arXiv subjects

Armen Sedrakian

Publications and source records attributed to Armen Sedrakian.

At least 37 records · Page 2Linked to original sources

arXiv2025

Bayesian inferences on covariant density functionals from multimessenger astrophysical data: Nucleonic models

[Background] Bayesian inference frameworks incorporating multi-messenger astrophysical constraints have recently been applied to covariant density functional (CDF) models to constrain their parameters. Among these, frameworks utilizing CDFs with density-dependent meson-nucleon couplings furnishing the equation of state (EoS) of compact star (CS) matter have been explored. [Purpose] The aforementioned inference framework has not yet incorporated astrophysical objects with potentially extreme high masses or ultra-small radii among its constraints, leaving its flexibility and predictive power under such extreme parameters still unknown. [Method] We apply the Bayesian inference framework based on CDFs with density dependent couplings. The astrophysical data is expanded to include not only the latest multi-messenger constraints from NICER and gravitational wave events but also the highest measured mass to date for the ``black widow" pulsar PSR J0952-0607 and the mass-radius estimates for the ultra-compact, low-mass object HESS J1731-347. [Results] Our systematic Bayesian analysis indicates that our CDF models can support higher maximum masses for CSs, reaching up to $2.4$-$2.5\,M_{\odot}$. However, achieving sufficient softening of the EoS in the low-density regime to accommodate the HESS J1731-347 data remains challenging. Nonetheless, we are able to impose tighter constraints on the parameter space of CDF models, ensuring consistency with current nuclear experimental and astrophysical data. [Conclusions] CDF models with density-dependent meson-nucleon couplings encompass a wide range of nuclear and astrophysical phenomena, providing a robust theoretical framework for interpreting compact objects. However, the predicted lower limit for the radii of low-mass stars is approximately 12 km, which stems from the restricted degrees of freedom in the isovector sector.

nucl-th↗

arXiv2025

Superconductivity in magnetars: Exploring type-I and type-II states in toroidal magnetic fields

We present a first two-dimensional general-relativistic analysis of superconducting regions in axially symmetric highly magnetized neutron star (magnetar) models with toroidal magnetic fields. We investigate the topology and distribution of type-II and type-I superconducting regions for varying toroidal magnetic field strengths and stellar masses by solving the Einstein-Maxwell equations using the XNS code. Our results reveal that the outer cores of low- to intermediate-mass magnetars sustain superconductivity over larger regions compared to higher-mass stars with non-trivial distribution of type-II and type-I regions. Consistent with previous one-dimensional (1D) models, we find that regardless of the gravitational mass, the inner cores of magnetars with toroidal magnetic fields are devoid of $S$-wave proton superconductivity. Furthermore, these models contain non-superconducting, torus-shaped regions - a novel feature absent in previous 1D studies. Finally, we speculate on the potential indirect effects of superconductivity on continuous gravitational wave emissions from millisecond pulsars, such as PSR J1843-1113, highlighting their relevance for future gravitational wave detectors.

astro-ph.SR↗

arXiv2025

Bayesian constraints on covariant density functional equations of state of compact stars with new NICER mass-radius measurements

Recent advancements in astrophysical observations of compact stars, particularly the new and updated NICER constraints, have provided mass-radius ($M$-$R$) data for pulsars spanning masses from 1 to $2\,M_{\odot }$. These data offer a unique opportunity to test modern theories of dense matter using multi-messenger constraints. Covariant density functional (CDF) models of nuclear matter, which capture a broad range of nuclear and astrophysical phenomena, provide a robust theoretical framework to interpret these observations. This study applies the Bayesian framework to a class of CDF models with density-dependent meson-nucleon couplings, specifically those based on nucleonic degrees of freedom. By incorporating the latest multi-messenger constraints, we impose tighter limits on the parameter space of these models and assess their consistency with observational data. Our analysis advances previous efforts by refining the density-dependence parameterization and integrating recent $M$-$R$ ellipses. This enables more stringent evaluations of dense matter models in light of new astrophysical observations.

