arXiv subjects

Milva G. Orsaria

Publications and source records attributed to Milva G. Orsaria.

At least 19 recordsLinked to original sources

arXiv2026

Systematic study of the morphology and length of slow stable hybrid star branches

We introduce and systematically study the length of the slow stable hybrid star branch as a quantitative measure of the extended stability region that arises in hybrid neutron stars when the hadron-quark phase conversion is slow compared to the radial oscillation timescale. Combining generalized piecewise-polytropic hadronic equations of state of varying stiffness with a constant-speed-of-sound quark-matter model, we construct a large set of hybrid equations of state spanning a broad range of transition pressures, energy-density jumps, and quark-matter speeds of sound. We identify four morphological types for the slow stable branch in the mass-radius plane: waterfall branches that descend monotonically from the hadronic maximum mass, bridges that connect the hadronic branch to a second unconditionally stable hybrid branch, tails that extend briefly beyond the maximum mass of an unconditionally stable hybrid branch, and tail-bridges that combine features of the latter two. Their prevalence is governed primarily by the transition pressure and the energy-density jump, while the branch length is also significantly influenced by the stiffness of the hadronic sector and the quark-matter speed of sound. Imposing current astrophysical and microphysical constraints shows that viable long branches are predominantly of waterfall type, and that stiff hadronic equations of state -- strongly disfavored under the rapid-conversion assumption -- remain compatible with all current constraints within the slow-conversion framework. In the plane of transition baryon density versus density jump, slow stable configurations open a new region of viable parameter space inaccessible under rapid conversions.

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arXiv2026

The Delta-isobar masquerade: intrahadronic phase transitions and their quark-mimicking signatures in neutron stars

We investigate the conditions under which $Δ(1232)$ isobars trigger a first-order phase transition within purely hadronic neutron-star matter, using the SW4L relativistic mean-field parametrization. For scalar-vector coupling differences $0.15 \lesssim x_{σΔ} - x_{ωΔ} \lesssim 0.2$ and $x_{σΔ} \gtrsim 1.3$, the onset of $Δ^-$ resonances produces a van der Waals-like instability driven by a self-amplifying feedback in the scalar meson sector, in which the $Δ^-$ particle fraction acts as the order parameter of a Landau-type transition. A Maxwell construction yields a sharp density discontinuity at baryon densities $n_b \sim (1.3$-$2)\,n_0$, separating a $Δ$-free outer core from a $Δ$-rich inner core. The resulting neutron-star sequences satisfy all current multimessenger constraints: maximum masses $M_{\rm max} \approx 2.15$-$2.25\,M_\odot$, radii $R_{1.4} \approx 11$-$12$ km, and tidal deformabilities $Λ_{1.4} \approx 190$-$480$, compatible with NICER observations and GW170817. We compute, for the first time for a $Δ$-induced interface, the $\ell = 2$ composition $g$-mode eigenfrequencies, obtaining $ν_g \sim 400$-$1100$ Hz with gravitational-wave damping times $τ_g \sim 10^3$-$10^9$ s. These frequencies overlap quantitatively with those predicted for hadron-quark phase-transition interfaces, demonstrating that the mass-radius ``knee'', reduced tidal deformability, and $g$-mode spectrum conventionally regarded as signatures of quark deconfinement can be reproduced by a purely intrahadronic mechanism. This extends the masquerade problem from static observables to the domain of gravitational-wave asteroseismology, implying that a future detection of a discontinuity $g$-mode alone would not suffice to identify quark matter in neutron-star cores.

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arXiv2025

Towards a unified hadron-quark equation of state for neutron stars within the relativistic mean-field model

The equation of state of dense matter remains a central challenge in astrophysics and high-energy physics, particularly at supra-nuclear densities where exotic degrees of freedom like hyperons or deconfined quarks are expected to appear. Neutron stars provide a unique natural laboratory to probe this regime. In this work, we present EVA--01, a novel equation of state that provides a unified description of dense matter by incorporating both hadron and quark degrees of freedom within a single relativistic mean-field Lagrangian, from which the equation of state is derived at finite temperature. The model extends the density-dependent formalism by introducing a Polyakov-loop-inspired scalar field to dynamically govern the hadron-quark phase transition, following the approach of chiral mean-field models. The resulting model is consistent with a wide range of theoretical and observational constraints, including those from chiral effective field theory, massive pulsars, gravitational-wave events, and NICER data. We analyze its thermodynamic properties by constructing the QCD phase diagram, identifying the deconfinement, chiral, and nuclear liquid-gas transitions. As a first application, we model the evolution of proto-neutron stars using isentropic snapshots and explore the implications of the slow stable hybrid star hypothesis. Our findings establish EVA--01 as a robust and versatile framework for exploring dense matter, bridging the gap between microphysical models and multimessenger astrophysical observations.

