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1. Possible binary neutron star merger history of the primary of GW230529

   https://doi.org/10.1103/c9l3-gw6w  

   Mahapatra, Parthapratim; Chattopadhyay, Debatri; Gupta, Anuradha; Antonini, Fabio; Favata, Marc; Sathyaprakash, B S; Arun, K G (June 2025, Physical Review D)

   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 ${67}_{-61}^{+163}$ 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. 

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2. A Unified Picture of Short and Long Gamma-Ray Bursts from Compact Binary Mergers

   https://doi.org/10.3847/2041-8213/ad096e  

   Gottlieb, Ore; Metzger, Brian D; Quataert, Eliot; Issa, Danat; Martineau, Tia; Foucart, Francois; Duez, Matthew D; Kidder, Lawrence E; Pfeiffer, Harald P; Scheel, Mark A (November 2023, The Astrophysical Journal Letters)

   Abstract The recent detections of the ∼10 s longγ-ray bursts (GRBs) 211211A and 230307A followed by softer temporally extended emission (EE) and kilonovae point to a new GRB class. Using state-of-the-art first-principles simulations, we introduce a unifying theoretical framework that connects binary neutron star (BNS) and black hole–NS (BH–NS) merger populations with the fundamental physics governing compact binary GRBs (cbGRBs). For binaries with large total masses,M_tot≳ 2.8M_⊙, the compact remnant created by the merger promptly collapses into a BH surrounded by an accretion disk. The duration of the pre-magnetically arrested disk (MAD) phase sets the duration of the roughly constant power cbGRB and could be influenced by the disk mass,M_d. We show that massive disks (M_d≳ 0.1M_⊙), which form for large binary mass ratiosq≳ 1.2 in BNS orq≲ 3 in BH–NS mergers, inevitably produce 211211A-like long cbGRBs. Once the disk becomes MAD, the jet power drops with the mass accretion rate as $\dot{M}\sim t^{-2}$ , establishing the EE decay. Two scenarios are plausible for short cbGRBs. They can be powered by BHs with less massive disks, which form for otherqvalues. Alternatively, for binaries withM_tot≲ 2.8M_⊙, mergers should go through a hypermassive NS (HMNS) phase, as inferred for GW170817. Magnetized outflows from such HMNSs, which typically live for ≲1 s, offer an alternative progenitor for short cbGRBs. The first scenario is challenged by the bimodal GRB duration distribution and the fact that the Galactic BNS population peaks at sufficiently low masses that most mergers should go through an HMNS phase. 

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3. Numerical relativity simulations of the neutron star merger GW190425: microphysics and mass ratio effects

   https://doi.org/10.1093/mnras/stac2333  

   Camilletti, Alessandro; Chiesa, Leonardo; Ricigliano, Giacomo; Perego, Albino; Lippold, Lukas Chris; Padamata, Surendra; Bernuzzi, Sebastiano; Radice, David; Logoteta, Domenico; Guercilena, Federico Maria (September 2022, Monthly Notices of the Royal Astronomical Society)

   ABSTRACT GW190425 was the second gravitational wave (GW) signal compatible with a binary neutron star (BNS) merger detected by the Advanced LIGO and Advanced Virgo detectors. Since no electromagnetic counterpart was identified, whether the associated kilonova was too dim or the localization area too broad is still an open question. We simulate 28 BNS mergers with the chirp mass of GW190425 and mass ratio 1 ≤ q ≤ 1.67, using numerical-relativity simulations with finite-temperature, composition dependent equations of state (EOS) and neutrino radiation. The energy emitted in GWs is $$\lesssim 0.083\mathrm{\, M_\odot }c^2$$ with peak luminosity of 1.1–$$2.4\times ~10^{58}/(1+q)^2\, {\rm {erg \, s^{-1}}}$$. Dynamical ejecta and disc mass range between 5 × 10−6–10−3 and 10−5–$$0.1 \mathrm{\, M_\odot }$$, respectively. Asymmetric mergers, especially with stiff EOSs, unbind more matter and form heavier discs compared to equal mass binaries. The angular momentum of the disc is 8–$$10\mathrm{\, M_\odot }~GM_{\rm {disc}}/c$$ over three orders of magnitude in Mdisc. While the nucleosynthesis shows no peculiarity, the simulated kilonovae are relatively dim compared with GW170817. For distances compatible with GW190425, AB magnitudes are always dimmer than ∼20 mag for the B, r, and K bands, with brighter kilonovae associated to more asymmetric binaries and stiffer EOSs. We suggest that, even assuming a good coverage of GW190425’s sky location, the kilonova could hardly have been detected by present wide-field surveys and no firm constraints on the binary parameters or EOS can be argued from the lack of the detection. 

