arXiv subjects

Monika Sinha

Publications and source records attributed to Monika Sinha.

At least 37 records · Page 2Linked to original sources

arXiv2023

(Anti)kaon condensation in strongly magnetized dense matter

Recent observations of several massive pulsars, with masses near and above $2~M_\odot$, point towards the existence of matter at very high densities, compared to normal matter that we are familiar with in our terrestrial world. This leads to the possibility of appearance of exotic degrees of freedom other than nucleons inside the core of the neutrons stars (NS). Another significant property of NSs is the presence of high surface magnetic field, with highest range of the order of $\sim~10^{16}$ G. We study the properties of highly dense matter with the possibility of appearance of heavier strange and non-strange baryons, and kaons in presence of strong magnetic field. We find that the presence of a strong magnetic field stiffens the matter at high density, delaying the kaon appearance and, hence, increasing the maximum attainable mass of NS family.

astro-ph.HE↗

arXiv2023

Hybrid stars are compatible with recent astrophysical observations

Compact stars (CS) are stellar remnants of massive stars. Inside CSs the density is so high that matter is in subatomic form composed of nucleons. With increase of density of matter towards the centre of the objects other degrees of freedom like hyperons, heavier non-strange baryons, meson condensates may appear. Not only that at higher densities, the nucleons may get decomposed into quarks and form deconfined strange quark matter (SQM). If it is so then CSs may contain SQM in the core surrounded by nucleonic matter forming hybrid stars (HSs). However, the nature and composition of matter inside CSs can only be inferred from the astrophysical observations of these CSs. Recent astrophysical observations in terms of CS mass-radius (M-R) relation and gravitational wave (GW) observation indicate that the matter should be soft in the intermediate density range and stiff enough at higher density range to attain the maximum possible mass above $2~M_\odot$ which is not compatible with pure hadronic equation of states (EOSs). Consequently, we study the HS properties with different models of SQM and find that within vector bag model considering density dependent bag parameter, the model goes well with the astrophysical observations so far.

astro-ph.HE↗

arXiv2023

Tension between implications from PREX-2 data and gravitational tidal response on dense matter equation of state

Recently an improved value of neutron skin thickness of $^{208}\text{Pb}$ was reported in Lead Radius EXperiment-2 (PREX-2) to be $R_{\text{skin}}=R_n - R_p=(0.283\pm 0.071)$ fm which corresponds to high estimations of nuclear symmetry energy ($E_{\text{sym}}$) and its slope ($L_{\text{sym}}$). The updated values of $E_{\text{sym}}$ and $L_{\text{sym}}$ commensurating to the neutron star observable estimations lie exterior to the astrophysical observed range. The higher values of $L_{\text{sym}}$ at $n_0$ deduced from recent PREX-2 data correlates to matter being easily deformable (yielding higher radius values) around intermediate matter densities leading to higher values of $\tildeΛ$ creating a tension between the terrestrial and astrophysical observations. In this study, we exploit this tension to constrain the $Δ$-scalar meson coupling parameter space.

astro-ph.HE↗

arXiv2022

Compact star merger events with stars composed of interacting strange quark matter

We investigate the properties of stars participating in double compact star merger events considering interacting model of stable strange quark matter. We model the matter making it compatible with the recent astrophysical observations of compact star mass-radius and gravitational wave events. In this context we consider modified MIT bag model and vector bag model with and without self interaction. We find new upper bound on tidal deformability of $1.4~M_\odot$ strange star corresponding to the upper bound of effective tidal deformability inferred from gravitational wave event. Range of compactness of $1.4~M_\odot$ strange star is obtained as ${0.175}\leq{C_{1.4}}\leq{0.199}$. Radius range of $1.5M_\odot$ primary star is deduced to be ${10.57}\leq{R_{1.5}}\leq{12.04}$ km, following stringent GW170817 constraints. GW190425 constraints provide with upper limit on radius of $1.7$ solar mass strange star that it should be less than $13.41$ $\text{km}$.