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arXiv2025

Exploring the role of $d^*$ hexaquarks on quark deconfinement and hybrid stars

We investigate the impact of the $d^*$(2380) hexaquark on the equation of state (EoS) of dense matter within hybrid stars (HSs) using the Chiral Mean-Field model (CMF). The hexaquark is included as a new degree of freedom in the hadronic phase, and its influence on the deconfinement transition to quark matter is explored. We re-parametrize the CMF model to ensure compatibility with recent astrophysical constraints, including the observation of massive pulsars and gravitational wave events. Our results show that the presence of $d^*$ significantly modifies the EoS, leading to a softening at high densities and a consequent reduction in the predicted maximum stellar masses. Furthermore, we examine the possibility of a first-order deconfinement phase transition within the context of the extended stability branch of slow stable HSs (SSHSs). We find that the presence of hexaquarks can delay the deconfinement phase transition and reduce the associated energy density gap, affecting the structure and stability of HSs. Our results suggest that, as the hexaquark appearance tends to destabilize stellar configurations, fine tuning of model parameters is required to obtain both the presence of hexaquarks and quark deconfinement in these systems. In this scenario, the SSHS branch plays a crucial role in obtaining HSs with hexaquarks that satisfy current astrophysical constraints. Our work provides new insights into the role of exotic particles like $d^*$ in dense matter and the complex interplay between hadronic and quark degrees of freedom inside compact stellar objects.

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arXiv2025

Consistent crust-core interpolation and its effect on non-radial neutron star oscillations

To model the structure of neutron stars (NSs) theoretically,it is common to consider layers with different density regimes. Matching the equation of state (EoS) for the crust and core and obtaining a suitable description of these extreme conditions are crucial for understanding the properties of these compact objects. In this work, we construct ten different NS EoSs incorporating three distinct crust models, which are connected to the core using a thermodynamically and causally consistent formalism. For cold NSs, we propose a linear relationship between pressure and energy density in a narrow region between the crust and core, effectively establishing an interpolation function in the pressure-baryonic chemical potential plane. We then compare this EoS matching method with the classical approach, which neglects causal and thermodynamic consistency. We solve the Tolman-Oppenheimer-Volkoff equation to obtain the mass-radius relationship and compare our results with observational constraints on NSs. Furthermore, we investigate the influence of the new matching formalism on non-radial oscillation frequencies and damping times. Our findings suggest that the method used to glue the crust and core EoS impacts NS observables, such as the radius, oscillation frequencies, and damping times of non-radial modes, which may be crucial for interpreting future gravitational wave observations from neutron star mergers or isolated pulsars. The effects are particularly noticeable for low-mass NSs, regardless of the specific EoS model chosen. In particular, we find that the $p_1$ oscillation mode exhibits significant differences in frequencies among alternative matching methods, whereas the fundamental $f$-mode remains unaffected by changes in crust models or interpolation schemes.

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arXiv2024

Fast Pulsars, Neutron Stars, and Astrophysical Strange Quark Matter Objects

This book chapter explores key aspects of neutron stars, pulsar glitches, tidal deformability, fast pulsars, the equation of state, and strange quark matter stars. Challenges in directly measuring neutron star radius have led to reliance on spectroscopic and timing techniques, with uncertainties addressed through careful source selection and theoretical modeling. Pulsar glitches reveal insights into the equation of state through angular momentum transfer within the neutron star. Tidal deformability is crucial in gravitational-wave astronomy, exemplified by the GW170817 event. Fast pulsars, instrumental in astrophysical testing, are classified into ordinary pulsars, millisecond pulsars, and magnetars. The EOS is vital for understanding neutron star internal structure, explored through various models. The chapter delves into the theoretical framework for rotating neutron stars, addressing uniform and differential rotation scenarios and their impacts on mass and radius. Additionally, the intriguing concept of quark stars and strange dwarfs is investigated. The various topics discussed in this book chapter contribute to a broader understanding of dense matter physics, astrophysical phenomena, and the potential for transformative discoveries through advanced observational techniques and technologies like gravitational wave detectors, radio telescopes, and X-ray telescopes.

