arXiv subjects

Andreas Konstantinou

Publications and source records attributed to Andreas Konstantinou.

10 recordsLinked to original sources

arXiv2026

Rotational effects in quark stars: comparing different models

We investigate the rotational properties of self-bound strange quark stars using two representative quark matter equations of state (EOS): the vector MIT bag model and the density-dependent quark mass (DDQM) model. Through general-relativistic calculations of uniformly rotating sequences, we analyze their mass--radius relations, moments of inertia, quadrupole moments, surface redshifts, Keplerian frequencies, and energy components. A central result of this work is the full decomposition of the stellar energy budget in rotating strange stars, separating gravitational, internal, rotational, and binding energy contributions. Rotation amplifies the intrinsic EOS differences: the MIT model supports more massive ($M_{\max} \gtrsim 3.3\,M_\odot$) compact stars with larger moments of inertia and greater resistance to deformation, while the DDQM model produces larger radii, less massive stars limited by mass-shedding at lower frequencies. Combined measurements of mass, radius, and frequency can thus break the EOS degeneracy; massive, rapidly rotating pulsars favors MIT-like EOS, whereas larger radii in canonical stars point to a DDQM-like model. These rotational observables, soon to be tightly constrained by NICER and next-generation gravitational-wave detectors, offer a means to test the existence and composition of self-bound quark matter in compact stars.

astro-ph.HE↗

arXiv2026

Scalar and Tensor Form Factors for $\Lambda \rightarrow p\ell \bar{\nu}_\ell$ from Lattice QCD

We present a determination of the scalar and tensor $\Lambda\to p$ transition form factors using lattice QCD. These form factors are relevant for semileptonic hyperon decays in the presence of extensions of the Standard Model that include scalar and tensor interactions. The calculation is carried out using a gauge ensemble of twisted mass fermions at the physical pion mass, following the same strategy as our recent study on vector and axial form factors for the same transition. We provide the complete set of form factors as functions of $q^2$ employing a model-independent parametrization. We examine their impact on searches for non-standard charged-current interactions via the muon-to-electron decay-rate ratio $R^{\mu e}=\Gamma(\Lambda\to p\mu\bar\nu_\mu)/\Gamma(\Lambda\to pe\bar\nu_e)$, where scalar and tensor contributions enter linearly and are helicity-enhanced relative to the electron channel. We compare this first-principles prediction for the decay-rate ratio with recent experimental measurements, thereby enabling improved constraints on non-standard charged-current interactions.

hep-lat↗

arXiv2026

Recent results on the $Λ\rightarrow p\ell \barν_\ell$ semileptonic decay

We present a lattice-QCD determination of the $Λ\to p$ vector and axial-vector form factors, providing theoretical input for studies of the semileptonic decay $Λ\to p\ell\barν_\ell$. The calculation is carried out on a single gauge ensemble with physical light, strange, and charm quark masses and delivers a precise determination of the complete set of transition form factors, including second-class contributions. Using these form factors, we compute decay rates for both the electronic and muonic channels, as well as their ratio, which offers a sensitive test of lepton-flavor universality and possible non-standard scalar or tensor interactions. This decay mode provides a theoretically well-controlled avenue for extracting the CKM matrix element $|V_{us}|$ from the baryon sector. Our estimate of $|V_{us}|$ is obtained by combining our recent lattice-QCD results with recent measurements of the relevant branching fraction reported by BESIII and LHCb.

hep-lat↗

arXiv2026

Rotational enhancement and stability of protoquark stars during thermal evolution

We present the first systematic study of rigidly rotating protoquark stars based on isentropic equations of state (EOS) within the density-dependent quark mass (DDQM) framework. Using a quasi-static equilibrium approach, we follow the Kelvin--Helmholtz evolution from hot, lepton-rich matter to a cold, catalyzed quark star (QS). Rotation substantially enhances the maximum stable mass (by up to $\sim 40\%$), equatorial radius, and key rotational observables, with the ratio of rotational kinetic to gravitational potential energy, $T_{\rm kin}/|W|$, reaching $0.18$--$0.19$ near the Keplerian limit, indicating a heightened susceptibility to gravitational-wave--emitting instabilities. Thermal evolution introduces a clear ordering: all stellar properties peak during the lepton-rich stages and decrease monotonically as the star cools. Compared to hadronic stars, rotating proto-QSs exhibit larger radii, higher moments of inertia, and stronger quadrupolar deformation, producing a distinct signature in the mass--radius--spin plane. The EOS parameters are constrained using current astrophysical observations, including mass--radius measurements from HESS~J1731--347 and PSR~J0030+0451, the high-mass constraint from PSR~J0740+6620, and mass-radius constraints inferred from GW170817. The results demonstrate that future multimessenger observations must account for both thermal history and rotation to identify quark matter (QM) in compact stars robustly.

