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1. Differential rotation in compact objects with hyperons and delta isobars

   https://doi.org/10.1002/asna.20230160  

   Farrell, Delaney; Weber, Fridolin; Li, Jia Jie; Sedrakian, Armen (February 2024, Astronomische Nachrichten)

   Abstract Neutron stars may experience differential rotation on short, dynamical timescales following extreme astrophysical events like binary neutron star mergers. In this work, the masses and radii of differentially rotating neutron star models are computed. We employ a set of equations of states for dense hypernuclear and ‐admixed‐hypernuclear matter obtained within the framework of CDF theory in the relativistic Hartree‐Fock (RHF) approximation. Results are shown for varying meson‐ couplings, or equivalently the ‐potential in nuclear matter. A comparison of our results with those obtained for nonrotating stars shows that the maximum mass difference between differentially rotating and static stars is independent of the underlying particle composition of the star. We further find that the decrease in the radii and increase in the maximum masses of stellar models when ‐isobars are added to hyperonuclear matter (as initially observed for static and uniformly rotating stars) persist also in the case of differentially rotating neutron stars. 

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2. Inferring the neutron star equation of state with nuclear-physics informed semiparametric models

   https://doi.org/10.1088/1361-6382/ae1094  

   Ng, Sunny; Legred, Isaac; Suleiman, Lami; Landry, Philippe; Traylor, Lyla; Read, Jocelyn (October 2025, Classical and Quantum Gravity)

   Abstract Over the past decade, an abundance of information from neutron-star observations, nuclear experiments and theory has transformed our efforts to elucidate the properties of dense matter. However, at high densities relevant to the cores of neutron stars (NSs), substantial uncertainty about the dense matter equation of state (EoS) remains. In this work, we present a semiparametric EoS framework aimed at better integrating knowledge across these domains in astrophysical inference. We use a meta-model and realistic crust at low densities, and Gaussian process (GP) extensions at high densities. Comparisons between our semiparametric framework to fully nonparametric EoS representations show that imposing nuclear theoretical and experimental constraints through the meta-model up to nuclear saturation density results in constraints on the pressure up to twice nuclear saturation density. We also show that our GP trained on EoS models with nucleonic, hyperonic, and quark compositions extends the range of EoS explored at high density compared to a piecewise polytropic extension schema, under the requirements of causality of matter and of supporting the existence of heavy pulsars (PSRs). We find that maximum Tolman–Oppenheimer–Volkoff masses above $3.2{\mathrm{M}}_{\odot }$can be supported by causal EoS compatible with nuclear constraints at low densities. We then combine information from existing observations of heavy PSR masses, gravitational waves emitted from binary NS mergers, and x-ray pulse profile modeling of millisecond PSRs within a Bayesian inference scheme using our semiparametric EoS prior. With information from all public NS Interior Composition ExploRer PSRs (including PSR J0030+0451, PSR J0740+6620, PSR J0437–4715, and PSR J0614–3329), we find an astrophysically favored pressure at two times nuclear saturation density of $P\left(2{\rho }_{\mathrm{nuc}}\right)={1.98}_{-1.08}^{+2.13}\times {10}^{34}$dyn cm⁻², a radius of a $1.4{\mathrm{M}}_{\odot }$NS value of $R_{1.4}={11.4}_{-0.60}^{+0.98}$ km, and $M_{\max }={2.31}_{-0.23}^{+0.35}{\mathrm{M}}_{\odot }$at the 90% credible level (C.L). 

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3. Differential rotation in neutron stars at finite temperatures

   https://doi.org/10.3389/fphy.2024.1474615  

   Farrell, Delaney; Weber, Fridolin; Negreiros, Rodrigo (October 2024, Frontiers in Physics)

   IntroductionThis paper investigates the impact of differential rotation on the bulk properties and onset of rotational instabilities in neutron stars at finite temperatures up to 50 MeV. MethodsUtilizing the relativistic Brueckner-Hartree-Fock (RBHF) formalism in full Dirac space, the study constructs equation of state (EOS) models for hot neutron star matter, including conditions relevant for high temperatures. These finite-temperature EOS models are applied to compute the bulk properties of differentially rotating neutron stars with varying structural deformations. ResultsThe findings demonstrate that the stability of these stars against bar-mode deformation, a key rotational instability, is only weakly dependent on temperature. Differential rotation significantly affects the maximum mass and radius of neutron stars, and the threshold for the onset of bar-mode instability shows minimal sensitivity to temperature changes within the examined range. DiscussionThese findings are crucial for interpreting observational data from neutron star mergers and other high-energy astrophysical events. The research underscores the necessity of incorporating differential rotation and finite temperature effects in neutron star models to predict their properties and stability accurately. 

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4. Exploring Massive Neutron Stars Towards the Mass Gap: Constraining the High Density Nuclear Equation of State

   https://doi.org/10.1134/S1063772923140214  

   Zuraiq, Zenia; Mukhopadhyay, Banibrata; Weber, Fridolin (December 2023, Astronomy Reports)

   Due to the high-density nuclear matter equation of state (EOS) being as yet unknown, neutron stars (NSs) do not have a confirmed limiting “Chandrasekhar” type maximum mass. However, observations of NSs (PSR J1614-2230, PSR J0348+0432, PSR J0740+6620, PSR J0952–0607) indicate that the NS’s limiting mass, if there is any, could be well over 2M⊙. On the other hand, there seems to be an observational mass gap (of around 2.5−5M⊙ ) between the lightest black hole and the heaviest NS. Therefore, the “massive NSs” are prime candidates to fill that gap. Several NS EOSs have been proposed using both microscopic and phenomenological approaches. In this project, we look at a class of phenomenological nuclear matter EOSs—relativistic mean field models—and see what kind of NS is formed from them. We compute the max- imum mass supported by each model EOS to observe if the mass of the NS is indeed in the “massive” NS (>2M⊙) regime. We also observe the effects of including exotic particles (hyperons, Δs) in the NS EOS and how that affects the NS mass. However, only by introducing the magnetic field, i.e. for magnetized anisotro- pic NSs, we find the mass exceeding 2.5M⊙. Using tidal deformability constraints from gravitational wave observations, we place a further check on how physical the EOS and NSs are. 

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5. Kilonova Emissions from Neutron Star Merger Remnants: Implications for the Nuclear Equation of State

   https://doi.org/10.3847/1538-4357/add148  

   Lund, Kelsey A; Somasundaram, Rahul; McLaughlin, Gail C; Miller, Jonah M; Mumpower, Matthew R; Tews, Ingo (June 2025, The Astrophysical Journal)

   Abstract Multimessenger observations of binary neutron star mergers can provide valuable information on the nuclear equation of state (EOS). Here, we investigate the extent to which electromagnetic observations of the associated kilonovae allow us to place constraints on the EOS. For this, we use state-of-the-art three-dimensional general-relativistic magnetohydrodynamics simulations and detailed nucleosynthesis modeling to connect properties of observed light curves to properties of the accretion disk, and hence, the EOS. Using our general approach, we use multimessenger observations of GW170817/AT2017gfo to study the impact of various sources of uncertainty on inferences of the EOS. We constrain the radius of a 1.4M_⊙neutron star to lie within 10.30 ≤R_1.4≤ 13.0 km and the maximum mass to beM_TOV≤ 3.06M_⊙. 

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