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arXiv2024

Neutron star stability beyond the mass peak: assessing the role of out-of-equilibrium perturbations

We investigate the radial stability of neutron stars under conditions where their composition may or may not remain in chemical equilibrium during oscillations. Using different equations of state that include nucleons, hyperons, and/or $Δ$ resonances, we compute stellar configurations and examine their fundamental mode frequencies in two limiting scenarios. In one limit, nuclear reactions are fast enough to maintain chemical equilibrium throughout the pulsation, resulting in a lower effective adiabatic index, $Γ_{\mathrm{EQ}}$, and softer stellar responses. In the opposite limit, nuclear reactions are too slow to adjust particle abundances during oscillations, yielding a higher index, $Γ_{\mathrm{FR}}$, and stiffer stellar responses. We find that the equilibrium scenario triggers dynamic instability at the maximum mass configuration, whereas the frozen composition scenario allows stable solutions to persist beyond this mass, extending the stable branch. This effect is modest for simpler equations of state, but becomes increasingly pronounced for more complex compositions, where the emergence of new particle species at high densities leads to a significant disparity between $Γ_{\mathrm{EQ}}$ and $Γ_{\mathrm{FR}}$. Realistic conditions, in which different nuclear reactions have distinct timescales, will place the effective $Γ$ between these two extreme values. Short-timescale reactions push the star toward the equilibrium limit, potentially restricting the length of the stable branch. Conversely, slow reactions preserve a frozen composition, allowing the stable branch to grow. Thus, the actual extent of the stable configuration range depends critically on the interplay between nuclear-reaction timescales and the star's fundamental oscillation period.

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arXiv2024

Could a slow stable hybrid star explain the central compact object in HESS~J1731-347?

We explore an alternative explanation for the low-mass ultra-compact star in the supernova remnant HESS~J1731-347 using a model-agnostic approach to construct hybrid equations of state. The hadronic part of the hybrid equation of state is constructed using a generalized piecewise polytropic scheme, while the quark phase is described by the generic constant speed of sound model. We assume an abrupt first-order hadron-quark phase transition with a slow conversion speed between phases. Our equations of state align with modern Chiral Effective Field Theory calculations near nuclear saturation density and are consistent with perturbative Quantum Chromodynamics calculations at high densities. Using this theoretical framework, we derive a wide range of hybrid equations of state capable of explaining the light compact object in HESS~J1731-347 in a model-independent manner, without fine-tuning. These equations of state are also consistent with modern astronomical constraints from high-mass pulsar timing, NICER observations, and multimessenger astronomy involving gravitational waves. Our results support the hypothesis that the compact object in HESS~J1731-347 could plausibly be a slow stable hybrid star.

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arXiv2023

On the Destabilization of High-Mass Neutron Stars by the Emergence of $d^*$-Hexaquarks

We study the effects of the first nontrivial hexaquark, $d^*$(2380), on the equation of state of dense neutron star matter and investigate the consequences of its existence for neutron stars. The matter in the core regions of neutron stars is described using density-dependent relativistic mean-field theory. Our results show that within the parameter spaces examined in our paper, (i) the critical density at which the $d^*$ condensate emerges lies between 4 and 5 times the nuclear saturation density, (ii) $d^*$ hexaquarks are found to exist only in rather massive neutron stars, (iii) only relatively small fractions of the matter in the core of a massive neutron star may contain hexaquarks.

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arXiv2023

Constraining self-interacting fermionic dark matter in admixed neutron stars using multimessenger astronomy

We investigate the structure of admixed neutron stars with a regular hadronic component and a fraction of fermionic self-interacting dark matter. Using two limiting equations of state for the dense baryonic interior, constructed from piecewise generalised polytropes, and an asymmetric self-interacting fermionic dark component, we analyse different scenarios of admixed neutron stars depending on the mass of dark fermions $m_χ$, interaction mediators $m_ϕ$, and self-interacting strengths $g$. We find that the contribution of dark matter to the masses and radii of neutron stars leads to tension with mass estimates of the pulsar J0453+1559, the least massive neutron star, and with the constraints coming from the GW170817 event. We discuss the possibilities of constraining dark matter model parameters $g$ and $y \equiv m_χ/m_ϕ$, using current existing knowledge on neutron star estimations of mass, radius, and tidal deformability, along with the accepted cosmological dark matter freeze-out values and self-interaction cross-section to mass ratio, $σ_\mathrm{SI}/m_χ$, fitted to explain Bullet, Abell, and dwarf galaxy cluster dynamics. By assuming the most restrictive upper limit, $σ_\mathrm{SI}/m_χ< 0.1$ cm$^2$/g, along with dark matter freeze-out range values, the allowed $g$-$y$ region is $0.01 \lesssim g \lesssim 0.1$, with $0.5 \lesssim y \lesssim 200$. For the first time, the combination of updated complementary restrictions is used to set constraints on self-interacting dark matter.

