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we report ab initio calculations of the s wave pairing gap in neutron matter calculated using realistic nuclear hamiltonians that include two- and three-body interactions. we use a trial state, properly optimized to capture the essential pairing correlations, from which we extract ground state properties by means of auxiliary field diffusion monte carlo simulations. we extrapolate our results to the thermodynamic limit by studying the finite-size effects in the symmetry-restored projected bardeen-cooper-schrieffer (pbcs) theory and compare our results to other ab initio studies done in the past. our quantum monte carlo results for the pairing gap show a modest suppression with respect to the mean-field bcs values. these results can be connected to cold atom experiments, via the unitarity regime where fermionic superfluidity assumes a unified description, and they are important in the prediction of thermal properties and the cooling of neutron stars. | the 1s0 pairing gap in neutron matter |
we model the thermal states of both isolated neutron stars and accreting neutron stars in x-ray transients in quiescence and confront them with observations. we use an equation of state calculated using realistic two-body and three-body nucleon interactions, and superfluid nucleon gaps obtained using the same microscopic approach in the bcs approximation. consistency with low-luminosity accreting neutron stars is obtained, as the direct urca process is operating in neutron stars with mass larger than 1.1 m⊙ for the employed equation of state. in addition, proton superfluidity and sufficiently weak neutron superfluidity, obtained using a scaling factor for the gaps, are necessary to explain the cooling of middle-aged neutron stars and to obtain a realistic distribution of neutron star masses. | thermal states of neutron stars with a consistent model of interior |
accreting neutron stars (nss) are one of the main targets for continuous gravitational wave searches, as asymmetric accretion may lead to quadrupolar deformations, or `mountains', on the crust of the star, which source gravitational wave (gw) emission at twice the rotation frequency. the gw torque may also impact on the spin evolution of the star, possibly dictating the currently observed spin periods of nss in low-mass x-ray binaries and leading to the increased spin-down rate observed during accretion in psr j1023+0038. previous studies have shown that deformed reaction layers in the crust of the ns lead to thermal and compositional gradients that can lead to gw emission. however, there are no realistic constraints on the level of asymmetry that is expected. in this paper, we consider a natural source of asymmetry, namely the magnetic field, and calculate the density and pressure perturbations that are expected in the crust of accreting nss. in general, we find that only the outermost reaction layers of the ns are strongly perturbed. the mass quadrupole that we estimate is generally small and cannot explain the increase of spin-down rate of psr j1023+0038. however, if strong shallow heating sources are present at low densities in the crust, as cooling observations suggest, these layers will be strongly perturbed and the resulting quadrupole could explain the observed spin-down of psr j1023+0038, and lead to observable gw signals from systems with higher accretion rates. | asymmetric accretion and thermal `mountains' in magnetized neutron star crusts |
neutron star models with maximum mass close to 2 m⊙ reach high central densities, which may activate nucleonic and hyperon direct urca neutrino emission. to alleviate the tension between fast theoretical cooling rates and thermal luminosity observations of moderately magnetized, isolated thermally emitting stars (with lγ ≳ 1031 erg s-1 at t ≳ 105.3 yr), some internal heating source is required. the power supplied by the internal heater is estimated for both a phenomenological source in the inner crust and joule heating due to magnetic field decay, assuming different superfluidity models and compositions of the outer stellar envelope. it is found that a thermal power of w(t) ≈ 1034 erg s-1 allows neutron star models to match observations of moderately magnetized, isolated stars with ages t ≳ 105.3 yr. the requisite w(t) can be supplied by joule heating due to crust-confined initial magnetic configurations with (i) mixed poloidal-toroidal fields, with surface strength bdip = 1013 g at the pole of the dipolar poloidal component and ~90 per cent of the magnetic energy stored in the toroidal component; and (ii) poloidal-only configurations with bdip = 1014 g. | fast cooling and internal heating in hyperon stars |
recent observations have found several candidates for old warm neutron stars whose surface temperatures are above the prediction of the standard neutron star cooling scenario, and, thus, require some heating mechanism. motivated by these observations, we study the non-equilibrium beta process in the minimal cooling scenario of neutron stars, which inevitably occurs in pulsars. this out-of-equilibrium process yields the late-time heating in the core of a neutron star, called the rotochemical heating, and significantly changes the time evolution of the neutron star surface temperature. to perform a realistic analysis of this heating effect, we include the proton-singlet- and neutron-triplet-pairing gaps simultaneously in the calculation of the rate and emissivity of this process, where the dependence of these pairing gaps on the nucleon density is also taken into account. we then compare the predicted surface temperature of neutron stars with the latest observational data. we show that the simultaneous inclusion of both proton and neutron gaps is advantageous for the explanation of the old warm neutron stars since it enhances the heating effect. it is then found that the observed surface temperatures of the old warm millisecond pulsars, j2124-3358 and j0437-4715, are explained for various choices of nucleon gap models. the same set-up is compatible with the observed temperatures of ordinary pulsars, including old warm ones, j0108-1431 and b0950+08, by choosing the initial rotational period of each neutron star accordingly. in particular, the upper limit on the surface temperature of j2144-3933 can be satisfied if its initial period is ≳ 10 ms. | cooling theory faced with old warm neutron stars: role of non-equilibrium processes with proton and neutron gaps |
the neutron-proton effective mass splitting in neutron-rich nucleonic matter reflects the spacetime nonlocality of the isovector nuclear interaction. it affects the neutron/proton ratio during the earlier evolution of the universe, cooling of proto-neutron stars, structure of rare isotopes and dynamics of heavy-ion collisions. while there is still no consensus on whether the neutron-proton effective mass splitting is negative, zero or positive and how it depends on the density as well as the isospin-asymmetry of the medium, significant progress has been made in recent years in addressing these issues. there are different kinds of nucleon effective masses. in this mini-review, we focus on the total effective masses often used in the non-relativistic description of nuclear dynamics. we first recall the connections among the neutron-proton effective mass splitting, the momentum dependence of the isovector potential and the density dependence of the symmetry energy. we then make a few observations about the progress in calculating the neutron-proton effective mass splitting using various nuclear many-body theories and its effects on the isospin-dependence of in-medium nucleon-nucleon cross-sections. perhaps, our most reliable knowledge so far about the neutron-proton effective mass splitting at saturation density of nuclear matter comes from optical model analyses of huge sets of nucleon-nucleus scattering data accumulated over the last five decades. the momentum dependence of the symmetry potential from these analyses provide a useful boundary condition at saturation density for calibrating nuclear many-body calculations. several observables in heavy-ion collisions have been identified as sensitive probes of the neutron-proton effective mass splitting in dense neutron-rich matter based on transport model simulations. we review these observables and comment on the latest experimental findings. | neutron-proton effective mass splitting in neutron-rich matter and its impacts on nuclear reactions |
new-generation x-ray polarimeters currently under development promise to open a new window in the study of high-energy astrophysical sources. among them, neutron stars (nss) appear particularly suited for polarization measurements. radiation from the (cooling) surface of an ns is expected to exhibit a large intrinsic polarization degree due to the star strong magnetic field (≈1012-1015 g), which influences the plasma opacity in the outermost stellar layers. the polarization fraction and polarization angle as measured by an instrument, however, do not necessary coincide with the intrinsic ones derived from models of surface emission. this is due to the effects of quantum electrodynamics in the highly magnetized vacuum around the star (the vacuum polarization) coupled with the rotation of the stokes parameters in the plane perpendicular to the line of sight induced by the non-uniform magnetic field. here, we revisit the problem and present an efficient method for computing the observed polarization fraction and polarization angle in the case of radiation coming from the entire surface of an ns, accounting for both vacuum polarization and geometrical effects due to the extended emitting region. our approach is fairly general and is illustrated in the case of blackbody emission from an ns with either a dipolar or a (globally) twisted magnetic field. | polarization of neutron star surface emission: a systematic analysis |
we report on a spectroscopic analysis of the x-ray emission from igr j17062-6143 in the aftermath of its 2020 june intermediate duration type i x-ray burst. using the neutron star interior composition explorer, we started observing the source 3 hr after the burst was detected with the monitor of all-sky x-ray image gas slit camera, and monitored the source for the subsequent 12 days. we observed the tail end of the x-ray burst-cooling phase, and find that the x-ray flux is severely depressed relative to its historic value for a three-day period directly following the burst. we interpret this intensity dip as the inner accretion disk gradually restoring itself after being perturbed by the burst irradiation. superimposed on this trend we observed a 1.5 day interval during which the x-ray flux is sharply lower than the wider trend. this drop in flux could be isolated to the nonthermal components in the energy spectrum, suggesting that it may be caused by an evolving corona. additionally, we detected a 3.4 kev absorption line at 6.3σ significance in a single 472 s observation while the burst emission was still bright. we tentatively identify the line as a gravitationally redshifted absorption line from burning ashes on the stellar surface, possibly associated with 40ca or 44ti. | on the impact of an intermediate duration x-ray burst on the accretion environment in igr j17062-6143 |
the dissipation of intense crustal electric currents produces high joule heating rates in cooling neutron stars. here, it is shown that joule heating can counterbalance fast cooling, making it difficult to infer the presence of hyperons (which accelerate cooling) from measurements of the observed thermal luminosity lγ. models with and without hyperon cores match lγ of young magnetars (with poloidal-dipolar field bdip ≳ 1014 g at the polar surface and lγ ≳ 1034 erg s-1 at t ≲ 105 yr) as well as mature, moderately magnetized stars (with bdip ≲ 1014 g and 1031 erg s-1 ≲ lγ ≲ 1032 erg s-1 at t ≳ 105 yr). in magnetars, the crustal temperature is almost independent of hyperon direct urca cooling in the core, regardless of whether the latter is suppressed or not by hyperon superfluidity. the thermal luminosities of light magnetars without hyperons and heavy magnetars with hyperons have lγ in the same range and are almost indistinguishable. likewise, lγ data of neutron stars with bdip ≲ 1014 g but with strong internal fields are not suitable to extract information about the equation of state as long as hyperons are superfluid, with maximum amplitude of the energy gaps of the order ≈1 mev. | thermal luminosity degeneracy of magnetized neutron stars with and without hyperon cores |
we study the timing and spectral properties of the low-magnetic field, transient magnetar swift j1822.3-1606 as it approached quiescence. we coherently phase-connect the observations over a time-span of ∼500 d since the discovery of swift j1822.3-1606 following the swift-burst alert telescope (bat) trigger on 2011 july 14, and carried out a detailed pulse phase spectroscopy along the outburst decay. we follow the spectral evolution of different pulse phase intervals and find a phase and energy-variable spectral feature, which we interpret as proton cyclotron resonant scattering of soft photon from currents circulating in a strong (≳1014 g) small-scale component of the magnetic field near the neutron star surface, superimposed to the much weaker (∼3 × 1013 g) magnetic field. we discuss also the implications of the pulse-resolved spectral analysis for the emission regions on the surface of the cooling magnetar. | the outburst decay of the low magnetic field magnetar swift j1822.3-1606: phase-resolved analysis and evidence for a variable cyclotron feature |
we study the effects of strangeness on the quark sector of a hybrid-star equation of state. since the model we use to describe quarks is the same as the one we use to describe hadrons, we can also study the effects of strangeness on the chiral symmetry restoration and deconfinement phase transitions (first order or crossover). finally, we analyze the combined effects of hyperons and quarks on global properties of hybrid stars such as mass, radius, and cooling profiles. it is found that a large amount of strangeness in the core is related to the generation of twin-star solutions, which can have the same mass as the lower or zero strangeness counterpart, but with smaller radii. | role of strangeness in hybrid stars and possible observables |
thermonuclear x-ray bursts from accreting neutron stars power brief but strong irradiation of their surroundings, providing a unique way to study accretion physics. we analyze maxi/gas slit camera and swift/xrt spectra of a day-long flash observed from igr j17062-6143 in 2015. it is a rare case of recurring bursts at a low accretion luminosity of 0.15% eddington. spectra from maxi, chandra, and nustar observations taken between the 2015 burst and the previous one in 2012 are used to determine the accretion column. we find it to be consistent with the burst ignition column of 5 × 1010 g cm-2, which indicates that it is likely powered by burning in a deep helium layer. the burst flux is observed for over a day, and decays as a straight power law: f ∝ t -1.15. the burst and persistent spectra are well described by thermal emission from the neutron star, comptonization of this emission in a hot optically thin medium surrounding the star, and reflection off the photoionized accretion disk. at the burst peak, the comptonized component disappears, when the burst may dissipate the comptonizing gas, and it returns in the burst tail. the reflection signal suggests that the inner disk is truncated at ∼102 gravitational radii before the burst, but may move closer to the star during the burst. at the end of the burst, the flux drops below the burst cooling trend for 2 days, before returning to the pre-burst level. | x-ray reflection and an exceptionally long thermonuclear helium burst from igr j17062-6143 |
crustal cooling of accretion-heated neutron stars provides insight into the stellar interior of neutron stars. the neutron star x-ray transient, ks 1731-260, was in outburst for 12.5 years before returning to quiescence in 2001. we have monitored the cooling of this source since then through chandra and xmm-newton observations. here we present a 150 ks chandra observation of ks 1731-260 taken in 2015 august, about 14.5 years into quiescence and 6 years after the previous observation. we find that the neutron star surface temperature is consistent with the previous observation, suggesting that crustal cooling has likely stopped and the crust has reached thermal equilibrium with the core. using a theoretical crust thermal evolution code, we fit the observed cooling curves and constrain the core temperature (tc= 9.35 ± 0.25 × 107 k), composition (q {}{imp}={4.4}-0.5+2.2), and level of extra shallow heating required (q sh = 1.36 ± 0.18 mev/nucleon). we find that the presence of a low thermal conductivity layer, as expected from nuclear pasta, is not required to fit the cooling curve well, but cannot be excluded either. | the thermal state of ks 1731-260 after 14.5 years in quiescence |
