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1. Gravitational scattering of two neutron stars

   https://doi.org/10.1103/d7j1-wsp4  

   Fontbuté, Joan; Bernuzzi, Sebastiano; Rettegno, Piero; Albanesi, Simone; Tichy, Wolfgang (December 2025, Physical Review D)

   We present the first numerical relativity simulations of the gravitational scattering of two neutron stars. Constraint-satisfying initial data for two equal-mass nonspinning sequences are constructed at fixed energy and various initial angular momenta (impact parameter) and evolved with Einstein equations through the scattering process. The strong-field scattering dynamics are explored up to scattering angles of 220° and the threshold of dynamical captures. The transition to bound orbits is aided by significant mass ejecta up to baryon mass $\sim 0.1M_{\odot }$ . A quantitative comparison with predictions of the scattering angle from state-of-the-art effective-one-body and post-Minkowskian calculations indicates quantitative agreement for large initial angular momenta, although significant discrepancies in the tidal contribution emerge toward the capture threshold. Gravitational waveforms and radiated energy are in qualitative agreement with the analogous black hole problem and state-of-the-art effective-one-body predictions. Toward the capture threshold, waveforms from scattering dynamics carry a strong imprint of matter effects, including the stars’ $f$ -mode excitations during the close encounter. Overall, our simulations open a new avenue to study tidal interactions in the relativistic two-body problem. 

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2. Time–frequency structures in post-merger binary black hole gravitational wave signals

   https://doi.org/10.1088/1361-6382/ae49dc  

   Henshaw, Chad; Heranval, Alice; Cadonati, Laura (March 2026, Classical and Quantum Gravity)

   Abstract Gravitational wave signals from asymmetric binary black hole systems have been shown to exhibit additional chirps beyond the primary merger chirp in the post-merger region of the time–frequency domain. These secondary post-merger chirps correlate to the evolving geometry of the common horizon that forms as the binary merges and were previously studied through numerical relativity simulation in a zero-spin regime. In this work, we investigate the post-merger time–frequency structure in systems with both aligned and precessing spin using widely available waveform models. We find that the inclusion of strong aligned spin $(\xi =0.75)$ induces further post-merger time–frequency peaks. Additionally we show that even a mildly precessing spin $({\chi }_{\mathrm{p}}=0.25)$ strongly affects the distribution of post-merger radiative power across the celestial sky of the final black hole. Our results support the theory of a correlation between the post-merger signal and horizon geometry. 

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3. Measurement prospects for the pair-instability mass cutoff with gravitational waves

   https://doi.org/10.1103/sqy9-nnqq  

   Mould, Matthew; Heinzel, Jack; Álvarez-López, Sofía; Plunkett, Cailin; Wolfe, Noah E; Vitale, Salvatore (May 2026, Physical Review D)

   Pair-instability supernovae leave behind no compact remnants, resulting in a predicted gap in the distribution of stellar black-hole masses. Gravitational waves from binary black-hole mergers probe the relevant mass range and analyses of the LIGO–Virgo–KAGRA catalog (GWTC-4) indicate a possible mass cutoff at $40-50M_{\odot }$ . However, the robustness of this result remains unclear. To this end, using full Bayesian parameter estimation, we simulate gravitational-wave catalogs with and without such a mass cutoff and then test whether its presence or absence is correctly inferred with parametric population models. For catalogs similar to GWTC-4, confident identification of a cutoff is not guaranteed, but the best constraints among our simulations are compatible with results from GWTC-4 when the model includes a cutoff. Conversely, spurious identification of a cutoff is unlikely. For catalogs expected by the end of the O4 observing run, uncertainty in the cutoff mass is reduced by $\gtrsim 20\%$ , but a cutoff at $40-50M_{\odot }$ yields only a lower bound on the ${}^{12}\mathrm{C}(\alpha ,\gamma {)}^{16}\mathrm{O}$ reaction rate, our most stringent constraints on the S factor at 300 keV being $S_{300}\gtrsim 125$ keV b at 90% credibility. Relative uncertainties on the Hubble parameter $H_0$ from gravitational-wave data alone can still be up to 100%. We also analyze GWTC-4 with the nonparametric population model, finding that some mass features are more prominent than in parametric models but a sharp cutoff is not required. However, the parametric model passes a likelihood-based predictive test in GWTC-4 and the results are consistent with those from our simulated catalogs with a cutoff. Such tests are necessary to make astrophysical claims from gravitational-wave catalogs. 

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4. Revisiting GW150914 with a non-planar, eccentric waveform model

   https://doi.org/10.1088/1361-6382/adfe50  

   Gamba, Rossella; Lange, Jacob; Chiaramello, Danilo; Tissino, Jacopo; Tibrewal, Snehal (September 2025, Classical and Quantum Gravity)

   Abstract The first direct detection of gravitational waves by the LIGO collaboration, GW150914, marked the start of a new exciting era in astronomy, enabling the study of the Universe through a new messenger. Since then, the field has grown rapidly, with the development of increasingly more sophisticated techniques to detect, analyze and interpret the signals. In this paper we revisit GW150914, presenting updated estimates of its source parameters using a waveform model developed within the EOB formalism, able to describe gravitational-wave emission from generic non-circular, non-planar binaries. We provide a comprehensive analysis of the signal and its properties, considering and contrasting various scenarios for the source: from the simplest, aligned-spin quasi-circular (QC) binary black hole (BBH) merger, to more complex scenarios, including precession, eccentricity or both. Unsurprisingly, we find that the signal is consistent with a QC (e < 0.08 at 15 Hz), slowly spinning $({\chi }_{\text{eff}}=-{0.03}_{-0.13}^{+0.12})$ BBH merger, a-posteriori validating a considerable body of works. This is the first analysis performed with an inspiral-merger-ringdown model containing both eccentricity and precession. 

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5. Search for planetary-mass ultracompact binaries using data from the first part of the LIGO–Virgo–KAGRA fourth observing run

   https://doi.org/10.1103/s551-7pch  

   Abac, A G; Abouelfettouh, I; Acernese, F; Ackley, K; Adamcewicz, C; Adhicary, S; Adhikari, D; Adhikari, N; Adhikari, R X; Adkins, V K; et al (July 2026, Physical Review D)

   We present a search for gravitational waves from inspiraling, planetary-mass ultracompact binaries using data from the first part of the fourth observing run of LIGO, Virgo, and KAGRA. Finding no evidence of such systems, we determine the maximum distance reach for such objects and their merger rate densities. Then, we identify classes of primordial black hole mass distributions for which these rate limits can be translated into relevant constraints on the mass distribution of primordial black holes, assuming that they are all composed of dark matter, in the mass range $[{10}^{-6},{10}^{-3}]M_{\odot }$ . Our constraints for the class of primordial black hole mass distribution functions $f\left(m\right)$ we consider reach down to $f\left(m\right)\sim 0.1$ . They are consistent with existing microlensing results in the planetary-mass range, provide a complementary probe to subsolar mass objects, and are publicly available [1]. 

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