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arXiv2022

Hybrid magnetized stars in the context of multi-messenger astronomy

The most recent detections of LIGO/Virgo and NICER have placed strong constraints on neutron stars' properties. In this work, we study neutron stars modeling them as hybrid stars, compact objects with a quark matter core surrounded by layers of hadronic matter. In addition, we consider the presence of strong magnetic fields, motivated by magnetars detection, like the recently discovered Swift J1818.0-1607. We incorporate the effects of the anomalous magnetic moment of the constituent particles into the equation of state of dense matter and analyze the implications of different hadron-quark phase transitions on the dynamic stability of these compact objects. This study shows that the constraints on the mass, radius, and tidal deformability, imposed by observations of massive pulsars and gravitational waves can be satisfied within the framework of our model.

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arXiv2022

Oscillating magnetised hybrid stars under the magnifying glass of multi-messenger observations

We model neutron stars as magnetised hybrid stars with an abrupt hadron-quark phase transition in their cores, taking into account current constraints from nuclear experiments and multi-messenger observations. We include magnetic field effects considering the Landau level quantisation of charged particles and the anomalous magnetic moment of neutral particles. We construct the magnetised hybrid equation of state, and we compute the particle population, the matter magnetisation and the transverse and parallel pressure components. We integrate the stable stellar models, considering the dynamical stability for \emph{rapid} or \emph{slow} hadron-quark phase conversion. Finally, we calculate the frequencies and damping times of the fundamental and $g$ non-radial oscillation modes. The latter, a key mode to learn about phase transitions in compact objects, is only obtained for stars with slow conversions. For low magnetic fields, we find that one of the objects of the GW170817 binary system might be a hybrid star belonging to the slow extended stability branch. For magnetars, we find that a stronger magnetic field always softens the hadronic equation of state. Besides, only for some parameter combinations a stronger magnetic field implies a higher hybrid star maximum mass. Contrary to previous results, the incorporation of anomalous magnetic moment does not affect the studied astrophysical quantities. We discuss possible imprints of the microphysics of the equation of state that could be tested observationally in the future, and that might help infer the nature of dense matter and hybrid stars.

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arXiv2021

Hybrid stars with color superconducting cores in an extended FCM model

We investigate the influence of repulsive vector interactions and color superconductivity on the structure of neutron stars using an extended version of the field correlator method (FCM) for the description of quark matter. The hybrid equation of state is constructed using the Maxwell description, which assumes a sharp hadron-quark phase transition. The equation of state of hadronic matter is computed for a density-dependent relativistic lagrangian treated in the mean-field approximation, with parameters given by the SW4L nuclear model. This model described the interactions among baryons in terms of $σ, ω, ρ, σ^*$, and $ϕ$ mesons. Quark matter is assumed to be in either the CFL or the 2SC+s color superconducting phase. The possibility of sequential (hadron-quark, quark-quark) transitions in ultra-dense matter is investigated. Observed data related to massive pulsars, gravitational-wave events, and NICER are used to constrain the parameters of the extended FCM model. The successful equations of state are used to explore the mass-radius relationship, radii, and tidal deformabilities of hybrid stars. A special focus lies on investigating consequences that slow or fast conversions of quark-hadron matter have on the stability and the mass-radius relationship of hybrid stars. We find that if slow conversion should occur, a new branch of stable massive stars would exist whose members have radii that are up to 1.5~km smaller than those of conventional neutron stars of the same mass. Such objects could be possible candidates for the stellar high-mass object of the GW190425 binary system.