the presence of superfluidity in neutron star interiors can affect the cooling of neutron stars in intricate ways, enhancing certain mechanisms and suppressing others. model calculations employing realistic nuclear potentials in bardeen-cooper-schrieffer theory generally suggest the development of a 3p2-3f2 pairing gap, and therefore the presence of superfluidity in dense neutron star matter. improved models that go beyond conventional mean-field calculations by including polarization effects suggest a suppression of the triplet gap. we have evaluated the pairing interaction by summing the "parquet" feynman diagrams, which include both ladder and ring diagrams systematically, plus a set of important nonparquet diagrams, making this the most comprehensive diagram-based approach presently available. our results suggest a radical suppression of the 3p2-3f2 triplet pairing gap and an enhancement of 3p0 pairing. | triplet pairing in neutron matter |
with our neutron star crust cooling code nscool, we track the thermal evolution of the neutron star in aql x-1 over the full accretion outburst history from 1996 until 2015. for the first time, we model many outbursts (23 outbursts were detected) collectively and in great detail. this allows us to investigate the influence of previous outbursts on the internal temperature evolution and to test different neutron star crust cooling scenarios. aql x-1 is an ideal test source for this purpose, because it shows frequent, short outbursts and thermally dominated quiescence spectra. the source goes into outburst roughly once a year for a few months. assuming that the quiescent swift/x-ray telescope observations of aql x-1 can be explained within the crust cooling scenario, we find three main conclusions. first, the data are well reproduced by our model if the envelope composition and shallow heating parameters are allowed to change between outbursts. this is not the case if both shallow heating parameters (strength and depth) are tied throughout all accretion episodes, supporting earlier results that the properties of the shallow heating mechanism are not constant between outbursts. secondly, from our models, shallow heating could not be connected to one specific spectral state during outburst. thirdly, and most importantly, we find that the neutron star in aql x-1 does not have enough time between outbursts to cool down to crust-core equilibrium and that heating during one outburst influences the cooling curves of the next. | a cooling neutron star crust after recurrent outbursts: modelling the accretion outburst history of aql x-1 |
we present the detection of 51 thermonuclear x-ray bursts observed from 4u 1636-536 by the neutron star interior composition explorer (nicer) over the course of a 3 yr monitoring campaign. we perform time-resolved spectroscopy for 40 of these bursts and show the existence of a strong soft excess in all the burst spectra. the excess emission can be characterized by the use of a scaling factor (the famethod) to the persistent emission of the source, which is attributed to the increased mass accretion rate onto the neutron star due to poynting-robertson drag. the soft excess emission can also be characterized by the use of a model taking into account the reflection of the burst emission off the accretion disk. we also present time-resolved spectral analysis of five x-ray bursts simultaneously observed by nicer and astrosat, which confirm the main results with even greater precision. finally, we present evidence for compton cooling using seven x-ray bursts observed contemporaneously with nustar, by means of a correlated decrease in the hard x-ray lightcurve of 4u 1636-536 as the bursts start. | burst-disk interaction in 4u 1636-536 as observed by nicer |
context. the possible presence of amorphous and heterogeneous phases in the inner crust of a neutron star is expected to reduce the electrical conductivity of the crust, potentially with significant consequences on the magneto-thermal evolution of the star. in cooling simulations, the disorder is quantified by an impurity parameter, which is often taken as a free parameter.aims: we aim to give a quantitative prediction of the impurity parameter as a function of the density in the crust, performing microscopic calculations including up-to-date microphysics of the crust.methods: a multicomponent approach was developed at a finite temperature using a compressible liquid-drop description of the ions with an improved energy functional based on recent microscopic nuclear models and optimized on extended thomas-fermi calculations. thermodynamic consistency was ensured by adding a rearrangement term, and deviations from the linear mixing rule were included in the liquid phase.results: the impurity parameter is consistently calculated at the crystallization temperature as determined in the one-component plasma approximation for the different functionals. our calculations show that at the crystallization temperature, the composition of the inner crust is dominated by nuclei with charge number around z ≈ 40, while the range of the z distribution varies from about 20 near the neutron drip to about 40 closer to the crust-core transition. this reflects on the behavior of the impurity parameter that monotonically increases with density reaching up to around 40 in the deeper regions of the inner crust.conclusions: our study shows that the contribution of impurities is non-negligible, thus potentially having an impact on the transport properties in the neutron-star crust. the obtained values of the impurity parameter represent a lower limit; larger values are expected in the presence of nonspherical geometries and/or fast cooling dynamics. the tables of the impurity parameter shown in fig. 6 are only available at the cds via anonymous ftp to http://cdsarc.u-strasbg.fr (ftp://130.79.128.5) or via http://cdsarc.u-strasbg.fr/viz-bin/cat/j/a+a/640/a77 | inner crust of a neutron star at the point of crystallization in a multicomponent approach |
the korea-ibs-daegu-skku energy density functional (kids-edf) models, constructed from the perturbative expansion of the energy density in nuclear matter, have been successfully and widely applied in describing the properties of finite nuclei and infinite nuclear matter. in the present work, we extend the applications of the kids-edf models to investigate the implications of the nucleon effective mass and nuclear symmetry energy for the properties of neutron stars (nss) and neutrino interaction with the ns constituent matter in the linear response approximation (lra). at fixed neutrino energy and momentum transfer, we analyze the total differential cross section of neutrinos, the neutrino mean free path (nmfp), and the ns mass-radius (m-r) relations. remarkable results are given by the kids0-m*87 and sly4 models, in which mn*/m ≲1 , and their nmfps are quite higher in comparison with those obtained from the kids0, kids-a, and kids-b models, which result in mn*/m ≳1 . for the kids0, kids-a, and kids-b models, we obtain λ ≲rns , indicating that these models could predict the slow ns cooling and neutrino trapping in nss. in contrast, the kids0-m*87 and sly4 models yield λ ≳rns and thus we expect faster ns cooling and a small possibility of neutrino trapping within nss. we also calculate the nmfp as a function of the neutrino energy and the nuclear matter density and find that the nmfp decreases as the density and neutrino energy increase, which is consistent with the result obtained in the brussels-montreal skyrme (bsk17 and bsk18) models at saturation density. | effect of nucleon effective mass and symmetry energy on the neutrino mean free path in a neutron star |
we derive analytic approximations for the neutrino luminosities and the heat capacities of neutron stars with isothermal nucleon cores as functions of the mass and radius of stars. the neutrino luminosities are approximated for the three basic neutrino emission mechanisms, and the heat capacities for the five basic combinations of the partial heat capacities. the approximations are valid for a wide class of equations of state of dense nucleon matter. the results significantly simplify the theoretical interpretation of observations of cooling neutron stars as well as of quasistationary thermal states of neutron stars in x-ray transients. for illustration, we present an analysis of the neutrino cooling functions of nine isolated neutron stars taking into account the effects of their magnetic fields and of the presence of light elements in their heat blanketing envelopes. these results allow one to investigate the superfluid properties of neutron star cores. | neutrino luminosities and heat capacities of neutron stars in analytic form |
strong magnetic fields in the magnetospheres of neutron stars (nss) (especially magnetars) and other astrophysical objects may release their energy in violent, intense episodes of magnetic reconnection. while reconnection has been studied extensively, the extreme field strength near nss introduces new effects: radiation cooling and electron-positron pair production. using massively parallel particle-in-cell simulations that self-consistently incorporate these new radiation and quantum-electrodynamic effects, we investigate relativistic magnetic reconnection in the strong-field regime. we show that reconnection in this regime can efficiently convert magnetic energy to x-ray and gamma-ray radiation and thus power bright, high-energy astrophysical flares. rapid radiative cooling causes strong plasma and magnetic field compression in compact plasmoids. in the most extreme cases, the field can approach the quantum limit, leading to copious pair production. | bright gamma-ray flares powered by magnetic reconnection in qed-strength magnetic fields |
the thermal evolution of hypernuclear compact stars is studied for stellar models constructed on the basis of covariant density functional theory in hartree and hartree-fock approximation. parametrizations of both types are consistent with the astrophysical mass constraints on compact stars and available hypernuclear data. we discuss the differences of these density functionals and highlight the effects they have on the composition and on the cooling of hypernuclear stars. it is shown that hypernuclear stars computed with density functional models that have a low symmetry energy slope, l, are fairly consistent with the cooling data of observed compact stars. the class of stellar models based on larger l values gives rise to the direct urca process at low densities, which leads to significantly faster cooling. we conjecture high-density pairing for protons and λ's in the p-wave channel and provide simple scaling arguments to obtain these gaps. as a consequence the most massive stellar models with masses 1.8 ≤ m/m⊙ ≤ 2 experience slower cooling by hyperonic durca processes which involve λ's and protons. | cooling of hypernuclear compact stars: hartree-fock models and high-density pairing |
we examine the gravitational wave frequencies from neutron stars during thermal evolution, adopting the relativistic cowling approximation. we particularly focus on the neutron star models, in which the direct urca (rapid cooling process) does not work, without the superfluidity and superconductivity. for such models, the cooling curve barely depends on the equation of state (eos) and mass of the neutron star, while we show that the gravitational wave frequencies strongly depend on the both properties. then, we find that the frequencies of the fundamental and the first pressure modes multiplied with the stellar mass are well expressed as a function of the stellar compactness almost independently of the eos. we also find that the frequency of the first gravity mode in the later phase of the thermal evolution is strongly correlated with the stellar compactness. in addition, we derive the empirical formula, estimating the threshold mass for the onset of the direct urca inside the neutron star as a function of the nuclear saturation parameter. this formula will give us a constraint on the neutron star properties, if it is observationally found that the direct urca occurs (or does not work) inside the neutron star. | gravitational wave asteroseismology on cooling neutron stars |
high luminosity accretion on to a strongly magnetized neutron star results in a radiation pressure dominated, magnetically confined accretion column. we investigate the dynamics of these columns using 2d radiative relativistic magnetohydrodynamic simulations, restricting consideration to modest accretion rates where the height of the column is low enough that cartesian geometry can be employed. the column structure is dynamically maintained through high-frequency oscillations of the accretion shock at ≃ 10-25 khz. these oscillations arise because it is necessary to redistribute the power released at the accretion shock through bulk vertical motions, both to balance the cooling and to provide vertical pressure support against gravity. sideways cooling always dominates the loss of internal energy. in addition to the vertical oscillations, photon bubbles form in our simulations and add additional spatial complexity to the column structure. they are not themselves responsible for the oscillations, and they do not appear to affect the oscillation period. however, they enhance the vertical transport of radiation and increase the oscillation amplitude in luminosity. the time-averaged column structure in our simulations resembles the trends in standard 1d stationary models, the main difference being that the time-averaged height of the shock front is lower because of the higher cooling efficiency of the 2d column shape. | radiative relativistic magnetohydrodynamic simulations of neutron star column accretion in cartesian geometry |
aims: in this work, we study the structure of neutron stars under the effect of a poloidal magnetic field and determine the limiting largest magnetic field strength that induces a deformation such that the ratio between the polar and equatorial radii does not exceed 2%. we consider that, under these conditions, the description of magnetic neutron stars in the spherical symmetry regime is still satisfactory.methods: we described different compositions of stars (nucleonic, hyperonic, and hybrid) using three state-of-the-art relativistic mean field models (nl3ωρ, mbf, and cmf, respectively) for the microscopic description of matter, all in agreement with standard experimental and observational data. the structure of stars was described by the general relativistic solution of both einstein's field equations assuming spherical symmetry and einstein-maxwell's field equations assuming an axi-symmetric deformation.results: we find a limiting magnetic moment on the order of 2 × 1031 am2, which corresponds to magnetic fields on the order of 1016 g at the surface and 1017 g at the center of the star, above which the deformation due to the magnetic field is above 2%, and therefore not negligible. we show that the intensity of the magnetic field developed in the star depends on the equation of state (eos), and, for a given baryonic mass and fixed magnetic moment, larger fields are attained with softer eos. we also show that the appearance of exotic degrees of freedom, such as hyperons or a quark core, is disfavored in the presence of a very strong magnetic field. as a consequence, a highly magnetized nucleonic star may suffer an internal conversion due to the decay of the magnetic field, which could be accompanied by a sudden cooling of the star or a gamma ray burst. | limiting magnetic field for minimal deformation of a magnetized neutron star |
the existence of superfluidity of the neutron component in the core of a neutron star, associated specifically with triplet p-wave pairing, is currently an open question that is central to interpretation of the observed cooling curves and other neutron-star observables. ab initio theoretical calculations aimed at resolving this issue face unique challenges in the relevant high-density domain, which reaches beyond the saturation density of symmetrical nuclear matter. these issues include uncertainties in the three-nucleon (3n) interaction and in the effects of strong short-range correlations—and more generally of in-medium modification of nucleonic self-energies and interactions. a survey of existing solutions of the gap equations in the triplet channel demonstrates that the net impact on the gap magnitude of 3n forces, coupled channels, and mass renormalization shows extreme variation dependent on specific theoretical inputs, in some cases even pointing to the absence of a triplet gap, thus motivating a detailed analysis of competing effects within a well-controlled model. in the present study, we track the effects of the 3n force and in-medium modifications in the representative case of the ^3p_2 channel, based on the argonne v_{18} two-nucleon (2n) interaction supplemented by 3n interactions of the urbana ix family. sensitivity of the results to the input interaction is clearly demonstrated. we point out consistency issues with respect to the simultaneous treatment of 3n forces and in-medium effects, which warrant further investigation. we consider this pilot study as the first step toward a systematic and comprehensive exploration of coupled-channel ^3p f_2 pairing using a broad range of 2n and 3n interactions from the current generation of refined semi-phenomenological models and models derived from chiral effective field theory. | three-nucleon forces and triplet pairing in neutron matter |