astro-ph.HE↗

arXiv2025

The Effect of a Self-bound Equation of State on the Structure of Rotating Compact Stars

This paper investigates how a self bound equation of state (EOS), which describes strange quark stars, affects the rotational properties of compact stars, focusing on deviations from universal relations governing gravitational mass and radius changes due to rotation. The analysis reveals significant deviations in stars with higher surface-to-center total energy-density ratios, $\frac{ε_s}{ε_c+c^2P_c}$, challenging the established universal relations. For Newtonian stars, hydrostatic equilibrium ensures that the difference between the gravitational potential at the center, $Φ_c$, and at the poles, $Φ_p$, remains constant within sequences of rotating neutron stars characterized by the same central and polar specific enthalpy ($Φ_c - Φ_p = -h_c +h_p$). Combined with the scaling $Φ\propto R_e^2$, where $R_e$ denotes the equatorial radius, this condition naturally leads to a quasi-universal behavior in the rotational change of radius within these sequences. Similarly, in general relativistic stars, the hydrostatic equilibrium maintains that $Φ^{GR}_{p} - Φ^{GR}_{c}$ remains unchanged within these sequences, where $Φ^{GR}$ is one of the metric potentials. Inspired by this theoretical framework, a toy model has been developed to capture the dependence of gravitational mass and radius deviations on the surface-to-central total energy density ratio. Subsequently, an improved set of empirical universal relations has been proposed, for accurately modeling rapidly rotating compact stars with self-bound EOSs.

astro-ph.HE↗

arXiv2025

Rotating Proto-Neutron Stars Admixed with Mirror Dark Matter: A two fluid approach

This work investigates the impact of mirror dark matter (DM) on the global properties of rotating neutron stars (NSs) across evolutionary stages, from hot, lepton-rich protoneutron stars (PNSs) to cold, catalyzed NSs along the Kelvin-Helmholtz timescale. The baryonic matter (BM) is modeled using a relativistic mean-field (RMF) approach with density-dependent couplings, while the dark sector mirrors the visible sector with analogous thermodynamic conditions. Using a two-fluid formalism with purely gravitational DM-BM interaction, we find that rotation enlarges the star, whereas DM admixture increases compactness and enhances gravitational stability. However, increased compactness due to DM lowers the threshold for rotational instabilities, making DM-admixed stars more susceptible. Rotation decreases {central temperature behavior} by redistributing thermal energy over a larger volume and reducing central density, while DM raises temperatures by deepening the gravitational potential and increasing thermal energy. Stars become more prone to collapse and rotational instabilities as frequency ($ν$) rises and the polar-to-equatorial radius ratio ($r_p/r_e$) decreases, especially near the Keplerian limit ($ν_K$). DM-admixed stars also show higher surface gravitational redshifts due to their compactness. Our results qualitatively agree with universal relations primarily derived for rotating cold stars. These findings highlight competing effects of rotation and DM on NS thermal evolution, structure, and observables, potentially offering indirect probes of DM within NSs.

astro-ph.HE↗

arXiv2025

Study of the $Λ\to p \ell \barν_\ell$ semileptonic decay in lattice QCD

We present the first lattice QCD determination of the $Λ\to N$ vector and axial-vector form factors, which are essential inputs for studying the semileptonic decay $Λ\to p \ell \barν_\ell$. This channel provides a clean, theoretically controlled avenue for extracting the CKM matrix element $|V_{us}|$ from the baryon sector. Our analysis uses a gauge ensemble with physical light, strange, and charm quark masses and yields the most precise determination to date of the full set of transition form factors -- including second-class contributions -- as well as the associated couplings, radii, and the ratio of muon-to-electron decay rates, an observable sensitive to possible non-standard scalar and tensor interactions. We compare our non-perturbative results with next-to-next-to-leading order expansions in the small parameter $δ= (m_Λ- m_N)/m_Λ\approx 0.16$. We find that the common phenomenological approximation of neglecting the $q^2$-dependence of the form factors leads to a $\sim 4\%$ deviation in the decay rate. This underscores the critical importance of precise, fully non-perturbative form factor inputs for achieving the sub-percent precision targets of upcoming experimental programs.