hep-ph↗

arXiv2025

Confronting new NICER mass-radius measurements with phase transition in dense matter and twin compact stars

The (re)analysis of data on the X-ray emitting pulsars PSR J0030+0451 and J0740+6620, as well as new results on PSR J0437-4715 and J1231-1411, are confronted with the predictions of the equation of state (EoS) models allowing for strong first-order phase transition for the mass-radius ($M$-$R$) diagram. We use models that are based on a covariant density functional (CDF) EoS for nucleonic matter at low densities and a quark matter EoS, parameterized by the speed of sound, at higher densities. To account for the variations in the ellipses for PSR J0030+0451 obtained from different analyses, we examined three scenarios to assess their consistency with our models, focusing particularly on the potential formation of twin stars. We found that in two scenarios, where the ellipses for PSR J0030+0451 and J0437-4715 with masses close to the canonical mass $\sim 1.4\,M_{\odot}$ are significantly separated, our models allow for the presence of twin stars as a natural explanation for potential differences in the radii of these stars.

astro-ph.HE↗

arXiv2025

Josephson currents in neutron stars

We demonstrate that the interface between $S$-wave and $P$-wave paired superfluids in neutron stars induces a neutron supercurrent, a charge-neutral analog of the Josephson junction effect in electronic superconductors. The proton supercurrent entrainment by the neutron superfluid generates, in addition to the neutral supercurrent, a charged current across the interface. Beyond this stationary effect, the motion of the neutron vortex lines responding to secular changes in the neutron star's rotation rate induces a time-dependent oscillating Josephson current across this interface when proton flux tubes are dragged along with them. We show that such motion produces radiation from the interface once clusters of proton flux tubes intersect the interface. The power of radiation exceeds by orders of magnitude the Ohmic dissipation of currents in neutron stars. This effect appears to be phenomenologically significant enough to heat the star and alter its cooling rate during the photon cooling era.

astro-ph.HE↗

arXiv2024

Strongly interacting matter in extreme magnetic fields

Magnetic fields are ubiquitous across different physical systems of current interest; from the early Universe, compact astrophysical objects and heavy-ion collisions to condensed matter systems. A proper treatment of the effects produced by magnetic fields during the dynamical evolution of these systems, can help to understand observables that otherwise show a puzzling behavior. Furthermore, when these fields are comparable to or stronger than Λ_QCD, they serve as excellent probes to help elucidate the physics of strongly interacting matter under extreme conditions of temperature and density. In this work we provide a comprehensive review of recent developments on the description of QED and QCD systems where magnetic field driven effects are important. These include the modification of meson static properties such as masses and form factors, the chiral magnetic effect, the description of anomalous transport coefficients, superconductivity in extreme magnetic fields, the properties of neutron stars, the evolution of heavy-ion collisions, as well as effects on the QCD phase diagram. We describe recent theory and phenomenological developments using effective models as well as LQCD methods. The work represents a state-of-the-art review of the field, motivated by presentations and discussions during the "Workshop on Strongly Interacting Matter in Strong Electromagnetic Fields" that took place in the European Centre for Theoretical Studies in Nuclear Physics and Related Areas (ECT*) in the city of Trento, Italy, September 25-29, 2023.

nucl-th↗

arXiv2024

Thermal Conductivity and Thermal Hall Effect in Dense Electron-Ion Plasma

In this study, we examine thermal conductivity and the thermal Hall effect in electron-ion plasmas relevant to hot neutron stars, white dwarfs, and binary neutron star mergers, focusing on densities found in the outer crusts of neutron stars and the interiors of white dwarfs. We consider plasma consisting of single species of ions, which could be either iron $\isotope[56]{Fe}$ or carbon $\isotope[12]{C}$ nuclei. The temperature range explored is from the melting temperature of the solid $T\sim10^9$~K up to $10^{11}$~K. This covers both degenerate and non-degenerate electron regimes. We find that thermal conductivity increases with density and temperature for which we provide analytical scaling relations valid in different regimes. The impact of magnetic fields on thermal conductivity is also analyzed, showing anisotropy in low-density regions and the presence of the thermal Hall effect characterized by the Righi--Leduc coefficient. The transition from a degenerate to non-degenerate regime is characterized by a minimum ratio of thermal conductivity to temperature, which is analogous to the minimum observed already in the case of electrical conductivity. We provide also formulas fit to our numerical results, which can be used in dissipative magneto-hydrodynamics simulations of warm compact stars.