aql x-1 is a prolific transient neutron star low-mass x-ray binary that exhibits an accretion outburst approximately once every year. whether the thermal x-rays detected in intervening quiescent episodes are the result of cooling of the neutron star or due to continued low-level accretion remains unclear. in this work, we use swift data obtained after the long and bright 2011 and 2013 outbursts, as well as the short and faint 2015 outburst, to investigate the hypothesis that cooling of the accretion-heated neutron star crust dominates the quiescent thermal emission in aql x-1. we demonstrate that the x-ray light curves and measured neutron star surface temperatures are consistent with the expectations of the crust cooling paradigm. by using a thermal evolution code, we find that ≃1.2-3.2 mev nucleon-1 of shallow heat release describes the observational data well, depending on the assumed mass-accretion rate and temperature of the stellar core. we find no evidence for varying strengths of this shallow heating after different outbursts, but this could be due to limitations of the data. we argue that monitoring aql x-1 for up to ≃1 yr after future outbursts can be a powerful tool to break model degeneracies and solve open questions about the magnitude, depth, and origin of shallow heating in neutron star crusts. | constraining the properties of neutron star crusts with the transient low-mass x-ray binary aql x-1 |
type-i x-ray burst oscillations are powered by thermonuclear energy released on the neutron star (ns) surface in low-mass x-ray binaries (lmxbs), where the burst oscillation frequencies are close to the ns spin rates. in this work, we report the detection of oscillation at 584.65 hz during the cooling tail of type-i x-ray bursts observed from the accreting ns lmxb 4u 1730-22 on 2022 march 20, by the neutron star interior composition explorer telescope. the oscillation signal showed a strong leahy power, p m ~ 54.04, around 584.65 hz, which has single-trial and multiple-trial confidence levels of 7.05σ and 4.73σ, respectively. the folded pulse profile of the oscillation in the 0.2-10 kev band showed a sinusoidal shape with the fractional rms amplitude of (8.0 ± 1.1)%. we found the oscillation frequency showed insignificant upward drifting, i.e., less than 0.3 hz, during the cooling tail, similar to the behavior appearing in accreting millisecond x-ray pulsars (amxp), and indicate the source could be an amxp spinning at 1.71 ms. | discovery of a 584.65 hz burst oscillation in the low-mass x-ray binary 4u 1730-22 |
binary neutron star mergers are believed to eject significant masses with a diverse range of velocities. once these ejected materials begin to be decelerated by a homogeneous medium, relativistic electrons are mainly cooled down by synchrotron radiation, generating a multiwavelength long-lived afterglow. analytic and numerical methods illustrate that the outermost matter, the merger shock-breakout material, can be parametrized by power-law velocity distributions \propto {≤ft({β }{{c}}{{γ }}\right)}-{α s}. considering that the shock-breakout material is moving on-axis toward the observer and the relativistic jet off-axis, we compute the light curves during the relativistic and the lateral expansion phase. as a particular case, we successfully describe the x-ray, optical, and radio light curves alongside the spectral energy distribution from the recently discovered gravitational-wave transient gw170817, when the merger shock-breakout material moves with mildly relativistic velocities and achieves the near-newtonian phase and the jet moves with relativistic velocities. future electromagnetic counterpart observations of this binary system could be able to evaluate different properties of these light curves. | light curves of a shock-breakout material and a relativistic off-axis jet from a binary neutron star system |
using a theoretical model, we track the thermal evolution of a cooling neutron star crust after an accretion-induced heating period with the goal of constraining the crustal parameters. we present for the first time a crust cooling model - nscool - that takes into account detailed variability during the full outburst based on the observed light curve. we apply our model to ks 1731-260. the source was in outburst for ∼12 yr during which it was observed to undergo variations on both long (years) and short (days-weeks) time-scales. our results show that ks 1731-260 does not reach a steady state profile during the outburst due to fluctuations in the derived accretion rate. additionally, long time-scale outburst variability mildly affects the complete crust cooling phase, while variations in the final months of the outburst strongly influence the first ∼40 d of the calculated cooling curve. we discuss the consequences for estimates of the neutron star crust parameters, and argue that detailed modelling of the final phase of the outburst is key to constraining the origin of the shallow heat source. | neutron star crust cooling in ks 1731-260: the influence of accretion outburst variability on the crustal temperature evolution |
context. it is thought that ultraluminous x-ray sources (ulxs) are mainly powered by super-eddington accreting neutron stars or black holes as shown by the recent discovery of x-ray pulsations and relativistic winds.aims: this work presents a follow-up study of the spectral evolution over two decades of the pulsing ulx ngc 1313 x-2 in order to understand the structure of the accretion disc. the primary objective is to determine the shape and nature of the dominant spectral components by investigating their variability with the changes in the source luminosity.methods: we performed a spectral analysis over the canonical 0.3-10.0 kev energy band of all the high signal-to-noise xmm-newton observations (96% of the available data), and we tested a number of different spectral models, which should approximate super-eddington accretion discs. the baseline model consists of two thermal blackbody components with different temperatures plus an exponential cutoff powerlaw.results: the baseline model provides a good description of the x-ray spectra. in particular, the hotter and brighter (lx ∼ 6-9 × 1039 erg s−1) thermal component describes the emission from the super-eddington inner disc and the cutoff powerlaw describes the contribution from the accretion column of the neutron star. instead, the cooler component describes the emission from the outer region of the disc close to the spherisation radius and the wind. the luminosity-temperature relation for the cool component follows a negative trend, which is not consistent with l ∝ t4, as is expected from a sub-eddington thin disc of shakura-sunayev. this is not consistent with l ∝ t2 either, as is expected for an advection-dominated disc. however, this would rather agree with a wind-dominated x-ray emitting region. instead, the (lx, tdisk) relation for the hotter component is somewhere in between the first two theoretical scenarios.conclusions: our findings agree with the super-eddington scenario and provide further detail on the disc structure. the source spectral evolution is qualitatively similar to that seen in ngc 1313 x-1 and holmberg ix x-1, indicating a common structure and evolution among archetypal ulxs. | broadband x-ray spectral variability of the pulsing ulx ngc 1313 x-2 |
the x-ray binary 4u 1954+31 has been classified as a low-mass x-ray binary containing an m giant and a neutron star (ns). it has also been included in the rare class of x-ray symbiotic binaries. the gaia parallax, infrared colors, spectral type, abundances, and orbital properties of the m star demonstrate that the cool star in this system is not a low-mass giant but a high-mass m supergiant. thus, 4u 1954+31 is a high-mass x-ray binary (hmxb) containing a late-type supergiant. it is the only known binary system of this type. the mass of the m i is ${9}_{-2}^{+6}$ m⊙ giving an age of this system in the range 12-50 myr with the ns no more than 43 myr old. the spin period of the ns is one of the longest known, 5 hr. the existence of m i plus ns binary systems is in accord with stellar evolution theory, with this system a more evolved member of the hmxb population. | the m supergiant high-mass x-ray binary 4u 1954+31 |
the radio pulsar gleam-x j162759.5-523504.3 has an extremely long spin period ($p = 1091.17\, \mbox{s}$), and yet seemingly continues to spin-down rapidly ($\dot{p} < 1.2 \times 10^{-9}\, \mbox{ss}^{-1}$). the magnetic field strength that is implied, if the source is a neutron star undergoing magnetic dipole braking, could exceed $10^{16}\, \mbox{g}$. this object may therefore be the most magnetized neutron star observed to date. in this paper, a critical analysis of a magnetar interpretation for the source is provided. (i) a minimum polar magnetic field strength of $b \sim 5 \times 10^{15}\, \mbox{g}$ appears to be necessary for the star to activate as a radio pulsar, based on conventional 'death valley' assumptions. (ii) back-extrapolation from magnetic braking and hall-plastic-ohm decay suggests that a large angularize momentum reservoir was available at birth to support intense field amplification. (iii) the observational absence of x-rays constrains the star's field strength and age, as the competition between heating from field decay and urca cooling implies a surface luminosity as a function of time. if the object is an isolated, young ($\sim 10\, \mbox{kyr}$) magnetar with a present-day field strength of $b \gtrsim 10^{16}\, \mbox{g}$, the upper limit ($\approx 10^{30}\, \mbox{erg s}^{-1}$) set on its thermal luminosity suggests it is cooling via a direct urca mechanism. | evolutionary implications of a magnetar interpretation for gleam-x j162759.5-523504.3 |
the study of transiently accreting neutron stars provides a powerful means to elucidate the properties of neutron star crusts. we present extensive numerical simulations of the evolution of the neutron star in the transient low-mass x-ray binary maxi j0556-332. we model nearly 20 observations obtained during the quiescence phases after four different outbursts of the source in the past decade, considering the heating of the star during accretion by the deep crustal heating mechanism complemented by some shallow heating source. we show that cooling data are consistent with a single source of shallow heating acting during the last three outbursts, while a very different and powerful energy source is required to explain the extremely high effective temperature of the neutron star, ~350 ev, when it exited the first observed outburst. we propose that a gigantic thermonuclear explosion, a "hyperburst" from unstable burning of neutron-rich isotopes of oxygen or neon, occurred a few weeks before the end of the first outburst, releasing ~1044 ergs at densities of the order of 1011 g cm-3. this would be the first observation of a hyperburst, and these would be extremely rare events, as the buildup of the exploding layer requires about a millennium of accretion history. despite its large energy output, the hyperburst did not produce, due to its depth, any noticeable increase in luminosity during the accretion phase and is only identifiable by its imprint on the later cooling of the neutron star. | a "hyperburst" in the maxi j0556-332 neutron star: evidence for a new type of thermonuclear explosion |
in this work i briefly review some of the effects of hyperons on the properties of neutron and proto-neutron stars. in particular, i revise the problem of the strong softening of the eos, and the consequent reduction of the maximum mass, induced by the presence of hyperons, a puzzle which has become more intringuing and difficult to solve because of the recent measurements of the unusually high masses of the millisecond pulsars psr j1903+0327 (1.667 ± 0.021m⊙), psr j1614-2230 (1.97 ± 0.04m⊙), and psr j0348+0432 (2.01 ± 0.04m⊙). some of the solutions proposed to tackle this problem are discussed. finally, i re-examine also the role of hyperons on the cooling properties of newly born neutron stars and on the so-called r-mode instability. | hyperons in neutron stars |
young pulsars are thought to be highly magnetized neutron stars (nss). the crustal magnetic field of a ns usually decays at different timescales in the forms of hall drift and ohmic dissipation. the magnetization parameter ωbτ is defined as the ratio of the ohmic timescale τohm to the hall drift timescale τhall. during the first several million years, the inner temperature of the newly born neutron star cools from t=109k to t=1.0×108k, and the crustal conductivity increases by three orders of magnitude. in this work, we adopt a unified equations of state for cold non-accreting neutron stars with the hartree–fock–bogoliubov method, developed by pearson et al. (2018), and choose two fiducial dipole magnetic fields of b=1.0×1013g and b=1.0×1014g, four different temperatures, t, and two different impurity concentration parameters, q, and then calculate the conductivity of the inner crust of nss and give a general expression of magnetization parameter for young pulsars: ωbτ≃(1‑50)b0/(1013g) by using numerical simulations. it was found when b≤1015 g, due to the quantum effects, the conductivity increases slightly with the increase in the magnetic field, the enhanced magnetic field has a small effect on the matter in the low-density regions of the crust, and almost has no influence the matter in the high-density regions. then, we apply the general expression of the magnetization parameter to the high braking-index pulsar psr j1640-4631. by combining the observed arrival time parameters of psr j1640-4631 with the magnetic induction equation, we estimated the initial rotation period p0, the initial dipole magnetic field b0, the ohm dissipation timescale τohm and hall drift timescale τhall. we model the magnetic field evolution and the braking-index evolution of the pulsar and compare the results with its observations. it is expected that the results of this paper can be applied to more young pulsars. | estimation of electrical conductivity and magnetization parameter of neutron star crusts and applied to the high-braking-index pulsar psr j1640-4631 |
upgrades to improve the sensitivity of gravitational wave detectors enable more frequent detections and more precise source parameter estimation. unlike other advanced interferometric detectors such as advanced ligo and advanced virgo, kagra requires a different approach for the upgrade since it is the only detector which employs cryogenic cooling of the test masses. in this paper, we describe possible kagra upgrades with technologies focusing on different detector bands and compare the impacts on the detection of compact binary coalescences. we show that either fivefold improvement in the 100 m⊙- 100 m⊙ binary black hole range, a factor of 1.3 improvement in the binary neutron star range, or a factor of 1.7 improvement in the sky localization of binary neutron stars is quite feasible with upgrades that do not require changes in the existing cryogenic or vacuum infrastructure. we also show that twofold broadband sensitivity improvement is possible by applying multiple upgrades to the detector. | prospects for improving the sensitivity of the cryogenic gravitational wave detector kagra |
monitoring the cooling of neutron-star crusts heated during accretion outbursts allows us to infer the physics of the dense matter present in the crust. we examine the crust cooling evolution of the low-mass x-ray binary mxb 1659-29 up to ∼505 days after the end of its 2015 outburst (hereafter outburst ii) and compare it with what we observed after its previous 1999 outburst (hereafter outburst i) using data obtained from the swift, xmm-newton, and chandra observatories. the observed effective surface temperature of the neutron star in mxb 1659 - 29 dropped from ∼92 ev to ∼56 ev from ∼12 days to ∼505 days after the end of outburst ii. the most recently performed observation after outburst ii suggests that the crust is close to returning to thermal equilibrium with the core. we model the crust heating and cooling for both its outbursts collectively to understand the effect of parameters that may change for every outburst (e.g. the average accretion rate, the length of outburst, the envelope composition of the neutron star at the end of the outburst) and those which can be assumed to be the same during these two outbursts (e.g. the neutron star mass, its radius). our modelling indicates that all parameters were consistent between the two outbursts with no need for any significant changes. in particular, the strength and the depth of the shallow heating mechanism at work (in the crust) were inferred to be consistent during both outbursts, contrary to what has been found when modelling the cooling curves after multiple outburst of another source, maxi j0556-332. this difference in source behaviour is not understood. we discuss our results in the context of our current understanding of cooling of accretion-heated neutron-star crusts, and in particular with respect to the unexplained shallow heating mechanism. a movie is available at https://www.aanda.org | consistent accretion-induced heating of the neutron-star crust in mxb 1659-29 during two different outbursts |
