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1. Observation of Gravitational Waves from the Coalescence of a 2.5–4.5 M _⊙ Compact Object and a Neutron Star

   https://doi.org/10.3847/2041-8213/ad5beb  

   Abac, A G; Abbott, R; Abouelfettouh, I; Acernese, F; Ackley, K; Adhicary, S; Adhikari, N; Adhikari, R X; Adkins, V K; Agarwal, D; et al (July 2024, The Astrophysical Journal Letters)

   Abstract We report the observation of a coalescing compact binary with component masses 2.5–4.5M_⊙and 1.2–2.0M_⊙(all measurements quoted at the 90% credible level). The gravitational-wave signal GW230529_181500 was observed during the fourth observing run of the LIGO–Virgo–KAGRA detector network on 2023 May 29 by the LIGO Livingston observatory. The primary component of the source has a mass less than 5M_⊙at 99% credibility. We cannot definitively determine from gravitational-wave data alone whether either component of the source is a neutron star or a black hole. However, given existing estimates of the maximum neutron star mass, we find the most probable interpretation of the source to be the coalescence of a neutron star with a black hole that has a mass between the most massive neutron stars and the least massive black holes observed in the Galaxy. We provisionally estimate a merger rate density of ${55}_{-47}^{+127}{\mathrm{Gpc}}^{-3}{\mathrm{yr}}^{-1}$ for compact binary coalescences with properties similar to the source of GW230529_181500; assuming that the source is a neutron star–black hole merger, GW230529_181500-like sources may make up the majority of neutron star–black hole coalescences. The discovery of this system implies an increase in the expected rate of neutron star–black hole mergers with electromagnetic counterparts and provides further evidence for compact objects existing within the purported lower mass gap. 

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2. A JWST View of the Overmassive Black Hole in NGC 4486B

   https://doi.org/10.3847/2041-8213/adf728  

   Tahmasebzadeh, Behzad; Taylor, Matthew A; Valluri, Monica; Yoshino, Haruka; Vasiliev, Eugene; Drinkwater, Michael J; Thompson, Solveig; Dage, Kristen; Côté, Patrick; Ferrarese, Laura; et al (August 2025, The Astrophysical Journal Letters)

   Abstract We present a new stellar dynamical measurement of the supermassive black hole in the compact elliptical galaxy NGC 4486B, based on integral field spectroscopy with JWST/NIRSpec. The two-dimensional kinematic maps reveal a resolved double nucleus and a velocity dispersion peak offset from the photometric center. Utilizing two independent methods—Schwarzschild orbit-superposition and Jeans anisotropic modeling—we tightly constrain the black hole mass by fitting the full line-of-sight velocity distribution. Our axisymmetric Schwarzschild models yield a best-fit black hole mass of ${\mathcal{M}}_{\mathrm{BH}}$ $=3.6_{-0.7}^{+0.7}\times 10^{8}M\odot$ , slightly lower but significantly more precise than previous estimates. However, since our models do not account for the nonequilibrium nature of the double nucleus, this value may represent a lower limit. Across all tested dynamical models, the inferred ${\mathcal{M}}_{\mathrm{BH}}$ /M_*ratio ranges from ∼4% to 13%, providing robust evidence for an overmassive SMBH in NGC 4486B. Combined with the galaxy’s location deep within the Virgo Cluster, our results support the interpretation that NGC 4486B is the tidally stripped remnant core of a formerly massive galaxy. As the JWST/NIRSpec field of view is insufficient to constrain the dark matter halo, we incorporate archival ground-based long-slit kinematics extending to 5″. While this provides some leverage on the dark matter content, the constraints remain relatively weak. We place only an upper limit on the dark matter fraction, with ${\mathcal{M}}_{\mathrm{DM}}$ / ${\mathcal{M}}_{*}$ ≲0.5 within 1 kpc—well beyond the effective radius. The inferred black hole mass remains unchanged with or without a dark matter halo. 

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3. Long–Time and Chaotic Dynamics in Microscopic Systems with Singular Interactions

   https://doi.org/10.1007/s00205-026-02189-1  

   Béjar-López, Alexis; Blaustein, Alain; Jabin, Pierre-Emmanuel; Soler, Juan (August 2026, Archive for Rational Mechanics and Analysis)