astro-ph.HE↗

arXiv2024

Short-Range Correlations and Urca Process in Neutron Stars

Recent measurements of high-momentum correlated neutron-proton pairs at JLab suggest that the dense nucleonic component of the compact stars contains a fraction of high-momentum neutron-proton pairs that is not accounted for in the familiar Fermi-liquid theory of the neutron-proton fluid mixture. We compute the rate of the Urca process in compact stars taking into account the non-Fermi liquid contributions to the proton's spectral widths induced by short-range correlations. The Urca rate differs strongly from the Fermi-liquid prediction at low temperatures, in particular, the high threshold on the proton fraction precluding the Urca process in neutron stars is replaced by a smooth increase with the proton fraction. This observation may have a profound impact on the theories of cooling of compact stars.

nucl-th↗

arXiv2024

Bulk viscosity of two-color superconducting quark matter in neutron star mergers

We study the bulk viscosity of moderately hot and dense, neutrino-transparent color superconducting quark matter arising from weak-interaction-driven direct URCA processes. The quark matter is modeled using the Nambu--Jona-Lasinio model improved to account for vector and 't Hooft interactions as well as antisymmetric pairing among the red/green up and down quarks. The unpaired excitations are the strange quarks and the blue up or down quarks. We compute the relaxation rates associated with $d$ and $s$-quark decay and electron capture processes on $u$ quark for blue color. The resulting bulk viscosity for density oscillations in the 1--10\,kHz range shows a resonant peak at $T\sim 5\,\MeV$, and the damping time may drop below 10 ms. This is short enough to affect the postmerger evolution and is very similar to the damping predicted in nuclear matter.

nucl-th↗

arXiv2024

Extremely small stars in scalar-tensor gravity: when stellar radius is less than Schwarzschild one

We show analytically that there exist compact stellar objects akin to neutron stars whose radius is smaller than the Schwarzschild radius defined by Arnowitt-Deser-Misner (ADM) mass. The radius of the compact object is defined by the radius where the energy density and the pressure of ordinary matter vanish, while clouds of scalar(s) can extend beyond this radius -- a situation that is often encountered in modified gravity theories, like $F(R)$ gravity and the scalar--Einstein--Gauss-Bonnet gravity. The clouds of scalar mode(s) give additional contributions to the ADM mass and as a result, the corresponding Schwarzschild radius given by the ADM mass can be larger than that of the compact object.

gr-qc↗

arXiv2024

Finite-temperature equations of state of compact stars with hyperons: three-dimensional tables

We construct tables of finite temperature equation of state (EoS) of hypernuclear matter in the range of densities, temperatures, and electron fractions that are needed for numerical simulations of supernovas, proto-neutron stars, and binary neutron star mergers and cast them in the format of {\sc CompOSE} database. The tables are extracted from a model that is based on covariant density functional (CDF) theory that includes the full $J^P=1/2^+$ baryon octet in a manner that is consistent with the current astrophysical and nuclear constraints. We employ a parameterization with three different values of the slope of the symmetry energy $L_{\rm sym}=30$, 50 and 70 MeV and fixed skewness $Q_{\rm sat}= 400$ MeV for above saturation matter. A model for the EoS of inhomogeneous matter is matched at sub-saturation density to the high-density hypernuclear EoS. We discuss the generic features of the resulting EoS and the composition of matter as a function of density, temperature, and electron fraction. The nuclear characteristics and strangeness fraction of these models are compared to the alternatives from the literature. The integral properties of static and rapidly rotating compact stars in the limit of zero temperature are discussed and confronted with the multimessenger astrophysical constraints.

nucl-th↗

arXiv2024

Electrical conductivity of warm neutron star crust in magnetic fields: Neutron-drip regime