we report results obtained from the study of 12 thermonuclear x-ray bursts in six astrosat observations of a neutron star x-ray binary and well-known x-ray burster, 4u 1636 - 536. burst oscillations (bos) at ~ 581 hz are observed with 4-5σ confidence in three of these x-ray bursts. the rising phase bos show a decreasing trend of the fractional rms amplitude at 3σ confidence, by far the strongest evidence of thermonuclear flame spreading observed with astrosat. during the initial 0.25 s of the rise a very high value ($34.0\pm 6.7{{{\ \rm per\ cent}}}$) is observed. the concave shape of the fractional amplitude profile provides a strong evidence of latitude-dependent flame speeds, possibly due to the effects of the coriolis force. we observe decay phase oscillations with amplitudes comparable to that observed during the rising phase, plausibly due to the combined effect of both surface modes, as well as the cooling wake. the doppler shifts due to the rapid rotation of the neutron star might cause hard pulses to precede the soft pulses, resulting in a soft lag. the distance to the source estimated using the photospheric radius expansion bursts is consistent with the known value of ~6 kpc. | thermonuclear x-ray bursts from 4u 1636 - 536 observed with astrosat |
after 15 yr, in late 2018, the magnetar xte j1810-197 underwent a second recorded x-ray outburst event and reactivated as a radio pulsar. we initiated an x-ray monitoring campaign to follow the timing and spectral evolution of the magnetar as its flux decays using swift, xmm-newton, nustar, and nicer observations. during the year-long campaign, the magnetar reproduced similar behaviour to that found for the first outburst, with a factor of 2 change in its spin-down rate from ~7.2 × 10-12 to ~1.5 × 10-11 s s-1 after two months. unique to this outburst, we confirm the peculiar energy-dependent phase shift of the pulse profile. following the initial outburst, the spectrum of xte j1810-197 is well modelled by multiple blackbody components corresponding to a pair of non-concentric, hot thermal caps surrounded by a cooler one, superposed to the colder star surface. we model the energy-dependent pulse profile to constrain the viewing and surface emission geometry and find that the overall geometry of xte j1810-197 has likely evolved relative to that found for the 2003 event. | the x-ray evolution and geometry of the 2018 outburst of xte j1810-197 |
the role of a strong magnetic field on the neutron-drip transition in the crust of a magnetar is studied. the composition of the crust and the neutron-drip threshold are determined numerically for different magnetic field strengths using the experimental atomic mass measurements from the 2012 atomic mass evaluation complemented with theoretical masses calculated from the brussels-montreal hartree-fock-bogoliubov nuclear mass model hfb-24. the equilibrium nucleus at the neutron-drip point is found to be independent of the magnetic field strength. as demonstrated analytically, the neutron-drip density and pressure increase almost linearly with the magnetic field strength in the strongly quantizing regime for which electrons lie in the lowest landau level. for weaker magnetic fields, the neutron-drip density exhibits typical quantum oscillations. in this case, the neutron-drip density can be either increased by about 14 % or decreased by 25 % depending on the magnetic field strength. these variations are shown to be almost universal, independently of the nuclear mass model employed. these results may have important implications for the physical interpretation of timing irregularities and quasiperiodic oscillations detected in soft gamma-ray repeaters and anomalous x-ray pulsars, as well as for the cooling of strongly magnetized neutron stars. | role of landau quantization on the neutron-drip transition in magnetar crusts |
experiments aimed at detecting ultralight dark matter typically rely on resonant effects, which are sensitive to the dark matter mass that matches the resonance frequency. in this study, we investigate the nucleon couplings of ultralight axion dark matter using a magnetometer operating in a nuclear magnetic resonance (nmr) mode. our approach involves the use of a $^{21}$ne spin-based sensor, which features the lowest nuclear magnetic moment among noble-gas spins. this configuration allows us to achieve an ultrahigh sensitivity of 0.73 ft/hz$^{1/2}$ at around 5 hz, corresponding to energy resolution of approximately 1.5$\times 10^{-23}\,\rm{ev/hz^{1/2}}$. our analysis reveals that under certain conditions it is beneficial to scan the frequency with steps significantly larger than the resonance width. the analytical results are in agreement with experimental data and the scan strategy is potentially applicable to other resonant searches. further, our study establishes stringent constraints on axion-like particles (alp) in the 4.5--15.5 hz compton-frequency range coupling to neutrons and protons, improving on prior work by several-fold. within a band around 4.6--6.6 hz and around 7.5 hz, our laboratory findings surpass astrophysical limits derived from neutron-star cooling. hence, we demonstrate an accelerated resonance search for ultralight dark matter, achieving an approximately 30-fold increase in scanning step while maintaining competitive sensitivity. | constraining ultralight dark matter through an accelerated resonant search |
nscool is a 1d (i.e., spherically symmetric) neutron star cooling code written in fortran 77. the package also contains a series of eoss (equation of state) to build stars, a series of pre-built stars, and a tov (tolman- oppenheimer-volkoff) integrator to build stars from an eos. it can also handle "strange stars" that have a huge density discontinuity between the quark matter and the covering thin baryonic crust. nscool solves the heat transport and energy balance equations in whole gr, resulting in a time sequence of temperature profiles (and, in particular, a teff - age curve). several heating processes are included, and more can easily be incorporated. in particular it can evolve a star undergoing accretion with the resulting deep crustal heating, under a steady or time-variable accretion rate. nscool is robust, very fast, and highly modular, making it easy to add new subroutines for new processes. | nscool: neutron star cooling code |
we study the thermal evolution of isolated neutron stars in scalar-tensor theories for the first time. whether rapid cooling due to the direct urca process occurs or not is an interesting question from the viewpoint of the temperature observation of isolated neutron stars. moreover, investigation of the cooling effect of nucleon superfluidity also has large uncertainties, though it is important in modern cooling theory. the cooling effect is typically influenced by the proton fraction and the central density. if a fifth force is mediated due to a modification of gravity, the relation between the central density and mass of neutron stars differs from that in general relativity, and the cooling curve is also naively expected to vary. we find that an unscreened fifth force near the surface of neutron stars changes the mass-central density relation, and the direct urca process can be triggered even for neutron stars with smaller mass. we also present cooling curves including nucleon superfluidity under the scalar-tensor theory. these results show that it might be useful to test gravitational theories with cooling observations of neutron stars. | neutron star cooling in modified gravity theories |
context. proto-neutron stars are born hot, with temperatures exceeding a few times 1010 k. in these conditions, the crust of the proto-neutron star is expected to be made of a coulomb liquid and composed of an ensemble of different nuclear species.aims: in this work, we perform a study of the beta-equilibrated proto-neutron-star crust in the liquid phase in a self-consistent multi-component approach. this also allows us to perform a consistent calculation of the impurity parameter, which is often taken as a free parameter in cooling simulations.methods: to this aim, we developed a self-consistent multi-component approach at finite temperature using a compressible liquid-drop description of the ions, with surface parameters adjusted to reproduce experimental masses. the treatment of the ion centre-of-mass motion was included through a translational free-energy term accounting for in-medium effects. the results of the self-consistent calculations of the multi-component plasma are systematically compared with those performed in a perturbative treatment as well as in the one-component plasma approximation.results: we show that the inclusion of non-linear mixing terms arising from the ion centre-of-mass motion leads to a breakdown of the ensemble equivalence between the one-component and multi-component approach. our findings also illustrate that the abundance of light nuclei becomes important and eventually dominates the whole distribution at higher density and temperature in the crust. this is reflected in the impurity parameter, which, in turn, may have a potential impact on neutron-star cooling. for practical application to astrophysical simulations, we also provide a fitting formula for the impurity parameter in the proto-neutron-star inner crust.conclusions: our results obtained within a self-consistent multi-component approach show important differences in the prediction of the proto-neutron-star composition with respect to those obtained with a one-component approximation or a perturbative multi-component approximation, particularly in the deeper region of the crust. this highlights the importance of a full, self-consistent multi-component plasma calculation for reliable predictions of the proto-neutron-star crust composition. the tables of the impurity parameter shown in fig. 15 are available at the cds via anonymous ftp to cdsarc.cds.unistra.fr (ftp://130.79.128.5) or via https://cdsarc.cds.unistra.fr/viz-bin/cat/j/a+a/677/a174 | the proto-neutron star inner crust in a multi-component plasma approach |
we analyse the origin of the magnetic field decay in normal radio pulsars found by us in a recent study. this decay has a typical time scale ∼ 4 × 105 yr and operates in the range ∼ 105 - few × 105 yr. we demonstrate that this field evolution may be either due to the ohmic decay related to the scattering from phonons, or due to the hall cascade which reaches the hall attractor. according to our analysis, the first possibility seems to be more reliable. so, we attribute the discovered field decay mainly to the ohmic decay by phonons, which is saturated at the age of a few× 105 yr when a neutron star cools down to the critical temperature below which the phonon scattering does not contribute much to the resistivity of the crust. some role of the hall effect and attractor is not excluded, and will be analysed in our further studies. | magnetic field decay in normal radio pulsars |
motivated by the various theoretical studies regarding the efficient capturing of dark matter by neutron stars, we explore the possible indirect effects of captured dark matter on the cooling mechanism of a neutron star. the equation of states for different configurations of dark matter admixed star at finite temperature is obtained using the relativistic mean-field formalism with the iopb-i parameter set. we show that the variation in the dark matter momentum vastly modifies the neutrino emissivity through specific neutrino generating processes of the star. the specific heat and the thermal conductivity of a dark matter admixed star have also been investigated to explore the propagation of cooling waves in the interior of the star. the dependence of theoretical surface temperature cooling curves on the equation of state and chemical composition of the stellar matter has also been discussed along with the observational data of thermal radiation from various sources. we observed that the dark matter admixed canonical stars with $k_{f}^{\rm dm} \gt 0.04$ comply with the fast cooling scenario. further, the metric for internal thermal relaxation epoch has also been calculated with different dark matter momentum and we deduced that increment of dark matter segment amplify the cooling and internal relaxation rates of the star. | thermal relaxation of dark matter admixed neutron star |
a review of fission product release theory is presented in support of fuel-failure monitoring analysis for the characterization and location of defective fuel. this work is used to describe: (i) the development of the steady-state visual_detect code for coolant activity analysis to characterize failures in the core and the amount of tramp uranium; (ii) a generalization of this model in the star code for prediction of the time-dependent release of iodine and noble gas fission products to the coolant during reactor start-up, steady-state, shutdown, and bundle-shifting manoeuvres; (iii) an extension of the model to account for the release of fission products that are delayed-neutron precursors for assessment of fuel-failure location; and (iv) a simplification of the steady-state model to assess the methodology proposed by wano for a fuel reliability indicator for water-cooled reactors. | fission product release modelling for application of fuel-failure monitoring and detection - an overview |
using a model for the equation of state and composition of dense matter and the magnitude of singlet proton superconductivity and triplet neutron superfluidity, we perform the first simultaneous fit of neutron star masses and radii determined from observations of quiescent low-mass x-ray binaries and luminosities and ages determined from observations of isolated neutron stars. we find that the vela pulsar strongly determines the values inferred for the superfluid/superconducting gaps and the neutron star radius. we find, regardless of whether or not the vela pulsar is included in the analysis, that the threshold density for the direct urca process lies between the central density of 1.7 and 2 solar mass neutron stars. we also find that two solar mass stars are unlikely to cool principally by the direct urca process because of the suppression by neutron triplet superfluidity. | simultaneous fitting of neutron star structure and cooling data |
the inner crust of a mature neutron star is composed of an elastic lattice of neutron-rich nuclei penetrated by free neutrons. these neutrons can flow relative to the crust once the star cools below the superfluid transition temperature. in order to model the dynamics of this system, which is relevant for a range of problems from pulsar glitches to magnetar seismology and continuous gravitational-wave emission from rotating deformed neutron stars, we need to understand general relativistic lagrangian perturbation theory for elastic matter coupled to a superfluid component. this paper develops the relevant formalism to the level required for astrophysical applications. | the dynamics of neutron star crusts: lagrangian perturbation theory for a relativistic superfluid-elastic system |
supercritical accretion onto compact objects may drive massive winds that are nearly spherical, optically thick, and eddington limited. blackbody emission from the photosphere is the direct observational signature of the wind. here we investigate whether or not it can explain the soft emission component seen in the energy spectra of ultraluminous x-ray sources (ulxs). based on high-quality xmm-newton spectra of 15 ulxs, we find that the soft component can be modeled as blackbody emission with a nearly constant luminosity, and the five known pulsating ulxs (pulxs) in the sample display a blackbody luminosity among the lowest. these are consistent with the scenario that the soft emission originates from the photosphere of the optically thick wind. however, the derived blackbody luminosity for pulxs is significantly above the eddington limit for neutron stars. a possible explanation is that a considerable fraction of the optically thick wind roots in the inner accretion flow, where the radiative flux could exceed the eddington limit due to a reduced scattering cross-section or enhanced radiation transfer with magnetic buoyancy. based on a wind model, the inferred mass accretion rate in these standard ulxs overlaps but is on average lower than that in luminous and very soft x-ray sources, which are also candidates with supercritical accretion. alternatively, it cannot be ruled out that the soft emission component is a result of the hard component, e.g., via down-scattering in a cool medium, as a weak correlation may exist between them. | linking soft excess in ultraluminous x-ray sources with optically thick wind driven by supercritical accretion |
