detector on April 25, 2019. This event reveals a compact binary coalescence with a total mass of approximately 3.4 solar masses, significantly exceeding the mass estimates of known binary neutron star (BNS) systems. The absence of detectable electromagnetic counterparts further adds intrigue to this gravitational-wave discovery.

Observation and Detection

GW190425 was initially identified by a single detector (LIGO Livingston) with a signal-to-noise ratio (S/N) of 12.9, marking it as a highly significant gravitational-wave signal. While the Virgo detector also collected data during this observation, its S/N was insufficient for contributory detection, though it aided parameter estimation. The rarity of single-detector observation warranted stringent verification against noise and instrumental disturbances.

Unique Mass Characteristics

The paper highlights that both the source-frame chirp mass (~1.44 solar masses) and the total system mass (~3.4 solar masses) are outliers. These masses are greater than those of any known Galactic BNS systems, thus leading to speculation about potential formation scenarios. While the component mass ranges and the total mass suggest the presence of neutron stars, the possibility remains that one or both components could be black holes, given the substantial mass.

Possible Origins and Formation Scenarios

Different formation scenarios are considered in light of GW190425's unique mass distribution. Canonical isolated binary evolution pathways and potential dynamical formation within dense star clusters are scrutinized. The signal's mass attributes compel theorizing about possible high-mass progenitor systems or formation in environments with significantly different physical characteristics, such as ultra-tight orbital mergers possibly facilitated by low-metallicity origins.

Neutron Star Matter Implications

The significant system mass enhances the potential to probe neutron star matter at previously inaccessible densities. However, due to the relatively low S/N of GW190425 compared to the precedent GW170817, constraints on neutron star properties such as tidal deformability, radius, and equation of state, are broader, limiting precise extrapolations on neutron star matter properties or potential core phase transitions.

Implications for Astrophysical Rates and the Mass Gap

The occurrence of GW190425 suggests an updated local BNS merger rate of 250–2810 Gpc^-3 yr^-1, impacted significantly by this event's peculiar characteristics. Additionally, the GW190425 system potentially challenges the classical mass gap between neutron stars and black holes, pushing for reconsiderations of theoretical supernova models or proposed primordial black hole mergers.

Conclusion and Future Prospects

GW190425 evidences a distinct type of compact object merger, diverging from known BNS systems both in mass and potentially in formation mechanism. Further observational data combined with refined models will be critical in demystifying such anomalous findings. The unique nature of this discovery urges a continuous investigation into the formation and evolution of similar high-mass coalescing binaries, potentially reshaping our understanding of stellar evolution, gravitational-wave sources, and neutron star physics.

The analysis provided accentuates the collaborative efforts within the gravitational-wave astrophysics community. Future detections of similar, high-mass BNS systems, combined with advancements in detector sensitivity and multi-messenger astrophysical follow-ups, will be vital in expanding the landscape of compact binary mergers.