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arXiv2020

Delta baryons and diquark formation in the cores of neutron stars

We investigate the hadron-quark phase transition in cold neutron stars in light of (i) the observed limits on the maximum-mass of heavy pulsars, (ii) constraints on the tidal properties inferred from the gravitational waves emitted in binary neutron-star mergers, and (iii) mass and radius constraints derived from the observation of hot spots on neutron star observed with NICER. Special attention is directed to the possible presence of $Δ(1232)$ baryons in neutron star matter. Our results indicate that this particle could make up a large fraction of the baryons in neutron stars and thus have a significant effect on the properties of such objects, particularly on their radii. This is partially caused by the low density appearance of $Δ$s for a wide range of theoretically defensible sets of meson-hyperon, SU(3) ESC08 model, and meson-$Δ$ coupling constants. The transition of hadronic matter to quark matter, treated in the 2SC+s condensation phase, is found to occur only in neutron stars very close to the mass peak. Nevertheless, quark matter may still constitute an appreciable fraction of the stars' total matter if the phase transition is treated as Maxwell-like (sharp), in which case the neutron stars located beyond the gravitational mass peak would remain stable against gravitational collapse. In this case, the instability against gravitational collapse is shifted to a new (terminal) mass different from the maximum-mass of the stellar sequence, giving rise to stable compact objects with the same gravitational masses as those of the neutron stars on the traditional branch, but whose radii are smaller by up to 1 km. All models for the equation of state of our study fall comfortably within the bound established very recently by Annala {\it et al.} (Nature Physics, 2020)

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arXiv2020

Estrellas híbridas con núcleos superconductores de color

The aim of this work is the study of hybrid stars interiors from the equations of state describing the matter composing them. We used modern hadronic equations of state to describe the matter in the outer core of these stars. The quark phase is modeled by an extended version of the Field Correlator Method, in which vector interactions among quarks and color superconductivity are included. We analyze the effect of these two contributions on the mass-radius relationship and tidal deformability. This is a relevant quantity for neutron stars after the GW170817 event, allowing to impose additional constraints to the equation of state of these compact objetcs.

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arXiv2019

Magnetised hybrid stars: effects of slow and rapid phase transitions at the quark-hadron interface

We study the influence of strong magnetic fields in hybrid stars, composed by hadrons and a pure quark matter core, and analyse their structure and stability as well as some possible evolution channels due to the magnetic field decay. Using an ad-hoc parametrisation of the magnetic field strength and taking into account Landau-quantization effects in matter, we calculate hybrid magnetised equations of state and some associated quantities, such as particle abundances and matter magnetisation, for different sets of parameters and different magnetic field strengths. Moreover, we compute the magnetised stable stellar configurations, the mass versus radius and the gravitational mass versus central energy density relationships, the gravitational mass versus baryon mass diagram, and the tidal deformability. Our results are in agreement with both, the $\sim 2M_\odot$ pulsars and the data obtained from GW170817. In addition, we study the stability of stellar configurations assuming that slow and rapid phase transitions occur at the sharp hadron-quark interface. We find that, unlike in the rapid transition scenario, where $\partial M/\partial ε_c < 0$ is a sufficient condition for instability, in the slow transition scenario there exists a connected extended stable branch beyond the maximum mass star, for which $\partial M/\partial ε_c < 0$. Finally, analysing the gravitational mass versus baryon mass relationship, we have calculated the energy released in transitions between stable stellar configurations. We find that the inclusion of the magnetic field and the existence of new stable branches allows the possibility of new channels of transitions that fulfil the energy requirements to explain Gamma Ray Bursts.

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arXiv2019

Phase transitions in neutron stars and their links to gravitational waves

The recent direct observation of gravitational wave event $GW170817$ and its $GRB170817A$ signal has opened up a new window to study neutron stars and heralds a new era of Astronomy referred to as the Multimessenger Astronomy. Both gravitational and electromagnetic waves from a single astrophysical source have been detected for the first time. This combined detection offers an unprecedented opportunity to place constraints on the neutron star matter equation of state. The existence of a possible hadron-quark phase transition in the central regions of neutron stars is associated with the appearance of g-modes, which are extremely important as they could signal the presence of a pure quark matter core in the centers of neutron stars. Observations of g-modes with frequencies between 1 kHz and 1.5 kHz could be interpreted as evidence of a sharp hadron-quark phase transition in the cores of neutron stars. In this article, we shall review the description of the dense matter composing neutron stars, the determination of the equation of state of such matter, and the constraints imposed by astrophysical observations of these fascinating compact objects.

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arXiv2019

Hybrid magnetized stars within the Field Correlator Method

We present preliminary results of the study of intense magnetic fields effects on hybrid stars. For the description of the hadronic phase, we use the relativistic mean field approximation. For the quark matter phase, we employ the Field Correlator Method formalism. Once the the equation of state is built, we discuss the pressure anisotropy due the presence of the strong magnetic field. Finally, we calculate the structure of the compact stars using magnetized hybrid equations of state and their oscillation modes related with the emission of gravitational waves.

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