1rxs j180408.9-342058 is a low-mass x-ray binary hosting a neutron star, which shows x-ray activity at very different mass-accretion regimes, from very faint to almost the eddington luminosity. in this work, we present a comprehensive x-ray study of this source using data from the neil gehrels swift observatory, nustar, and integral/jem-x. in order to follow the spectral evolution, we analysed the 2015 outburst using swift data and three nustar observations. besides the canonical hard and soft spectral states, we identified the rarely observed intermediate state. this was witnessed by the appearance of the accretion disc emission in the spectrum (at ktdisc ∼0.7 kev) and the simultaneous cooling of the hot corona. in addition, we also unveiled a hard tail above 30 kev in this state. in the hard state, a thermal comptonization model with two seed photons populations (kts,1 ∼ 1.5 kev and kts,2 ∼ 0.4 kev, respectively) and a hot comptonizing plasma, represents the physically best motivated scenario to describe the data. we also estimated a reflection fraction below 20 per cent in all states, while no constraints on the inclination and only lower limits on the inner disc radius could be inferred. finally, we studied a number of type-i x-ray bursts displayed from the source, one of them at the eddington limit (observed with jem-x). their characteristics, combined with the clocked behaviour observed during the intermediate state, point out h/he composition for the accreted material, which makes unlikely the helium dwarf nature for the companion. | new insights on the puzzling lmxb 1rxs j180408.9-342058: the intermediate state, the clocked type-i x-ray bursts, and much more |
timing observations of rapidly rotating neutron stars revealed a great number of glitches, observed from both canonical radio pulsars and magnetars. among them, 76 glitches have shown exponential relaxation(s) with characteristic decay times ranging from several days to a few months, followed by a more gradual recovery. glitches displaying exponential relaxation with single or multiple decay time constants are analysed in terms of a model based on the interaction of the vortex lines with the toroidal arrangement of flux tubes in the outer core of the neutron star. model results agree with the observed time-scales in general. thus, the glitch phenomenon can be used to deduce valuable information about neutron star structure, in particular on the interior magnetic field configuration which is unaccessible from surface observations. one immediate conclusion is that the magnetar glitch data are best explained with a much cooler core and therefore require that direct urca-type fast-cooling mechanisms should be effective for magnetars. | post-glitch exponential relaxation of radio pulsars and magnetars in terms of vortex creep across flux tubes |
the transient neutron star (ns) low-mass x-ray binary maxi j0556-332 provides a rare opportunity to study ns crust heating and subsequent cooling for multiple outbursts of the same source. we examine maxi, swift, chandra, and xmm-newton data of maxi j0556-332 obtained during and after three accretion outbursts of different durations and brightnesses. we report on new data obtained after outburst iii. the source has been tracked up to ∼1800 days after the end of outburst i. outburst i heated the crust strongly, but no significant reheating was observed during outburst ii. cooling from ∼333 ev to ∼146 ev was observed during the first ∼1200 days. outburst iii reheated the crust up to ∼167 ev, after which the crust cooled again to ∼131 ev in ∼350 days. we model the thermal evolution of the crust and find that this source required a different strength and depth of shallow heating during each of the three outbursts. the shallow heating released during outburst i was ∼17 mev nucleon-1 and outburst iii required ∼0.3 mev nucleon-1. these cooling observations could not be explained without shallow heating. the shallow heating for outburst ii was not well constrained and could vary from ∼0 to 2.2 mev nucleon-1, i.e., this outburst could in principle be explained without invoking shallow heating. we discuss the nature of the shallow heating and why it may occur at different strengths and depths during different outbursts. | different accretion heating of the neutron star crust during multiple outbursts in maxi j0556-332 |
context. measuring the abundances of neutron-capture elements in galactic disk stars is an important part of understanding key stellar and galactic processes. in the optical wavelength regime a number of different neutron-capture elements have been measured; however, only the s-process-dominated element cerium has been accurately measured for a large sample of disk stars from the infrared h band. the more r-process dominated element ytterbium has only been measured in a small subset of stars so far.aims: in this study we aim to measure the ytterbium (yb) abundance of local disk giants using the yb ii line at λair = 16 498 å. we also compare the resulting abundance trend with cerium and europium abundances for the same stars to analyse the s- and r-process contributions.methods: we analyse 30 k giants with high-resolution h band spectra using spectral synthesis. the very same stars have already been analysed using high-resolution optical spectra via the same method, but it was not possible to determine the abundance of yb from those spectra due to blending issues for stars with [fe/h] > −1. in the present analysis, we utilise the stellar parameters determined from the optical analysis.results: we determined the yb abundances with an estimated uncertainty for [yb/fe] of 0.1 dex. by comparison, we found that the [yb/fe] trend closely follows the [eu/fe] trend and has clear s-process enrichment in identified s-rich stars. this comparison confirms both that the validity of the yb abundances is ensured and that the theoretical prediction that the s-/r-process contribution to the origin of yb of roughly 40/60 is supported.conclusions: these results show that, with a careful and detailed analysis of infrared spectra, reliable yb abundances can be derived for a wider sample of cooler giants in the range −1.1 < [fe/h] < 0.3. this is promising for further studies of the production of yb and for the r-process channel, key for galactochemical evolution, in the infrared. tables 1 and 3 are also available at the cds via anonymous ftp to ftp://cdsarc.u-strasbg.fr (130.79.128.5) or via http://cdsarc.u-strasbg.fr/viz-bin/cat/j/a+a/665/a135 | chemical evolution of ytterbium in the galactic disk |
pulsar-like compact stars provide us a unique laboratory to explore properties of dense matter at supra-nuclear densities. one of the models for pulsar-like stars is that they are totally composed of "strangeons", and in this paper, we studied the pulsar glitches in a strangeon star model. strangeon stars would be solidified during cooling, and the solid stars would be natural to have glitches as the result of starquakes. based on the starquake model established before, we proposed that when the starquake occurs, the inner motion of the star which changes the moment of inertia and has impact on the glitch sizes, is divided into plastic flow and elastic motion. the plastic flow which is induced in the fractured part of the outer layer, would move tangentially to redistribute the matter of the star and would be hard to recover. the elastic motion, on the other hand, changes its shape and would recover significantly. under this scenario, we could understand the behaviors of glitches without significant energy releasing, including the crab and the vela pulsars, in an uniform model. we derive the recovery coefficient as a function of glitch size, as well as the time interval between two successive glitches as the function of the released stress. our results show consistency with observational data under reasonable ranges of parameters. the implications on the oblateness of the crab and the vela pulsars are discussed. | pulsar glitches in a strangeon star model |
the be/x-ray transients v0332+53 and 4u 0115+63 exhibited giant, type-ii outbursts in 2015. here we present swift/xrt follow-up observations at the end of those outbursts. surprisingly, the sources did not decay back to their known quiescent levels but stalled at a (slowly decaying) meta-stable state with luminosities a factor ∼10 above that observed in quiescence. the spectra in these states are considerably softer than the outburst spectra and appear to soften in time when the luminosity decreases. the physical mechanism behind these meta-stable states is unclear and they could be due to low-level accretion (either directly on to the neutron stars or on to their magnetospheres) or due to cooling of the accretion-heated neutron star crusts. based on the spectra, the slowly decreasing luminosities, and the spectral softening, we favour the crust cooling hypothesis but we cannot exclude the accretion scenarios. on top of this meta-stable state, weak accretion events were observed that occurred at periastron passage and may thus be related to regular type-i outbursts. | meta-stable low-level accretion rate states or neutron star crust cooling in the be/x-ray transients v0332+53 and 4u 0115+63 |
the first hours following a neutron star merger are considered to provide several ultraviolet (uv)/optical/near-infrared signals: β-decay emission from free neutrons, radioactive decay of shocked heavy elements in the cocoon and cocoon's cooling emission. here, we consider two additional emission sources: β-decay of free neutrons in the cocoon and synchrotron by the β-decay electrons. we present three-dimensional relativistic hydrodynamic simulations of jets that propagate in a multi-layer ejecta from the merger and calculate semi-analytically the resulting light curves. we find that the free neutrons emission at high latitudes is enhanced by the cocoon by a factor of a few to power a wide (≲60°) and brief (∼1 h) uv signal that can reach an absolute magnitude of ≳-15, comparable with the cooling emission. if the ejected neutron matter mass is m_n ≳ 10^{-4} {m_{⊙ }}, the synchrotron emission may yield a long (∼8 h) quasi-isotropic uv/optical signal with an absolute magnitude between -12 and -15, depending on the magnetic field. such a high mass of a mildly relativistic component may partly obscure the cocoon's shocked r-process elements, thereby attenuating its radioactive decay emission. future observations on these time-scales, including null detections, may place constraints on the ejected neutron matter mass and shed light on the ejecta and jet-cocoon characteristics. | electromagnetic signals from the decay of free neutrons in the first hours of neutron star mergers |
background: in order to study structure of protoneutron stars and those in subsequent cooling stages, it is of great interest to calculate inhomogeneous hot and cold nuclear matter in a variety of phases. the finite-temperature hartree-fock-bogoliubov (ft-hfb) theory is a primary choice for this purpose; however, its numerical calculation for superfluid (superconducting) many-fermion systems in three dimensions requires enormous computational costs. purpose: to study a variety of phases in the crust of hot and cold neutron stars, we propose an efficient method to perform the ft-hfb calculation with the three-dimensional (3d) coordinate-space representation. methods: recently, an efficient method based on the contour integral of green's function with the shifted conjugate-orthogonal conjugate-gradient method was proposed [phys. rev. c 95, 044302 (2017), 10.1103/physrevc.95.044302]. we extend the method to finite temperature, using the shifted conjugate-orthogonal conjugate-residual method. results: we benchmark the 3d coordinate-space solver of the ft-hfb calculation for hot isolated nuclei and fcc phase in the inner crust of neutron stars at finite temperature. the computational performance of the present method is demonstrated. different critical temperatures of the quadrupole and the octupole deformations are confirmed for 146ba. the robustness of the shape coexistence feature in 184hg is examined. for the neutron-star crust, deformed neutron-rich se nuclei embedded in the sea of superfluid low-density neutrons appear in the fcc phase at a nucleon density of 0.045 fm-3 and a temperature of kbt =200 kev. conclusions: the efficiency of the developed solver is demonstrated for nuclei and inhomogeneous nuclear matter at finite temperature. it may provide a standard tool for nuclear physics, especially for the structure of hot and cold neutron-star matter. | coordinate-space solver for finite-temperature hartree-fock-bogoliubov calculations using the shifted krylov method |
previous research centered on the hydrodynamics in x-ray pulsar accretion columns has largely focused on the single-fluid model, in which the super-eddington luminosity inside the column decelerates the flow to rest at the stellar surface. this type of model has been relatively successful in describing the overall properties of the accretion flows, but it does not account for the possible dynamical effect of the gas pressure. on the other hand, the most successful radiative transport models for pulsars generally do not include a rigorous treatment of the dynamical structure of the column, instead assuming an ad hoc velocity profile. in this paper, we explore the structure of x-ray pulsar accretion columns using a new, self-consistent, “two-fluid” model, which incorporates the dynamical effect of the gas and radiation pressures, the dipole variation of the magnetic field, the thermodynamic effect of all of the relevant coupling and cooling processes, and a rigorous set of physical boundary conditions. the model has six free parameters, which we vary in order to approximately fit the phase-averaged spectra in her x-1, cen x-3, and lmc x-4. in this paper, we focus on the dynamical results, which shed new light on the surface magnetic field strength, the inclination of the magnetic field axis relative to the rotation axis, the relative importance of gas and radiation pressures, and the radial variation of the ion, electron, and inverse-compton temperatures. the results obtained for the x-ray spectra are presented in a separate paper. | a new two-fluid radiation-hydrodynamical model for x-ray pulsar accretion columns |
recently, parthsarathy et al. analysed long-term timing observations of 85 young radio pulsars. they found that 15 objects have absolute values of braking indices ranging ∼10-3000, far from the classical value n = 3. they also noted a mild correlation between measured value of n and characteristic age of a radio pulsar. in this article, we systematically analyse possible physical origin of large braking indices. we find that a small fraction of these measurements could be caused by gravitational acceleration from an unseen ultra-wide companion of a pulsar or by precession. remaining braking indices cannot be explained neither by pulsar obliquity angle evolution, nor by complex high-order multipole structure of the poloidal magnetic field. the most plausible explanation is a decay of the poloidal dipole magnetic field which operates on a time-scale ∼104-105 yr in some young objects, but has significantly longer time-scale in other radio pulsars. this decay can explain both amplitude of measured n and some correlation between n and characteristic age. the decay can be caused by either enhanced crystal impurities in the crust of some isolated radio pulsars or, more likely, by enhanced resistivity related to electron scattering off phonons due to slow cooling of low-mass neutron stars. if this effect is indeed the main cause of the rapid magnetic field decay manifesting as large braking indices, we predict that pulsars with large braking indices are hotter in comparison to those with n ≈ 3. | braking indices of young radio pulsars: theoretical perspective |
thermal evolution of neutron stars (nss) is studied as a probe of physics beyond the standard model. we first review the standard cooling theory of nss in detail, with an emphasis on the roles of nucleon superfluidity. then we discuss non-standard evolution with axion and dark matter (dm); axion production enhances the cooling while dm accretion leads to heating of nss. to evaluate the effect of dm heating, we need to compare it to the rotochemical heating, which is caused by the out-of-equilibrium beta processes in a ns. in the dissertation, we also investigate the rotochemical heating in the presence of both neutron and proton superfluidity. | thermal evolution of neutron stars as a probe of physics beyond the standard model |