astro-ph.HE↗

arXiv2022

Direct URCA process in light of PREX-2

We study the implications of the recent development in nuclear symmetry energy constraints from PREX-2 data on dense matter equation of state and its impact on dURCA threshold density. In this work, we construct the equation of state within the framework of covariant density functional theory implementing coupling schemes of non-linear and density-dependent models and exploring the coupling parameter space of isovector-vector meson to baryons constrained by the isospin asymmetry parameter values deduced from recent PREX-2 data. The modified parameter sets are applied to evaluate the dense matter properties. We find that the updated data suggests the occurrence of dURCA process within neutron star even with mass as low as one solar mass.

nucl-th↗

arXiv2022

Influence of the nuclear symmetry energy slope on observables of compact stars with $Δ$-admixed hypernuclear matter

In this work, we study the effects of nuclear symmetry energy slope on neutron star dense matter equation of state and its impact on neutron star observables (mass-radius, tidal response). We construct the equation of state within the framework of covariant density functional theory implementing coupling schemes of non-linear and density-dependent models with viability of heavier non-nucleonic degrees of freedom. The slope of symmetry energy parameter ($L_{\text{sym}}$) is adjusted following density-dependence of isovector meson coupling to baryons. We find that smaller values of $L_{\text{sym}}$ at saturation favour early appearance of $Δ$-resonances in comparison to hyperons leading to latter's threshold at higher matter densities. We also investigate the dependence of $L_{\text{sym}}$ on tidal deformability and compactness parameter of a $1.4~M_\odot$ neutron star for different equation of states and observe similar converging behaviour for larger $L_{\text{sym}}$ values.

astro-ph.HE↗

arXiv2021

Baryonic dense matter in view of gravitational-wave observations

The detection of gravitational waves from the merger of binary neutron stars events (GW170817, GW190425) and subsequent estimations of tidal deformability play a key role in constraining the behaviour of dense matter. In addition, massive neutron star candidates ($\sim 2 M_{\odot}$) along with NICER mass-radius measurements also, set sturdy constraints on the dense matter equation of state. Strict bounds from gravitational waves and massive neutron stars observations constrain the theoretical models of nuclear matter comportment at large density regimes. On the other hand, model parameters providing the highly dense matter response are bounded by nuclear saturation properties. This work analyses coupling parametrizations from two classes based on covariant density functional models: Non-Linear and Density-Dependent schemes. Considering these constraints together, we study possible models and parametrization schemes with the feasibility of exotic degrees of freedom in dense matter which go well with the astrophysical observations as well as the terrestrial laboratory experiments. We show that most parametrizations with non-linear schemes do not support the observations and experiments while density-dependent scheme goes well with both. Astrophysical observations are well explained if the inclusion of heavier non-strange baryons is considered as one fraction of the dense matter particle spectrum.

astro-ph.HE↗

arXiv2021

Ambipolar decay of magnetic field in magnetars and the observed magnetar activities

Magnetars are comparatively young neutron stars with ultra-strong surface magnetic field in the range $10^{14-16}$ G. The old neutron stars have surface magnetic field some what less $\sim 10^8$ G which clearly indicates the decay of field with time. One possible way of magnetic field decay is by ambipolar diffusion. We describe the general procedure to solve for the ambipolar velocity inside the star core without any approximation. With a realistic model of neutron star we determine the ambipolar velocity configuration inside the neutron star core and hence find the ambipolar decay rate and time scale which is consistent with the magnetar observations.

astro-ph.HE↗

arXiv2021

Massive $Δ$-resonance admixed hypernuclear stars with anti-kaon condensations

In this work, we study the effect of (anti)kaon condensation on the properties of compact stars that develop hypernuclear cores with and without an admixture of $Δ$-resonances. We work within the covariant density functional theory with the parameters adjusted to $K$-atomic and kaon-nucleon scattering data in the kaonic sector. The density-dependent parameters in the hyperonic sector are adjusted to the data on $Λ$ and $Ξ^-$ hypernuclei data. The $Δ$-resonance couplings are tuned to the data obtained from their scattering off nuclei and heavy-ion collision experiments. We find that (anti)kaon condensate leads to a softening of the equation of state and lower maximum masses of compact stars than in the absence of the condensate. Both the $K^-$ and $\bar K^0$-condensations occur through a second-order phase transition, which implies no mixed-phase formation. For large values of (anti)kaon and $Δ$-resonance potentials in symmetric nuclear matter, we observe that condensation leads to an extinction of $ Ξ^{-,0}$ hyperons. We also investigate the influence of inclusion of additional hidden-strangeness $σ^{*}$ meson in the functional and find that it leads to a substantial softening of the equation of state and delay in the onset of (anti)kaons.