We compute the anisotropic electrical conductivity tensor of the inner crust of a compact star at non-zero temperature by extending a previous work on the conductivity of the outer crust. The physical scenarios, where such crust is formed, involve proto-neutron stars born in supernova explosions, binary neutron star mergers and accreting neutron stars. The temperature-density range studied covers the transition from a non-degenerate to a highly degenerate electron gas and assumes that the nuclei form a liquid, i.e., the temperature is above the melting temperature of the lattice of nuclei. The electronic transition probabilities include (a) the dynamical screening of electron-ion interaction in the hard-thermal-loop approximation for the QED plasma, (b) the correlations of the ionic component in a one-component plasma, and (c) finite nuclear size effects. The conductivity tensor is obtained from the Boltzmann kinetic equation in relaxation time approximation accounting for the anisotropies introduced by a magnetic field. The sensitivity of the results towards the matter composition of the inner crust is explored by using several compositions of the inner crust which were obtained using different nuclear interactions and methods of solving the many-body problem. The standard deviation of relaxation time and components of the conductivity tensor from the average are below $\le 10\%$ except close to crust-core transition, where non-spherical nuclear structures are expected. Our results can be used in dissipative magneto-hydrodynamics (MHD) simulations of warm compact stars.

astro-ph.HE↗

arXiv2024

Hybrid star models in the light of new multi-messenger data

Recent astrophysical mass inferences of compact stars HESS J1731-347 and PSR J0952-0607, with extremely small and large masses respectively, as well as the measurement of the neutron skin of Ca in the CREX experiment challenge and constrain the models of dense matter. We examine the concept of hybrid stars - objects containing quark cores surrounded by nucleonic envelopes - as models that account for these new data along with other inferences. We employ a family of 81 nucleonic equations of state (EoSs) with variable skewness and slope of symmetry energy at saturation density and a constant speed-of-sound EoS for quark matter. For each nucleonic EoS, a family of hybrid EoSs is generated by varying the transition density, the energy jump, and the speed of sound. These models are tested against the data from GW170817 and J1731-347, which favor low-density soft EoS and J0592-0607 and J0740+6620, which require high-density stiff EoS. The addition of J0592-0607's mass measurement to the constraints has no significant impact on the parameter space of the admissible EoS, but allows us to explore the potential effect of pulsars more massive than J0740+6620, if such exists. We then examine the occurrence of twin configurations and quantify the ranges of masses and radii that they can possess. It is shown that including J1731-347 data favors EoSs that predict low-mass twins with $M \lesssim 1.3\,M_{\odot}$ that can be realized if the deconfinement transition density is low. If combined with large speed of sound in quark matter such models allow for maximum masses of hybrid stars in $2.0$--$2.6\,M_{\odot}$.

astro-ph.HE↗

arXiv2024

New covariant density functionals of nuclear matter for compact star simulations

We generate three families of extended covariant density functionals of nuclear matter that have varying slope of symmetry energy and skewness at nuclear saturation density, but otherwise share the same basic parameters (symmetry energy, compressibility, saturation parameters, etc.) with the standard DDME2, DD2, and MPE functionals. Tables of the parameters of these new density functionals are given, which can be straightforwardly used in DDME2, DD2, and MPE parameterization-based codes. Furthermore, we provide tables of a large number of equations of state (81 for each family) that can be used in astrophysical simulations to assess the impact of variations of not-well-known slope of symmetry energy and skewness of nuclear systems on the astrophysics of compact objects. We also provide tables of computed integral parameters (mass, radius, and tidal deformability) that can be used, e.g., for modeling gravitational waveforms. Finally, for the extended DDME2-based parameterization, we implement a first-order phase transition to quark matter to obtain a family of equations of state that accommodates a phase transition to quark matter. Analogous tables of the equations of state and integral parameters are provided for this case as well.