   Abstract This paper investigates the long–time dynamics of interacting particle systems subject to singular interactions. We consider a microscopic system ofNinteracting point particles, where the time evolution of the joint distribution$$f_N(t)$$ $f_N\left(t\right)$ is governed by the Liouville equation. Our primary objective is to analyze the system’s behavior over extended time intervals, focusing on the stability, the potential chaotic dynamics and the impact of singularities. In particular, we aim to derive reduced models in the regime$$N \gg 1$$ $N\gg 1$ , exploring both the mean-field approximation and configurations far from chaos, where the mean-field approximation no longer holds. These reduced models do not always emerge but in these cases we prove that it is possible to derive uniform bounds in$$ L^2 $$ $L^2$ , both over time and with respect to the number of particles, on the marginals$$ \left( f_{k,N}\right) _{1\leqq k \leqq N}$$ ${\left(f_{k,N}\right)}_{1\leqq k\leqq N}$ , irrespective of the initial state’s chaotic nature. Furthermore, we extend previous results by considering a wide range of singular interaction kernels$$ K \in W^{\frac{-2}{d+2}, d+2} $$ $K\in W^{\frac{-2}{d+2},d+2}$ in dimension$$d\geqq 2$$ $d\geqq 2$ , surpassing the traditional$$ L^d $$ $L^d$ regularity barriers. Finally, we address the highly singular case of$$ K \in H^{-1} $$ $K\in H^{-1}$ under a threshold temperature regime, offering new insights into the behavior of such systems. 

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4. Tests of general relativity with GW230529: A neutron star merging with a lower mass-gap compact object

   https://doi.org/10.1103/r43k-51yq  

   Sänger, Elise M; Roy, Soumen; Agathos, Michalis; Birnholtz, Ofek; Buonanno, Alessandra; Dietrich, Tim; Haney, Maria; Julié, Félix-Louis; Pratten, Geraint; Steinhoff, Jan; et al (April 2026, Physical Review D)

   On May 29, 2023, the LIGO Livingston observatory detected the gravitational-wave signal GW230529_181500 from the merger of a neutron star with a lower mass-gap compact object. Its long inspiral signal provides a unique opportunity to test general relativity (GR) in a parameter space previously unexplored by strong-field tests. In this work, we performed parameterized inspiral tests of GR with GW230529_181500. Specifically, we search for deviations in the frequency-domain gravitational wave phase by allowing for agnostic corrections to the post-Newtonian coefficients. We performed tests with the Flexible Theory Independent and Test Infrastructure For General Relativity frameworks using several quasicircular waveform models that capture different physical effects (higher modes, spins, tides). We find that the signal is consistent with GR for all deviation parameters. Assuming the primary object is a black hole, we obtain particularly tight constraints on the dipole radiation at $-1\mathrm{PN}$ order of $\left|\delta {\hat{\phi }}_{-2}\right|\lesssim 8\times {10}^{-5}$ , which is a factor $\sim 17$ times more stringent than previous bounds from the neutron star–black hole merger GW200115_042309, as well as on the 0.5PN and 1PN deviation parameters. We discuss some challenges that arise when analyzing this signal, namely biases due to correlations with tidal effects and the degeneracy between the 0PN deviation parameter and the chirp mass. To illustrate the importance of GW230529_181500 for tests of GR, we mapped the agnostic $-1\mathrm{PN}$ results to a class of Einstein-scalar-Gauss-Bonnet (ESGB) theories of gravity. We also conducted an analysis probing the specific phase deviation expected in ESGB theory and obtain an upper bound on the Gauss-Bonnet coupling of ${\ell }_{\mathrm{GB}}\lesssim 0.51M_{\odot }$ ( $\sqrt{{\alpha }_{\mathrm{GB}}}\lesssim 0.28\mathrm{km}$ ), which is better than any previously reported constraint. 

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5. Neutron stars with exceptionally light QCD axions

   https://doi.org/10.1103/5zgm-z4v9  

   Kumamoto, Mia; Huang, Junwu; Drischler, Christian; Baryakhtar, Masha; Reddy, Sanjay (August 2025, Physical Review D)

   We present a comprehensive study of axion condensed neutron stars that arise in models of an exceptionally light axion that couples to quantum chromodynamics (QCD). These axions solve the strong-charge-parity ( $CP$ ) problem, but have a mass-squared lighter than that due to QCD by a factor of $ϵ<1$ . Inside dense matter, the axion potential is altered, and much of the matter in neutron stars resides in the axion condensed phase where the strong- $CP$ parameter $\theta =\pi$ and $CP$ remains a good symmetry. In these regions, masses and interactions of nucleons are modified, in turn changing the equation of state (EOS), structure, and phenomenology of the neutron stars. We take the first steps toward the study of the EOS of neutron star matter at $\theta =\pi$ within chiral effective field theory and use relativistic mean field theory to deduce the resulting changes to nuclear matter and the neutron star low-density EOS. We derive constraints on the exceptionally light axion parameter space based on observations of the thermal relaxation of accreting neutron stars, isolated neutron star cooling, and pulsar glitches, excluding the region up to $5\times {10}^{-7}\lesssim ϵ\lesssim 0.2$ for $m_a\gtrsim 2\times {10}^{-9}\mathrm{eV}$ . We comment on potential changes to the neutron star mass-radius relationship, and discuss the possibility of novel, nuclear-density compact objects with $\theta =\pi$ that are stabilized not by gravity but by the axion potential. 

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