astro-ph.HE↗

arXiv2020

Dense matter equation of state of a massive neutron star with anti-kaon condensation

Recent measurements of neutron star mass from several candidates (PSR J$1614-2230$, PSR J$0348+0432$, MSP J$0740+6620$) set the lower bound on the maximum possible mass for this class of compact objects $\sim 2$ M$_\odot$. Existence of stars with high mass brings the possibility of existence of exotic matter (hyperons, meson condensates) at the core region of the objects. In this work, we investigate the (anti)kaon ($K^-, \bar{K}^0$) condensation in $β-$equilibrated nuclear matter within the framework of covariant density functional theory. The functionals in the kaonic sector are constrained by the experimental studies on $K^-$ atomic, kaon-nucleon scattering data fits. We find that the equation of state softens with the inclusion of (anti)kaon condensates, which lowers the maximum mass of neutron star. In one of the density-independent coupling cases, the $K^-$ condensation is through a first-order phase transition type, which produces a $2$ M$_\odot$ neutron star. The first-order phase transition results in mixed phase region in the inner core of the stars. While $\bar{K}^0$ condensation appears via second-order phase transition for all the models we consider here.

astro-ph.HE↗

arXiv2020

Equation of state of strongly magnetized matter with hyperons and $Δ$-resonances

We construct a new equation of state for the baryonic matter under an intense magnetic field within the framework of covariant density functional theory. The composition of matter includes hyperons as well as $ Δ$-resonances. The extension of the nucleonic functional to the hypernuclear sector is constrained by the experimental data on $Λ$ and $Ξ$-hypernuclei. We find that the equation of state stiffens with the inclusion of the magnetic field, which increases the maximum mass of neutron star compared to the non-magnetic case. In addition, the strangeness fraction in the matter is enhanced. Several observables, like the Dirac effective mass, particle abundances, etc show typical oscillatory behavior as a function of the magnetic field and/or density which is traced back to the occupation pattern of Landau levels.

hep-ph↗

arXiv2017

From microphysics to dynamics of magnetars

MeV-scale magnetic fields in the interiors of magnetars suppress the pairing of neutrons and protons in the $S$-wave state. In the case of a neutron condensate the suppression is the consequence of the Pauli-paramagnetism of the neutron gas, i.e., the alignment of the neutron spins along the magnetic field. The proton $S$-wave pairing is suppressed because of the Landau diamagnetic currents of protons induced by the field. The Ginzburg-Landau and BCS theories of the critical magnetic fields for unpairing are reviewed. The macrophysical implications of the suppression (unpairing) of the condensates are discussed for the rotational crust-core coupling in magnetars and the neutrino-dominated cooling era of their thermal evolution.

astro-ph.HE↗

arXiv2015

Magnetar superconductivity versus magnetism: neutrino cooling processes

We describe the microphysics, phenomenology, and astrophysical implication of a $B$-field induced unpairing effect that may occur in magnetars, if the local $B$-field in the core of a magnetar exceeds a critical value $H_{c2}$. Using the Ginzburg-Landau theory of superconductivity, we derive the $H_{c2}$ field for proton condensate taking into the correction ($\le 30\%$) which arises from its coupling to the background neutron condensate. The density dependence of pairing of proton condensate implies that $H_{c2}$ is maximal at the crust-core interface and decreases towards the center of the star. As a consequence, magnetar cores with homogenous constant fields will be partially superconducting for "medium-field" magnetars ($10^{15}\le B\le 5 \times 10^{16}$ G) whereas "strong-field" magnetars ($B>5\times 10^{16}$ G) will be void of superconductivity. The neutrino emissivity of a magnetar's core changes in a twofold manner: (i)~the $B$-field assisted direct Urca process is enhanced by orders of magnitude, because of the unpairing effect in regions where $B\ge H_{c2}$; (ii)~the Cooper-pair breaking processes on protons vanish in these regions and the overall emissivity by the pair-breaking processes is reduced by a factor of only a few.

