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

TL;DR

GW190425 reports a highly significant GW signal consistent with a binary neutron star merger having a total mass around $m_{\text{tot}}\approx 3.4\,M_{\odot}$ and a chirp mass of $\mathcal{M}=1.44\,M_{\odot}$, past the range of known Galactic BNS systems. Bayesian parameter estimation across multiple waveform models indicates component masses in the NS regime under both low- and high-spin priors, though a black-hole–neutron-star or BH–BH interpretation cannot be ruled out due to limited tidal information. The event yields modest constraints on the neutron-star equation of state and tidal deformability, while implying a potentially high local BNS merger rate of order $10^{3}\,\mathrm{Gpc}^{-3}\mathrm{yr}^{-1}$; it also suggests formation channels requiring ultra-tight or low-metallicity progenitors and/or dynamical channels, though Galactic-BNS populations remain the reference. No electromagnetic counterpart was observed, and the analysis emphasizes robustness to waveform-systematics, guiding future observations to refine NS matter constraints and binary-population models.

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 $250-2810 \text{Gpc}^{-3}\text{yr}^{-1}$.

Figures (19)

• Figure 1: Combined$\mathrm{S} / \mathrm{N}-\xi^{2}$ noise probability density function for LHO, LLO, and Virgo in the BNS region, computed by adding the normalized 2D histograms of background triggers in the $\mathrm{S} / \mathrm{N}-\xi^{2} / \mathrm{S} / \mathrm{N}^{2}$ plane from the three detectors. The gold star indicates GW190425. There is no background present at the position of GW190425; it stands out above all of the background recorded in the Advanced LIGO and Virgo detectors in the first three observing runs. The background contains 169.5 days of data from O 1 and O 2 and the first 50 days of O3, at times when any of the detectors were operating. For comparison the LLO and LHO triggers for GW170817 are also shown in the plot as blue and red diamonds, respectively.
• Figure 2: Sky map for GW190425. The shaded patch is the sky map obtained from the Bayesian parameter estimation code LALINFERENCE (Veitch et al. 2015) (see Section 4) with the$90 \%$ confidence region bounded by the thin dotted contour. The thick solid contour shows the $90 \%$ confidence region from the lowlatency sky localization algorithm BAYESTAR (Singer & Price 2016).
• Figure 3: Posterior distribution of the component masses$m_{1}$ and $m_{2}$ in the source frame for the low-spin ( $\chi<0.05$; orange) and high-spin ( $\chi<0.89$; blue) analyses. Vertical lines in the one-dimensional plots enclose $90 \%$ of the probability and correspond to the ranges given in Table 1. The one-dimensional distributions have been normalized to have equal maxima. A dashed line marks the equal-mass bound in the two-dimensional plot.
• Figure 4: Joint posterior distribution of$\chi_{\text{eff }}$ and $q$ for the low-spin ( $\chi<0.05$; orange) and high-spin ( $\chi<0.89$; blue) prior. Vertical lines enclose the $90 \%$ credible interval for $\chi_{\text{eff }}$ and horizontal lines mark the $90 \%$ lower limits for $q$. The one-dimensional distributions have been normalized to have equal maxima. For comparison, the effective spins are shown for two Galactic BNS systems, PSR J1946+2052 (green) and PSR J0737-3039A/B (red), if extrapolated to their mergers. For PSR J1946+2052, it is assumed that the primary spin is perpendicular to the orbital plane and that the unmeasured secondary spin is negligible. Uncertainties in the pulsar $q$ and $\chi_{\text{eff }}$ values, calculated by marginalizing over mass and equation of state information, are smaller than the markers except for the mass ratio of PSR J1946+2052, which is shown with an error bar.
• Figure 5: Total system masses for GW190425 under different spin priors, and those for the 10 Galactic BNSs from Farrow et al. (2019) that are expected to merge within a Hubble time. The distribution of the total masses of the latter is shown and fit using a normal distribution shown by the dashed black curve. The green curves are for individual Galactic BNS total mass distributions rescaled to the same ordinate axis height of 1 .