we consider the local dynamics of a realistic neutron-star core, including composition gradients, superfluidity and thermal effects. the main focus is on the gravity g-modes, which are supported by composition stratification and thermal gradients. we derive the equations that govern this problem in full detail, paying particular attention to the input that needs to be provided through the equation of state and distinguishing between normal and superfluid regions. the analysis highlights a number of key issues that should be kept in mind whenever equation of state data is compiled from nuclear physics for use in neutron-star calculations. we provide explicit results for a particular stellar model and a specific nucleonic equation of state, making use of cooling simulations to show how the local wave spectrum evolves as the star ages. our results show that the composition gradient is effectively dominated by the muons whenever they are present. when the star cools below the superfluid transition, the support for g-modes at lower densities (where there are no muons) is entirely thermal. we confirm the recent suggestion that the g-modes in this region may be unstable, but our results indicate that this instability will be weak and would only be present for a brief period of the star's life. our analysis accounts for the presence of thermal excitations encoded in entrainment between the entropy and the superfluid component. finally, we discuss the complete spectrum, including the normal sound waves and, in superfluid regions, the second sound. | buoyancy and g-modes in young superfluid neutron stars |
observational evidence has been accumulating that thermonuclear x-ray bursts ignited on the surface of neutron stars influence the surrounding accretion flow. here, we exploit the excellent sensitivity of nustar up to 79 kev to analyse the impact of an x-ray burst on the accretion emission of the neutron star lmxb 4u 1608-52. the ≃200 s long x-ray burst occurred during a hard x-ray spectral state, and had a peak intensity of ≃30-50 per cent of the eddington limit with no signs of photospheric radius expansion. spectral analysis suggests that the accretion emission was enhanced up to a factor of ≃5 during the x-ray burst. we also applied a linear unsupervised decomposition method, namely non-negative matrix factorization (nmf), to study this x-ray burst. we find that the nmf performs well in characterizing the evolution of the burst emission and is a promising technique to study changes in the underlying accretion emission in more detail than is possible through conventional spectral fitting. for the burst of 4u 1608-52, the nmf suggests a possible softening of the accretion spectrum during the x-ray burst, which could potentially be ascribed to cooling of a corona. finally, we report a small (≃3 per cent) but significant rise in the accretion emission ≃0.5 h before the x-ray burst, although it is unclear whether this was related to the x-ray burst ignition. | probing the effects of a thermonuclear x-ray burst on the neutron star accretion flow with nustar |
aims: integral has been monitoring the galactic center region for more than a decade. over this time it has detected hundreds of type-i x-ray bursts from the neutron star low-mass x-ray binary 4u 1728-34, also known as the slow burster. our aim is to study the connection between the persistent x-ray spectra and the x-ray burst spectra in a broad spectral range.methods: we performed spectral modeling of the persistent emission and the x-ray burst emission of 4u 1728-34 using data from the integral jem-x and ibis/isgri instruments.results: we constructed a hardness intensity diagram to track spectral state variations. in the soft state, the energy spectra are characterized by two thermal components likely coming from the accretion disc and the boundary/spreading layer, together with a weak hard x-ray tail that we detect in 4u 1728-34 for the first time in the 40 to 80 kev range. in the hard state, the source is detected up to 200 kev and the spectrum can be described by a thermal comptonization model plus an additional component: either a powerlaw tail or reflection. by stacking 123 x-ray bursts in the hard state, we detect emission up to 80 kev during the x-ray bursts. we find that during the bursts the emission above 40 kev decreases by a factor of approximately three with respect to the persistent emission level.conclusions: our results suggest that the enhanced x-ray burst emission changes the spectral properties of the accretion disc in the hard state. the likely cause is an x-ray burst induced cooling of the electrons in the inner hot flow near the neutron star. | x-ray burst-induced spectral variability in 4u 1728-34 |
the theory of quasi-spherical subsonic accretion onto magnetized rotating neutron star is reviewed. different regimes of quasi-spherical accretion onto a neutron star: supersonic (bondi) accretion, which takes place when the captured matter cools down rapidly and falls supersonically towards the neutron-star magnetosphere, and subsonic (settling) accretion which occurs when the plasma remains hot until it meets the magnetospheric boundary. in subsonic accretion, which works at x-ray luminosities $\lesssim 4\times 10^{36}$~erg~s$^{-1}$, a hot quasi-spherical shell must form around the magnetosphere, and the actual accretion rate onto the neutron star is determined by the ability of the plasma to enter the magnetosphere due to the rayleigh-taylor instability. we show how the dimensionless parameters of the theory can be determined from observations of equilibrium x-ray pulsars (vela x-1, gx 301-2). we also discuss how in the settling accretion theory bright x-ray flares ($\sim 10^{38}-10^{40}$~ergs) observed in supergiant fast x-ray transients (sfxt) may be produced by sporadic capture of magnetized stellar-wind plasma. at sufficiently low accretion rates, magnetic reconnection can enhance the magnetospheric plasma entry rate, resulting in copious production of x-ray photons, strong compton cooling and ultimately in unstable accretion of the entire shell. a bright flare develops on the free-fall time scale in the shell, and the typical energy released in an sfxt bright flare corresponds to the mass of the shell. | wind accretion - observations vs theory |
we report on the aftermath of a magnetar outburst from the young, high magnetic field radio pulsar psr j1119-6127 that occurred on 2016 july 27. we present the results of a monitoring campaign using the neil gehrels swift x-ray telescope, nustar, and xmm-newton. after reaching a peak absorbed 0.5-10 kev flux of {45}-7+5× {10}-12 erg cm-2 s-1 on 2016 july 27, the pulsar’s x-ray flux declined by factor of ∼50 to {0.83}-0.06+0.06× {10}-12 erg cm-2 s-1 on 2016 december 12. the x-ray spectra are well described by a blackbody plus a hard power-law tail. during this time, the blackbody radius decreases monotonically by a factor of ∼4 over a span of nearly 200 days. we also report a highly pulsed hard x-ray emission component, which fades on a similar timescale to the soft x-ray flux, as predicted by models of relaxation of magnetospheric current twists. the previously reported spin-up glitch that accompanied this outburst was followed by a period of enhanced and erratic torque, leading to a net spin down of ∼3.5 × 10-4 hz, a factor of ∼24 over-recovery. we suggest that this and other radiatively loud magnetar-type glitch recoveries are dominated by magnetospheric processes, in contrast to conventional radio pulsar glitch recoveries which are dominated by internal physics. | the 2016 outburst of psr j1119-6127: cooling and a spin-down-dominated glitch |
we model neutron star cooling, in particular the current rapid cooldown of the neutron star cas a, with a microscopic nuclear equation of state featuring strong direct urca processes and using compatible nuclear pairing gaps as well as effective masses. several scenarios are possible to explain the features of cas a, but only large and extended proton 1s0 gaps and small neutron 3pf2 gaps are able to accommodate also the major part of the complete current cooling data. we conclude that the possibility of strong direct urca processes cannot be excluded from the cooling analysis. | cassiopeia a and direct urca cooling |
we here present a new version of the publicly available general relativistic magnetohydrodynamic (grmhd) code spritz, which now includes an approximate neutrino leakage scheme able to handle neutrino cooling and heating. the leakage scheme is based on the publicly available zelmanileak code, with a few modifications in order to properly work with spritz. we discuss the involved equations, physical assumptions, and implemented numerical methods, along with a large battery of general relativistic tests performed with and without magnetic fields. our tests demonstrate the correct implementation of the neutrino leakage scheme, paving the way for further improvements of our neutrino treatment and the first application to magnetized binary neutron star mergers. we also discuss the implementation in the spritz code of high-order methods for a more accurate evolution of hydrodynamical quantities. | spritz: general relativistic magnetohydrodynamics with neutrinos |
neutron stars cool down during their lifetime through the combination of neutrino emission from the interior and photon cooling from the surface. strongly magnetized neutron stars, called magnetars, are no exception, but the effect of their strong fields adds further complexities to the cooling theory. besides other factors, modelling the outermost hundred meters (the envelope) plays a crucial role in predicting their surface temperatures. in this letter, we revisit the influence of envelopes on the cooling properties of neutron stars, with special focus on the critical effects of the magnetic field. we explore how our understanding of the relation between the internal and surface temperatures has evolved over the past two decades, and how different assumptions about the neutron star envelope and field topology lead to radically different conclusions on the surface temperature and its cooling with age. in particular, we find that relatively old magnetars with core-threading magnetic fields are actually much cooler than a rotation-powered pulsar of the same age. this is at variance with what is typically observed in crustal-confined models. our results have important implications for the estimates of the x-ray luminosities of aged magnetars, and the subsequent population study of the different neutron star classes. | how bright can old magnetars be? assessing the impact of magnetized envelopes and field topology on neutron star cooling |
recent observations of old warm neutron stars suggest the presence of a heating source in these stars, requiring a paradigm beyond the standard neutron-star cooling theory. in this work, we study the scenario where this heating is caused by the friction associated with the creep motion of neutron superfluid vortex lines in the crust. as it turns out, the heating luminosity in this scenario is proportional to the time derivative of the angular velocity of the pulsar rotation, and the proportional constant $j$ has an approximately universal value for all neutron stars. this $j$ parameter can be determined from the temperature observation of old neutron stars because the heating luminosity is balanced with the photon emission at late times. we study the latest data of neutron star temperature observation and find that these data indeed give similar values of $j$, in favor of the assumption that the frictional motion of vortex lines heats these neutron stars. these values turn out to be consistent with the theoretical calculations of the vortex-nuclear interaction. | vortex creep heating in neutron stars |
social insects host a diversity of viruses. we examined new zealand populations of the globally widely distributed invasive argentine ant (linepithema humile) for rna viruses. we used metatranscriptomic analysis, which identified six potential novel viruses in the dicistroviridae family. of these, three contigs were confirmed by sanger sequencing as linepithema humile virus-1 (lhuv-1), a novel strain of kashmir bee virus (kbv) and black queen cell virus (bqcv), while the others were chimeric or misassembled sequences. we extended the known sequence of lhuv-1 to confirm its placement in the dicistroviridae and categorised its relationship to closest relatives, which were all viruses infecting hymenoptera. we examined further for known viruses by mapping our metatranscriptomic sequences to all viral genomes, and confirmed kbv, bqcv, lhuv-1 and deformed wing virus (dwv) presence using qrt-pcr. viral replication was confirmed for dwv, kbv and lhuv-1. viral titers in ants were higher in the presence of honey bee hives. argentine ants appear to host a range of' honey bee' pathogens in addition to a virus currently described only from this invasive ant. the role of these viruses in the population dynamics of the ant remain to be determined, but offer potential targets for biocontrol approaches. | single-stranded rna viruses infecting the invasive argentine ant, linepithema humile |
aims: we aim to study the possibility of a hadron-quark phase transition in the interior of neutron stars, taking into account different schematic evolutionary stages at finite temperature. we also discuss the strange quark matter stability in the quark matter phase. furthermore, we aim to analyze the astrophysical properties of hot and cold hybrid stars, considering the constraint on maximum mass given by the pulsars j1614-2230 and j0348+0432.methods: we have developed a computational code to construct semi-analytical hybrid equations of state at fixed entropy per baryon and to obtain different families of hybrid stars. an analytical approximation of the field correlator method is developed for the quark matter equation of state. for the hadronic equation of state we use a table based on the relativistic mean field theory, without hyperons. the phase transition was obtained imposing the maxwell conditions, by assuming a high surface tension at the interface hadron-quark. we solved the relativistic structure equations of hydrostatic equilibrium and mass conservation for hybrid star configurations.results: for the different equations of state obtained, we calculated the stability window for the strange quark matter, lepton abundances, temperature profiles and contours profiles for the maximum mass star depending on the parameters of the field correlator method. we also computed the mass-radius and gravitational mass-baryonic mass relationships for different hybrid star families.conclusions: we have analyzed different stages of hot hybrid stars as a first approximation of the cooling evolution of neutron stars with quark matter cores. we obtain cold hybrid stars with maximum masses ≥2 m⊙ for different combinations of the field correlator method parameters. in addition, our study based on the gravitational mass - baryonic mass plane shows a late phase transition between hadronic and quark matter during the proto-hybrid star evolution, in contrast with previous studies of proto-neutron stars. | constant entropy hybrid stars: a first approximation of cooling evolution |
we model neutron star cooling with several microscopic nuclear equations of state based on different nucleon-nucleon interactions and three-body forces, and compatible with the recent gw170817 neutron star merger event. they all feature strong direct urca processes. we find that all models are able to describe well the current set of cooling data for isolated neutron stars, provided that large and extended proton 1s0 gaps and no neutron 3pf2 gaps are active in the stellar matter. we then analyse the neutron star mass distributions predicted by the different models and single out the preferred ones. | neutron star cooling with microscopic equations of state |
we present the mass excesses of 52-57sc, obtained from recent time-of-flight nuclear mass measurements at the national superconducting cyclotron laboratory at michigan state university. the masses of 56sc and 57sc were determined for the first time with atomic mass excesses of -24.85 (59 )(-54+0) mev and -21.0 (1.3 ) mev , respectively, where the asymmetric uncertainty for 56sc was included due to possible contamination from a long-lived isomer. the 56sc mass indicates a small odd-even mass staggering in the a =56 mass chain towards the neutron drip line, significantly deviating from trends predicted by the global frdm mass model and favoring trends predicted by the unedf0 and unedf1 density functional calculations. together with new shell-model calculations of the electron-capture strength function of 56sc, our results strongly reduce uncertainties in model calculations of the heating and cooling at the 56ti electron-capture layer in the outer crust of accreting neutron stars. we find that, in contrast to previous studies, neither strong neutrino cooling nor strong heating occurs in this layer. we conclude that urca cooling in the outer crusts of accreting neutron stars that exhibit superbursts or high temperature steady-state burning, which are predicted to be rich in a ≈56 nuclei, is considerably weaker than predicted. urca cooling must instead be dominated by electron capture on the small amounts of adjacent odd-a nuclei contained in the superburst and high temperature steady-state burning ashes. this may explain the absence of strong crust urca cooling inferred from the observed cooling light curve of the transiently accreting x-ray source maxi j0556-332. | mass measurement of 56sc reveals a small a =56 odd-even mass staggering, implying a cooler accreted neutron star crust |