astro-ph.HE↗

arXiv2015

Dense matter is strong magnetic fields

Compact stars having strong magnetic fields (magnetars) have been observationally determined to have surface magnetic fields of order of 10^14-10^15 G, the implied internal field strength being several orders larger. We study the equation of state and composition of hypernuclear matter and quark matter - two forms of dense matter in strong magnetic fields. We find that the magnetic field has substantial influence on the properties of hypernuclear matter and quark matter for magnetic field B \ge 10^17 G and B \ge 10^18 G respectively. In particular the matter properties become anisotropic. Moreover, above a critical field B_cr, both hypernuclear and quark matter show instability, although the values of B_cr are different for two kinds of matter.

astro-ph.HE↗

arXiv2014

Upper critical field and (non)-superconductivity of magnetars

We construct equilibrium models of compact stars using a realistic equation of state and obtain the density range occupied by the proton superconductor in strong $B$-fields. We do so by combining the density profiles of our models with microscopic calculations of proton pairing gaps and the critical unpairing field $H_{c2}$ above which the proton type-II superconductivity is destroyed. We find that magnetars with interior homogeneous field within the range $0.1 \le B_{16}\le 2$, where $B_{16} = B/10^{16}$ G, are partially superconducting, whereas those with $B_{16} > 2$ are void of superconductivity. We briefly discuss the neutrino emissivity and superfluid dynamics of magnetars in the light of their (non)-superconductivity.

astro-ph.SR↗

arXiv2013

Strange quark matter in strong magnetic fields within a confining model

We construct an equation of state of strange quark matter in strong magnetic field within a confining model. The confinement is modeled by means of the Richardson potential for quark-quark interaction modified suitably to account for strong magnetic field. We compare our results for the equation of state and magnetization of matter to those derived within the MIT bag model. The differences between these models arise mainly due to the momentum dependence of the strong interaction between quarks in the Richardson model. Specifically, we find that the magnetization of strange quark matter in this model has much more pronounced de Haas-van Alfven oscillations than in the MIT bag model, which is the consequence of the (static) gluon-exchange structure of the confining potential.

astro-ph.HE↗

arXiv2013

Hypernuclear matter in strong magnetic field

Compact stars with strong magnetic fields (magnetars) have been observationally determined to have surface magnetic fields of order of 10^{14}-10^{15} G, the implied internal field strength being several orders larger. We study the equation of state and composition of dense hypernuclear matter in strong magnetic fields in a range expected in the interiors of magnetars. Within the non-linear Boguta-Bodmer-Walecka model we find that the magnetic field has sizable influence on the properties of matter for central magnetic field B \ge 10^{17} G, in particular the matter properties become anisotropic. Moreover, for the central fields B \ge 10^{18} G, the magnetized hypernuclear matter shows instability, which is signaled by the negative sign of the derivative of the pressure parallel to the field with respect to the density, and leads to vanishing parallel pressure at the critical value B_{\rm cr} \simeq 10^{19} G. This limits the range of admissible homogeneously distributed fields in magnetars to fields below the critical value B_{\rm cr}.

astro-ph.HE↗

arXiv2013

Constraining the central magnetic field of magnetars

The magnetars are believed to be highly magnetized neutron stars having surface magnetic field 10^{14} - 10^{15} G. It is believed that at the center, the magnetic field may be higher than that at the surface. We study the effect of the magnetic field on the neutron star matter. We model the nuclear matter with the relativistic mean field approach considering the possibility of appearance of hyperons at higher density. We find that the effect of magnetic field on the matter of neutron stars and hence on the mass-radius relation is important, when the central magnetic field is atleast of the order of 10^{17} G. Very importantly, the effect of strong magnetic field reveals anisotropy to the system. Moreover, if the central field approaches 10^{19} G, then the matter becomes unstable which limits the maximum magnetic field at the center of magnetars.

astro-ph.HE↗