nucl-th↗

arXiv2023

Differential Rotation in Compact Objects with Hyperons and Delta Isobars

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 $Δ$-admixed-hypernuclear matter obtained within the framework of CDF theory in the relativistic Hartree-Fock (RHF) approximation. Results are shown for varying meson-$Δ$ couplings, or equivalently the $Δ$-potential in nuclear matter. A comparison of our results with those obtained for non-rotating 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 $Δ$-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.

nucl-th↗

arXiv2023

Bulk viscosity from Urca processes: $npeμ$ matter in the neutrino-transparent regime

We study the bulk viscosity of moderately hot and dense, neutrino-transparent relativistic $npeμ$ matter arising from weak-interaction direct Urca processes. This work parallels our recent study of the bulk viscosity of $npeμ$ matter with a trapped neutrino component. The nuclear matter is modeled in a relativistic density functional approach with two different parametrizations -- DDME2 (which does not allow for the low-temperature direct-Urca process at any density) and NL3 (which allows for low-temperature direct-Urca process above a low-density threshold). We compute the equilibration rates of Urca processes of neutron decay and lepton capture, as well as the rate of the muon decay, and find that the muon decay process is subdominant to the Urca processes at temperatures $T\geq 3$MeV in the case of DDME2 model and $T\geq 1$MeV in the case of NL3 model. Thus, the Urca-process-driven bulk viscosity is computed with the assumption that pure leptonic reactions are frozen. As a result the electronic and muonic Urca channels contribute to the bulk viscosity independently and at certain densities the bulk viscosity of $npeμ$ matter shows instead of the standard one-peak (resonant) form a "flattened" shape. In the final step, we estimate the damping timescales of density oscillations by the bulk viscosity. We find that, e.g., at a typical oscillation frequency $f=1$kHz, the damping of oscillations is most efficient at temperatures $3\leq T\leq 5$MeV and densities $n_B\leq 2n_0$ where they can affect the evolution of the post-merger object.

nucl-th↗

arXiv2023

Universal relations for compact stars with heavy baryons

A set of hadronic equations of state derived from covariant density functional theory and constrained by terrestrial experiments, and astrophysical observations, in particular by the NICER experiment inferences is used to explore the universal relations among the global properties of compact stars containing heavy baryons at high densities. We confirm the validity of universal $I$-Love-$Q$ relations connecting the moment of inertia $(I)$, the tidal deformability ($Λ$), and the spin-induced quadrupole moment ($Q$) for isolated non-rotating stars. We further confirm the validity of the $I$-$C$-$Q$ relations connecting the moment of inertia, compactness $(C)$, and quadrupole moment for uniformly and slowly rotating stars, and extend the validity of these relations to maximally rotating sequences. We then investigate the relations between integral parameters of maximally rotating and static compact stars. The universalities are shown to persist for equations of state and compositions containing hyperons and $Δ$ degrees of freedom. When heavy baryons are included, however, the radial profiles of integrands in expressions of global properties exhibit ``bumps", which are not present in the case of nucleonic stars in which case the profiles are smooth. We determine the coefficients entering the universal relations in the case of hyperonic and $Δ$-resonance containing stars.

nucl-th↗

arXiv2023

Baryonic models of ultra-low-mass compact stars for the central compact object in HESS J1731-347

The recent attempt on mass and radius inference of the central compact object within the supernova remnant HESS J1731-347 suggests for this object an unusually low mass of $M = 0.77^{+0.20}_{-0.17}\,M_{\odot}$ and a small radius of $R = 10.4^{+0.86}_{-0.78}$\,km. We explore the ways such a result can be accommodated within models of dense matter with heavy baryonic degrees of freedom which are constrained by the multi-messenger observations. We find that to do so using only purely nucleonic models, one needs to assume a rather small value of the slope of symmetry energy $L_{\rm sym}$. Once heavy baryons are included higher values of the slope $L_{\rm sym}$ become acceptable at the cost of a slightly reduced maximum mass of static configuration. These two scenarios are distinguished by the particle composition and will undergo different cooling scenarios. In addition, we show that the universalities of the $I$-Love-$Q$ relations for static configurations can be extended to very low masses without loss in their accuracy.

nucl-th↗