we investigate the impacts of the neutrino cooling mechanism inside the neutron star (ns) core on the light curves of type i x-ray bursts and x-ray superbursts. from several observations of ns thermal evolution, physical processes of fast neutrino cooling, such as the direct urca (du) process, are indicated. they significantly decrease the surface temperature of nss, though the cooling effect could be suppressed by nucleon superfluidity. in the present study, focusing on the du process and nucleon superfluidity, we investigate the effects of ns cooling on the x-ray bursts using a general-relativistic stellar-evolution code. we find that the du process leads to a longer recurrence time and higher peak luminosity, which could be obstructed by the neutrons' superfluidity. we also apply our burst models to the comparison with clocked burster gs 1826-24, and to the recurrence time of a superburst triggered by carbon ignition. these effects are significant within a certain range of binary parameters and the uncertainty of the ns equation of state. | impacts of the direct urca and superfluidity inside a neutron star on type i x-ray bursts and x-ray superbursts |
we report detailed x-ray observations of the unique binary system ϵ lupi, the only known short-period binary consisting of two magnetic early-type stars. the components have comparably strong, but anti-aligned magnetic fields. the orbital and magnetic properties of the system imply that the magnetospheres overlap at all orbital phases, suggesting the possibility of variable inter-star magnetospheric interaction due to the non-negligible eccentricity of the orbit. to investigate this effect, we observed the x-ray emission from ϵ lupi, both near and away from periastron passage, using the neutron star interior composition explorer mission (nicer) x-ray telescope. we find that the system produces excess x-ray emission at the periastron phase, suggesting the presence of variable inter-star magnetospheric interaction. we also discover that the enhancement at periastron is confined to a very narrow orbital phase range ($\approx 5~{{\ \rm per\ cent}}$ of the orbital period), but the x-ray properties close to periastron phase are similar to those observed away from periastron. from these observations, we infer that the underlying cause is magnetic reconnection heating the stellar wind plasma, rather than shocks produced by wind-wind collision. finally, by comparing the behavior of ϵ lupi with that observed for cooler magnetic binary systems, we propose that elevated x-ray flux at periastron phase is likely a general characteristic of interacting magnetospheres irrespective of the spectral types of the constituent stars. | discovery of extraordinary x-ray emission from magnetospheric interaction in the unique binary stellar system ϵ lupi |
several candidates for accreting magnetars have been proposed recently by different authors. existence of such systems contradicts the standard magnetic field decay scenario where a large magnetic field of a neutron star reaches ≲a few ×1013 g at ages ≳1 myr. among other sources, the high-mass x-ray binary 4u 0114+65 seems to have a strong magnetic field around 1014 g. we develop a new bayesian estimate for the kinematic age and demonstrate that 4u 0114+65 has kinematic age 2.4-5 myr (95 per cent credential interval) since the formation of the neutron star. we discuss which conditions are necessary to explain the potential existence of magnetars in accreting high-mass binaries with ages about few myr and larger. three necessary ingredients are: the hall attractor to prevent rapid decay of dipolar field, relatively rapid cooling of the crust in order to avoid ohmic decay due to phonons, and finally, low values of the parameter q to obtain long ohmic time-scale due to impurities. if age and magnetic field estimates for proposed accreting magnetars are correct, then these systems set the strongest limit on the crust impurity for a selected sample of neutron stars and provide evidence in favour of the hall attractor. | how to make a mature accreting magnetar |
millisecond pulsars (msps) are old, fast spinning neutron stars (nss) thought to have evolved from classical pulsars in binary systems, where the rapid rotation is caused by the accretion of matter and angular momentum from their companion. during this transition between classical and msps, there is a magnetic field reduction of ∼4 orders of magnitude, which is not well understood. according to the standard scenario, the magnetic field is reduced as a consequence of accretion, either through ohmic dissipation or through screening by the accreted matter. we explored an alternative hypothesis in which the magnetic field is reduced through ambipolar diffusion before the accretion. this is particularly effective during the long epoch in which the pulsar has cooled, but has not yet started accreting. this makes the final magnetic field dependent on the evolution time of the companion star and thus its initial mass. we use observed binary systems to constrain the time available for the magnetic field decay based on the current pulsar companion: a helium white dwarf, a carbon-oxygen white dwarf, or another ns. based on a simplified model without baryon pairing, we show that the proposed process agrees with the general distribution of observed magnetic field strengths in binaries, but is not able to explain some mildly recycled pulsars where no significant decay appears to have occurred. we discuss the possibility of other formation channels for these systems and the conditions under which the magnetic field evolution would be set by the ns crust rather than the core. | on the weak magnetic field of millisecond pulsars: does it decay before accretion? |
whether fast cooling processes occur or not is crucial for the thermal evolution of neutron stars. in particular, the threshold of the direct urca process, which is one of the fast cooling processes, is determined by the interior proton fraction y_p, or the nuclear symmetry energy. since recent observations indicate the small radius of neutron stars, a low value is preferred for the symmetry energy. in this study, simulations of neutron star cooling are performed adopting three models for the equation of state (eos): togashi, shen, and ls220 eoss. the togashi eos has been recently constructed with realistic nuclear potentials under finite temperature, and found to account for the small radius of neutron stars. as a result, we find that, since the direct urca process is forbidden, the neutron star cooling is slow with use of the togashi eos. this is because the symmetry energy of togashi eos is lower than those of other eoss. hence, in order to account for observed age and surface temperature of isolated neutron stars with the use of the togashi eos, other fast cooling processes are needed regardless of the surface composition. | possibility of rapid neutron star cooling with a realistic equation of state |
type i x-ray bursts are thermonuclear flashes observed from the surfaces of accreting neutron stars (nss) in low mass x-ray binaries. oscillations have been observed during the rise and/or decay of some of these x-ray bursts. those seen during the rise can be well explained by a spreading hot spot model, but large amplitude oscillations in the decay phase remain mysterious because of the absence of a clear-cut source of asymmetry. to date there have not been any quantitative studies that consistently track the oscillation amplitude both during the rise and decay (cooling tail) of bursts. here we compute the light curves and amplitudes of oscillations in x-ray burst models that realistically account for both flame spreading and subsequent cooling. we present results for several such “cooling wake” models, a “canonical” cooling model where each patch on the ns surface heats and cools identically, or with a latitude-dependent cooling timescale set by the local effective gravity, and an “asymmetric” model where parts of the star cool at significantly different rates. we show that while the canonical cooling models can generate oscillations in the tails of bursts, they cannot easily produce the highest observed modulation amplitudes. alternatively, a simple phenomenological model with asymmetric cooling can achieve higher amplitudes consistent with the observations. | x-ray burst oscillations: from flame spreading to the cooling wake |
<p id="par1">nuclear-powered x-ray millisecond pulsars are the third type of millisecond pulsars, which are powered by thermonuclear fusion processes. the corresponding brightness oscillations, known as burst oscillations, are observed during some thermonuclear x-ray bursts, when the burning and cooling accreted matter gives rise to an azimuthally asymmetric brightness pattern on the surface of the spinning neutron star. apart from providing neutron star spin rates, this x-ray timing feature can be a useful tool to probe the fundamental physics of neutron star interior and surface. this chapter presents an overview of the relatively new field of nuclear-powered x-ray millisecond pulsars. | nuclear-powered x-ray millisecond pulsars |
in this paper, we investigate the cooling of neutron stars with relativistic and nonrelativistic models of dense nuclear matter. we focus on the effects of uncertainties originated from the nuclear models, the composition of elements in the envelope region, and the formation of superfluidity in the core and the crust of neutron stars. discovery of 2m⊙ neutron stars psr j1614-2230 and psr j0343+0432 has triggered the revival of stiff nuclear equation of state at high densities. in the meantime, observation of a neutron star in cassiopeia a for more than 10 years has provided us with very accurate data for the thermal evolution of neutron stars. both mass and temperature of neutron stars depend critically on the equation of state of nuclear matter, so we first search for nuclear models that satisfy the constraints from mass and temperature simultaneously within a reasonable range. with selected models, we explore the effects of element composition in the envelope region, and the existence of superfluidity in the core and the crust of neutron stars. due to uncertainty in the composition of particles in the envelope region, we obtain a range of cooling curves that can cover substantial region of observation data. | nuclear equation of state and neutron star cooling |
in view of new constraints put forth by recent observations and measurements in the realm of astrophysics and nuclear physics, we update the nonlinear realization of the σ model as to reflect such constraints. by doing this, we obtain new equations of state that may be used to describe neutron stars. such equations of state are obtained by investigating different ways by which the vector mesons self-interact. furthermore, we also investigate the role played by δ mesons in the model. as a result, we are able to develop equations of state that are in better agreement with data, such as nuclear compressibility and slope of the symmetry energy at saturation, star masses, radii, and cooling profiles. | reconciling nuclear and astrophysical constraints |
we present a modification of our previous model for the mechanisms underlying the glitch phenomena in pulsars and young neutron stars. accordingly, pulsars are born with embryonic cores comprising of purely incompressible superconducting gluon-quark superfluid (henceforth susu-cores). as the ambient medium cools and spins down due to emission of magnetic dipole radiation, the mass and size of susu-cores are set to grow discretely with time, in accordance with the onsager-feynmann analysis of superfluidity. presently, we propose that the spacetime embedding glitching pulsars is dynamical and of bimetric nature: inside susu-cores the spacetime must be flat, whereas the surrounding region, where the matter is compressible and dissipative, the spacetime is schwarzschild. it is further proposed that the topological change of spacetime is derived by the strong nuclear force, whose operating length scales is found to increase with time to reach o(1) cm at the end of the luminous lifetimes of pulsars. the model presented here model is in line with the recent radio and gw observations of pulsars and nss. | glitching pulsars: unraveling the interactions of general relativistic and quantum fields in the strong field regimes |
short range correlations (src) have been known to be an important aspect of nuclear theory for some time. recent works have re-ignited interest on this topic, particularly due to the fact that it has recently been demonstrated that src may be responsible for breaking pairing gaps in nuclear matter. in this work we revisit the concept of src for beta equilibrated matter in neutron stars. we construct two equivalent models, with and without src and proceed to investigate the thermal evolution of stars described by such models. we show that src play a major role in the thermal evolution of neutron stars. it will be shown that while the src largely leaves the macroscopic properties of the star unaltered, it significantly alters the proton fraction, thus leading to an early onset of the direct urca (du) process, which in turns leads to stars exhibiting much faster cooling. | short-range correlation effects on the neutron star cooling |
an ambitious goal of the astrophysical community is not only to constrain the equation of state (eos) of neutron star (ns) matter by confronting it with astrophysics observations, but ultimately also to infer the ns composition. nevertheless, the composition of the ns core is likely to remain uncertain unless we have an accurate determination of the nuclear symmetry energy at suprasaturation density (ρ >ρ0) . we investigate how the nucleonic direct urca (durca) processes can be used as an effective probe to constrain the high density nuclear symmetry energy. a large number of minimally constrained eoss has been constructed by applying a bayesian approach to study the correlations of the symmetry energy at different densities with a few selected properties of a ns. the nuclear symmetry energy above the baryon density 0.5 fm−3 (≈3 ρ0) is found to be strongly correlated with ns mass at which the onset of nucleonic durca neutrino cooling takes place in the core. this allows us to constrain the high density behavior of nuclear symmetry energy within narrow bounds. the pure neutron matter pressure constraint from chiral effective field theory rules out the onset of nucleonic durca in stars with a mass ≲1.4 m⊙. the onset of durca inside 1.6 m⊙ to 1.8 m⊙ ns implies a slope of the symmetry energy l at ≈2.5 ρ0 , respectively, between 54 and 48 mev. | inferring the nuclear symmetry energy at suprasaturation density from neutrino cooling |
we discuss the observation that under neutron star conditions of charge neutrality and $\beta-$equilibrium the contribution from the symmetry energy to the equation of state (eos) follows a universal behaviour. we call this behaviour the conjecture of a universal symmetry energy contribution (usec). we find that an usec holds provided the density dependence of the symmetry energy $e_s(n)$ follows a behaviour that limits the proton fraction $x(n)$ to values below the threshold for the direct urca (du) cooling process. the absence of du cooling in typical mass neutron stars appears to be supported by the phenomenology of neutron star cooling data and allows to constrain the behaviour of $e_s(n)$ at high densities. two classes of symmetry energy functions are investigated more in detail to elucidate the usec. we derive an analytic formula for the usec to the neutron star eos based on the result for the symmetry energy extracted from isobaric analog states of nuclei. | universal symmetry energy contribution to the neutron star equation of state |
the interior of mature neutron stars is expected to contain superfluid neutrons and superconducting protons. the influence of temperature and currents on superfluid properties is studied within the self-consistent time-dependent nuclear energy-density functional theory. we find that this theory predicts the existence of a regime in which nucleons are superfluid (the order parameter remains finite) even though the energy spectrum of quasiparticle excitations exhibits no gap. we show that the disappearance of the gap leads to a specific heat that is not exponentially suppressed at low temperatures as in the bardeen-cooper-schrieffer regime but can be comparable to that in the normal phase. introducing some dimensionless effective superfluid velocity, we show that the behavior of the specific heat is essentially universal and we derive general approximate analytical formulas for applications to neutron-star cooling simulations. | gapless superfluidity in neutron stars: thermal properties |
