Possible binary neutron star merger history of the primary of GW230529

TL;DR

The paper investigates whether the primary of GW230529, a low-mass-gap object, could be the remnant of a prior binary neutron star (BNS) merger. Using NR-based fitting formulas to map parent BNS properties to remnant mass and spin, the authors perform a Bayesian inference to reconstruct plausible parent BNS configurations compatible with GW230529’s observed parameters, finding a consistent parent with a heavier NS around $2.1$–$2.5\,M_{}$ and a lighter companion around $1.3\,M_{}$, with a tidal deformability $\Lambda_{1,p}$ hinting at EOS-compatible values. The inferred parent population resembles GW190425-like binaries, suggesting GW190425-like remnants could frequently seed GW230529-like mergers, and yielding a branching fraction $f_b \approx 0.18^{+0.57}_{-0.15}$ (90% credibility) for remnants that later merge within a Hubble time. The study also discusses natal kicks and retention in cluster environments, implying hierarchical mergers in dense environments or triples/quadruples as viable formation channels, while acknowledging limitations of the NR fits and the need for NS spin-inclusive models. Overall, the work demonstrates how GW data can constrain hierarchical BNS formation scenarios and informs future observations and modeling of low-mass-gap black holes.

Abstract

Black holes (BHs) with masses between $\sim 3-5M_{\odot}$, produced by a binary neutron star (BNS) merger, can further pair up with a neutron star or BH and merge again within a Hubble time. However, the astrophysical environments in which this can happen and the rate of such mergers are open questions in astrophysics. Gravitational waves may play an important role in answering these questions. In this context, we discuss the possibility that the primary of the recent LIGO-Virgo-KAGRA binary GW230529_181500 (GW230529, in short) is the product of a previous BNS merger. Invoking numerical relativity (NR)-based fitting formulas that map the binary constituents' masses and tidal deformabilities to the mass, spin, and kick velocity of the remnant BH, we investigate the potential parents of GW230529's primary. Our calculations using NR fits based on BNS simulations reveal that the remnant of a high-mass BNS merger similar to GW190425 is consistent with the primary of GW230529. This argument is further strengthened by the gravitational wave-based merger rate estimation of GW190425-like and GW230529-like populations. We show that around 18% (median) of the GW190425-like remnants could become the primary component in GW230529-like mergers. The dimensionless tidal deformability parameter of the heavier neutron star in the parent binary is constrained to $67^{+163}_{-61}$ at 90% credibility. Using estimates of the gravitational-wave kick imparted to the remnant, we also discuss the astrophysical environments in which these types of mergers can take place and the implications for their future observations.

Figures (5)

• Figure 1: A schematic depiction of the possible merger history of GW230529 inferred by the method proposed in Ref. Mahapatra:2024qsy and using the NR fits for BNS mergers in Ref. Coughlin:2018fis. The middle of the figure depicts the observed binary components of GW230529, indicating the masses (in units of solar masses, $M_{\odot}$) as well as the effective dimensionless spin parameters $(\chi_{\rm eff}, \chi_{p})$ as inferred from the LVK Collaboration analysis GW230529. The remnant mass and spin of GW230529 are obtained using the NR fit for NSBH systems in Ref. Zappa:2019ntl. While estimating the remnant mass and spin, we have assumed a uniform distribution between 0 and 3000 for the dimensionless tidal deformability of the secondary component. The lower part of the figure shows the parameters $(m_{1, \rm p}, m_{2, \rm p}, \Lambda_{\rm 1, p}, \Lambda_{2, \rm p})$ for the parents of the primary component of GW230529. The kick magnitude $V_{\rm kick,p}$ (in km/s) imparted to the primary of GW230529 is also shown. Those values are inferred via the method described in Sec. \ref{['sec:method']} and are among the main results of this paper. (We make use of the Flat prior as discussed in Sec. \ref{['sec:prior']}.) The numbers shown here quote the median parameter values and the upper and lower limits of the 90% credibility interval of the inferred posteriors. We also show the redshift and merger rate (in units of $\rm Gpc^{-3}yr^{-1}$) for GW230529.
• Figure 2: Posterior distributions of the component masses and tidal deformability parameters of the parent BNS of the primary of GW230529. The curves with different colors and line styles correspond to different prior choices for NS masses (listed in the legend). The colored vertical lines mark the 90% credible intervals. Different prior distributions are shown in gray with varying line styles. The median values and 90% confidence intervals for the different posterior distributions are also provided in the table.
• Figure 3: The left panel shows the posterior distributions of the inferred kick magnitude $V_{\rm kick,p}$ imparted to the primary component of GW230529. The curves with different colors and line styles correspond to different prior choices for NS masses (listed in the legend). The colored vertical lines mark the 90% credible intervals. Different prior distributions are shown in gray with varying line styles. The median values and 90% confidence intervals for the different posterior distributions are also provided in the table. The right panel shows the retention probability of the primary of GW230529 as a function of the escape speed of the host astrophysical environment. The shaded regions show the range of escape speeds for GCs and nuclear star clusters Antonini:2016gqe. The retention probability is computed directly from the cumulative distribution function of $V_{\rm kick,p}$ following Ref. Mahapatra:2021hme.
• Figure 4: Left: comparison of the masses of the parent BNS of GW230529's primary with two BNS events in GWTC-3 GW170817GW190425GWTC-3 and the recently discovered binary millisecond pulsar PSR J0514--4002E Barr:2024wwl. For convenience, we also show the masses of GW230529. The 90% credible intervals for the component masses of GW190425 and the masses of the parent BNS of GW230529's primary overlap. This suggests that GW190425-like binaries can serve as the potential parent of the primary of GW230529. Right: the 90% credible intervals and median values for the merger rate and redshift of the two GWTC-3 BNS events and GW230529.
• Figure 5: The posterior distribution on the relative branching fraction $f_b$ between lower-generation BNS mergers that produce BHs in the low-mass gap and higher-generation binary mergers containing low-mass gap BHs and NSs. The vertical dashed lines mark the 90% credible intervals and median values.