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4. Inferring the Neutron Star Maximum Mass and Lower Mass Gap in Neutron Star–Black Hole Systems with Spin

   https://doi.org/10.3847/1538-4357/ac7f99  

   Ye, Christine; Fishbach, Maya (September 2022, The Astrophysical Journal)

   Abstract Gravitational-wave (GW) detections of merging neutron star–black hole (NSBH) systems probe astrophysical neutron star (NS) and black hole (BH) mass distributions, especially at the transition between NS and BH masses. Of particular interest are the maximum NS mass, minimum BH mass, and potential mass gap between them. While previous GW population analyses assumed all NSs obey the same maximum mass, if rapidly spinning NSs exist, they can extend to larger maximum masses than nonspinning NSs. In fact, several authors have proposed that the ∼2.6 M ⊙ object in the event GW190814—either the most massive NS or least massive BH observed to date—is a rapidly spinning NS. We therefore infer the NSBH mass distribution jointly with the NS spin distribution, modeling the NS maximum mass as a function of spin. Using four LIGO–Virgo NSBH events including GW190814, if we assume that the NS spin distribution is uniformly distributed up to the maximum (breakup) spin, we infer the maximum nonspinning NS mass is 2.7 − 0.4 + 0.5 M ⊙ (90% credibility), while assuming only nonspinning NSs, the NS maximum mass must be >2.53 M ⊙ (90% credibility). The data support the mass gap’s existence, with a minimum BH mass at 5.4 − 1.0 + 0.7 M ⊙ . With future observations, under simplified assumptions, 150 NSBH events may constrain the maximum nonspinning NS mass to ±0.02 M ⊙ , and we may even measure the relation between the NS spin and maximum mass entirely from GW data. If rapidly rotating NSs exist, their spins and masses must be modeled simultaneously to avoid biasing the NS maximum mass. 

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5. Black hole–neutron star mergers: The first mass gap and kilonovae

   https://doi.org/10.1051/0004-6361/202039418  

   Drozda, P.; Belczynski, K.; O’Shaughnessy, R.; Bulik, T.; Fryer, C. L. (November 2022, Astronomy & Astrophysics)

   Observations of X-ray binaries indicate a dearth of compact objects in the mass range from ∼2 − 5  M ⊙ . The existence of this (first mass) gap has been used to discriminate between proposed engines behind core-collapse supernovae. From LIGO/Virgo observations of binary compact remnant masses, several candidate first mass gap objects, either neutron stars (NSs) or black holes (BHs), were identified during the O3 science run. Motivated by these new observations, we study the formation of BH-NS mergers in the framework of isolated classical binary evolution, using population synthesis methods to evolve large populations of binary stars (Population I and II) across cosmic time. We present results on the NS to BH mass ratios ( q  =  M NS / M BH ) in merging systems, showing that although systems with a mass ratio as low as q  = 0.02 can exist, typically BH-NS systems form with moderate mass ratios q  = 0.1 − 0.2. If we adopt a delayed supernova engine, we conclude that ∼30% of BH-NS mergers may host at least one compact object in the first mass gap (FMG). Even allowing for uncertainties in the processes behind compact object formation, we expect the fraction of BH-NS systems ejecting mass during the merger to be small (from ∼0.6 − 9%). In our reference model, we assume: (i) the formation of compact objects within the FMG, (ii) natal NS/BH kicks decreased by fallback, (iii) low BH spins due to Tayler-Spruit angular momentum transport in massive stars. We find that ≲1% of BH-NS mergers will have any mass ejection and about the same percentage will produce kilonova bright enough to have a chance of being detected with a large (Subaru-class) 8 m telescope. Interestingly, all these mergers will have both a BH and an NS in the FMG. 

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