hep-lat↗

arXiv2025

Rotating Neutron Stars with Dark Matter Halos

If dark matter (DM) exists in halos around rotating neutron stars (NSs), it will be essential to understand the effects of rotation on the distribution of DM and baryonic matter (BM) in the stars to interpret observations. In this work, we construct rapidly rotating dark matter admixed neutron stars (DANS) with DM halos using the two-fluid approximation, where the BM and DM interact only through gravity. Our goal is to describe rapidly rotating millisecond-period DANS spun up by the accretion of BM from a zero angular momentum state. We extend the Rapidly Rotating Neutron Star (RNS) code to compute axisymmetric configurations in which the BM rotates rigidly while the DM remains torque-free and differentially rotates through the frame-dragging of spacetime. For the first time, we examine in detail local and global definitions of mass in general relativity for two-fluid systems, showing how their differences affect the interpretation of baryonic and dark component masses. We compute energy density and frame-dragging frequency profiles for DANS with three different characteristic DM halos. We demonstrate that rapid BM rotation reduces DM halo sizes if central energy densities are kept constant between non-rotating and rotating models. We also construct sequences of DANS to create mass and radius curves and compare rotating and non-rotating cases. Finally, we quantify deviations in the spacetime metric outside the baryonic surfaces of these sequences of stars caused by the DM halos. We hypothesize that the size of this quantity could indicate whether a DM halo will significantly impact X-ray pulse profile modeling. These results provide a framework for assessing the observational consequences of DM halos around rapidly rotating NSs.

astro-ph.HE↗

arXiv2024

The Effect of a Dark Matter Core on the Structure of a Rotating Neutron Star

Neutron stars represent unique laboratories, offering insights into the physics of supranuclear-density matter and serving as potential hosts for dark matter. This study explores the impact of dark matter cores on rapidly rotating neutron stars through the two-fluid approximation, assuming minimal interaction between baryonic matter and dark matter. The investigation employs phenomenological models for fermionic and bosonic dark matter, revealing that universal relations governing mass and radius changes due to rotation remain largely unaffected in the presence of a dark matter core. Specifically, for a 5 % dark matter mass fraction, the percent deviations in total mass ($M_{tot}$), the baryonic equatorial radius ($R_{Be}$), and polar-to-equatorial baryonic radius ratio ($R_{ratioB}$) are within 3.9 %, 1.8 %, and 1.4 %, respectively. These findings suggest that the universal relations governing neutron star shape can be utilized to infer constraints on the properties of dark matter cores even in cases where the dark matter significantly softens the neutron star's equation of state.

astro-ph.HE↗

arXiv2022

Universal Relations for the Increase in the Mass and Radius of a Rotating Neutron Star

Rotation causes an increase in a neutron star's mass and equatorial radius. The mass and radius depend sensitively on the unknown equation of state (EOS) of cold, dense matter. However, the increases in mass and radius due to rotation are almost independent of the EOS. The EOS independence leads to the idea of neutron star universality. In this paper, we compute sequences of rotating neutron stars with constant central density. We use a collection of randomly generated EOS to construct simple correction factors to the mass and radius computed from the equations of hydrostatic equilibrium for non-rotating neutron stars. The correction factors depend only on the non-rotating star's mass and radius and are almost independent of the EOS. This makes it computationally inexpensive to include observations of rotating neutron stars in EOS inference codes. We also construct a mapping from the measured mass and radius of a rotating neutron star to a corresponding non-rotating star. The mapping makes it possible to construct a zero-spin mass-radius curve if the masses and radii of many neutron stars with different spins are measured. We show that the changes in polar and equatorial radii are symmetric, in that the polar radius shrinks at the same rate that the equatorial radius grows. This symmetry is related to the observation that the equatorial compactness (the ratio of mass to radius) is almost constant on one of the constant-density sequences.

astro-ph.HE↗