context. neutron stars in low-mass binary systems are subject to accretion when material originating from the companion star accumulates on the surface. in most cases, the justified and common assumption in studying the properties of the neutron star crust is the fully accreted crust approximation. however, observations of some x-ray transient sources indicate that the original crust has not been completely replaced by accreted material, but is partly composed of the compressed original crust.aims: the crust of an accreting neutron star beyond the fully accreted crust approximation was studied; a two-part (or hybrid) crust made of the original crust that is compressed and of the accreted material crashing onto it was reconstructed as a function of the accretion stage. the differences in the composition and energy sources for the fully accreted and hybrid crusts influence the cooling and transport properties.methods: a simple semi-empirical formula of a compressible liquid drop was used to compute the equation of state and composition of the hybrid crust. calculations were based on the single-nucleus model, with a more accurate treatment of the neutron drip point. we compared the nuclear reactions triggered by compression in the original crust and in the accreted matter part of the hybrid crust. we discuss another crust compression astrophysical phenomenon related to spinning neutron stars.results: the compression of the originally catalyzed outer crust triggers exothermic reactions (electron captures and pycnonuclear fusions) that deposit heat in the crust. the heat sources are cataloged as a function of the compression until the fully accreted crust approximation is reached. the pressure at which neutron drip occurs is a nonmonotonic function of the depth, leading to a temporary neutron drip anomaly. the additional potential source of energy for partially accreted crusts is the occurrence of a density inversion phenomenon between some compressed layers.conclusions: the original crust of a neutron star cannot be neglected for the initial period of accretion, when the original crust is not fully replaced by the accreted matter. the amount of heat associated with the compression of the original crust is on the same order of magnitude as that from the sources acting in the accreted part of the hybrid crust. appendix tables are also available at the cds via anonymous ftp to cdsarc.u-strasbg.fr (ftp://130.79.128.5) or via http://cdsarc.u-strasbg.fr/viz-bin/cat/j/a+a/662/a63 movies are available at https://www.aanda.org | partially accreted crusts of neutron stars |
we study the s10 proton pairing gap in β -equilibrated neutron star matter within the framework of chiral effective field theory. we focus on the role of three-body forces, which strongly modify the effective proton-proton spin-singlet interaction in dense matter. we find that three-body forces generically reduce both the size of the pairing gap and the maximum density at which proton pairing may occur. the pairing gap is computed within bardeen-cooper-schrieffer theory using a single-particle dispersion relation calculated up to second order in perturbation theory. model uncertainties are estimated by varying the nuclear potential (its order in the chiral expansion and high-momentum cutoff) and the choice of single-particle spectrum in the gap equation. we find that a second-order perturbative treatment of the single-particle spectrum suppresses the proton s10 pairing gap relative to the use of a free spectrum. we estimate the critical temperature for the onset of proton superconductivity to be tc=(3.2 -5.1 ) ×109 k, which is consistent with previous theoretical results in the literature and marginally within the range deduced from a recent bayesian analysis of neutron star cooling observations. | proton pairing in neutron stars from chiral effective field theory |
a superconductor of paired protons is thought to form in the core of neutron stars soon after their birth. minimum energy conditions suggest magnetic flux is expelled from the superconducting region due to the meissner effect, such that the neutron star core is largely devoid of magnetic fields for some nuclear equation of state and proton pairing models. we show via neutron star cooling simulations that the superconducting region expands faster than flux is expected to be expelled because cooling timescales are much shorter than timescales of magnetic field diffusion. thus magnetic fields remain in the bulk of the neutron star core for at least 106-107yr . we estimate the size of flux free regions at 107yr to be ≲100 m for a magnetic field of 1011g and possibly smaller for stronger field strengths. for proton pairing models that are narrow, magnetic flux may be completely expelled from a thin shell of approximately the above size after 105yr . this shell may insulate lower conductivity outer layers, where magnetic fields can diffuse and decay faster, from fields maintained in the highly conducting deep core. | dynamical onset of superconductivity and retention of magnetic fields in cooling neutron stars |
we model the cooling of hybrid neutron stars combining a microscopic nuclear equation of state in the brueckner-hartree-fock approach with different quark models. we then analyse the neutron star cooling curves predicted by the different models and single out the preferred ones. we find that the possibility of neutron p-wave pairing can be excluded in our scenario. | cooling of hybrid neutron stars with microscopic equations of state |
we use two-dimensional, general relativistic, viscous, radiation hydrodynamic simulations to study the impact of a type i x-ray burst on a hot and geometrically thick accretion disk surrounding an unmagnetized, non-rotating neutron star. the disk is initially consistent with a system in its low/hard spectral state, and is subject to a burst that rises to a peak luminosity of 1038 erg s-1 in 2.05 s. at the peak of the burst, the temperature of the disk has dropped by more than three orders of magnitude and its scale height has gone down by more than one order of magnitude. the simulations show that these effects predominantly happen due to compton cooling of the hot plasma, and clearly illustrate the potential cooling effects of bursts on accretion disk coronae. in addition, we demonstrate the presence of poynting-robertson drag, though it only enhances the mass accretion rate onto the neutron star by a factor of ∼3-4 compared to a simulation with no burst. simulations such as these are important for building a general understanding of the response of an accretion disk to an intense x-ray impulse, which, in turn, will be crucial for deciphering burst spectra. detailed analysis of such spectra offers the potential to measure neutron star radii, and hence constrain the neutron star equation of state, but only if the contributions coming from the impacted disk and its associated corona can be understood. | simulating the collapse of a thick accretion disk due to a type i x-ray burst from a neutron star |
geometry and dynamical structure of emission regions in accreting pulsars are shaped by the interplay between gravity, radiation, and strong magnetic field, which significantly affects the opacities of a plasma and radiative pressure under such extreme conditions. quantitative consideration of magnetic plasma opacities is therefore an essential ingredient of any self-consistent modelling of emission region structure of x-ray pulsars (xrps). we present results of computations of the rosseland and planck mean opacities of a strongly magnetized plasma with a simple chemical composition, namely the solar hydrogen/helium mix. we consider all relevant specific opacities of the magnetized plasma including vacuum polarization effect and contribution of electron-positron pairs where the pair number density is computed in the thermodynamic equilibrium approximation. the magnetic planck mean opacity determines the radiative cooling of an optically thin strongly magnetized plasma. it is by factor of three smaller than non-magnetic planck opacity at $k_{\rm b}t \lt 0.1\, e_{\rm cyc}$ and increases by a factor of 102-104 at $k_{\rm b}t \gt 0.3\, e_{\rm cyc}$ due to cyclotron thermal processes. we propose a simple approximate expression which has sufficient accuracy for the magnetic planck opacity description. we provide the rosseland opacity in a tabular form computed in the temperature range 1-300 kev, magnetic field range 3 × 1010-1015 g, and a broad range of plasma densities. we demonstrate that the scattering on the electron-positron pairs increases the rosseland opacity drastically at temperatures > 50 kev in the case of mass densities typical for accretion channel in xrps. | mean opacities of a strongly magnetized high-temperature plasma |
neutron stars in low-mass x-ray binaries exhibit oscillations during thermonuclear bursts, attributed to asymmetric brightness patterns on the burning surfaces. all models that have been proposed to explain the origin of these asymmetries (spreading hotspots, surface waves, and cooling wakes) depend on the accretion rate. by analysis of archival rxte data of six oscillation sources, we investigate the accretion rate dependence of the amplitude of burst oscillations. this more than doubles the size of the sample analyzed previously by muno et al., who found indications for a relationship between accretion rate and oscillation amplitudes. we find that burst oscillation signals can be detected at all observed accretion rates. moreover, oscillations at low accretion rates are found to have relatively small amplitudes ({a}{{rms}}≤slant 0.10) while oscillations detected in bursts observed at high accretion rates cover a broad spread in amplitudes (0.05≤slant {a}{{rms}}≤slant 0.20). in this paper we present the results of our analysis and discuss these in the light of current burst oscillation models. additionally, we investigate the bursts of two sources without previously detected oscillations. despite the fact that these sources have been observed at accretion rates where burst oscillations might be expected, we find their behavior not to be anomalous compared to oscillation sources. | the accretion rate dependence of burst oscillation amplitude |
background: global environmental change is exacerbating human vulnerability to adverse atmospheric conditions including air pollution, aeroallergens such as pollen, and extreme weather events. public information and advisories are a central component of responses to mitigate the human impacts of environmental hazards. digital technologies are emerging as a means of providing personalised, timely and accessible warnings. method: we describe airrater, an integrated online platform that combines symptom surveillance, environmental monitoring, and notifications of changing environmental conditions via a free smartphone app. it was developed and launched in tasmania, australia (population 510 000), with the aim of reducing health impacts and improving quality of life in people with conditions such as asthma and allergic rhinitis. we present environmental data, user uptake and results from three online evaluation surveys conducted during the first 22 months of operation, from october 2015 through august 2017. results: there were 3,443 downloads of the app from all regions of tasmania. of the 1,959 individuals who registered, 79% reported having either asthma or allergic rhinitis. downloads increased during adverse environmental conditions and following publicity. symptom reports per active user were highest during spring (72%), lowest in autumn (37%) and spiked during periods of reduced air quality. in response to online surveys, most users reported that the app was useful and had improved their understanding of how environmental conditions affect their health, and in some cases had prompted action such as the timely use of medication. conclusion: active engagement and consistent positive feedback from users demonstrates the potential for considerable individual, clinical and wider public health benefits from integrated and personalised monitoring systems such as airrater. the perceived health benefits require objective verification, and such systems need to address several challenges in providing timely, reliable and valid environmental data. | using smartphone technology to reduce health impacts from atmospheric environmental hazards |
background: knowing the difference between the neutron and proton densities of nuclei is a significant topic because of its importance for understanding neutron star structures and cooling mechanisms. the coherent-nuclear photoproduction of pions, (γ ,π0 ) , combined with elastic electron scattering, has been suggested to be a very accurate probe of density differences. purpose: study the (γ ,π0 ) reaction mechanism so as to better access the uncertainties involved in extracting the neutron density. methods: include the effects of final-state pion-nucleus charge-exchange reactions on the cross section and study the influence of the nonzero spatial extent of the proton. results: the effects of final-state charge-exchange increase the cross section between 6% and 5%, generally decreasing as the momentum transfer increases. this leads to an increase of the extracted neutron skin distance by about 50%. the validity of the previous treatments of the proton size is confirmed. conclusion: the model dependence of the theoretically computed cross section increases the total systematic uncertainty (experiment plus theory) in extracting the neutron skin from the (γ ,π0 ) cross section by at least a factor of three. | coherent-nuclear pion photoproduction and neutron radii |
inspired by the well-known anomaly in the lifetime of the neutron, we investigated its consequences inside neutron stars. we first assessed the viability of the neutron decay hypothesis suggested by fornal and grinstein within neutron stars, in terms of the equation of state and compatibility with observed properties. this was followed by an investigation of the constraint information on neutron star cooling that can be placed on the decay rate of the dark boson into standard model particles, in the context of various bsm ideas. | constraining dark boson decay using neutron stars |
phase diagrams of fully ionized binary ionic mixtures are considered within the framework of the linear mixing formalism taking into account recent advances in understanding quantum one-component plasma thermodynamics. we have followed a transformation of azeotropic phase diagrams into peritectic and eutectic types with increase of the charge ratio. for solid 12c/16o and 16o/20ne mixtures, we have found extensive miscibility gaps. their appearance seems to be a robust feature of the theory. the gaps evolve naturally into two-solid regions of eutectic phase diagrams at higher z2/z1. they do not depend on thermodynamic fit extensions beyond their applicability limits. the gaps are sensitive to binary mixture composition and physics, being strongly different for c/o and o/ne mixtures and for the three variants of corrections to linear-mixing solid-state energies available in the literature. when matter cools to its miscibility gap temperature, the exsolution process takes place. it results in a separation of heavier and lighter solid solutions. this may represent a significant reservoir of gravitational energy and should be included in future white dwarf (wd) cooling simulations. ion quantum effects mostly resulted in moderate modifications; however, for certain z2/z1, these effects can produce qualitative restructuring of the phase diagram. this may be important for the model with 22ne distillation in cooling c/o/ne wd proposed as a solution for the ultramassive wd cooling anomaly. | phase diagrams of binary ionic mixtures and white dwarf cooling |
we report the discovery of an extremely metal-poor (emp) giant, lamost j110901.22+075441.8, which exhibits a large excess of r-process elements with [eu/fe] ∼ +1.16. the star is one of the newly discovered emp stars identified from the lamost low-resolution spectroscopic survey and a high-resolution follow-up observation with the subaru telescope. stellar parameters and elemental abundances have been determined from the subaru spectrum. accurate abundances for a total of 23 elements including 11 neutron-capture elements from sr through dy have been derived for lamost j110901.22+075441.8. the abundance pattern of lamost j110901.22+075441.8 in the range of c through zn is in line with the “normal” population of emp halo stars, except that it shows a notable underabundance in carbon. the heavy element abundance pattern of lamost j110901.22+075441.8 is in agreement with other well studied cool r-ii metal-poor giants such as cs 22892-052 and cs 31082-001. the abundances of elements in the range from ba through dy match the scaled solar r-process pattern well. lamost j110901.22+075441.8 provides the first detailed measurements of neutron-capture elements among r-ii stars at such low metallicity with [fe/h] ≲ -3.4, and exhibits similar behavior as other r-ii stars in the abundance ratio of zr/eu as well as sr/eu and ba/eu. | discovery of a strongly r-process enhanced extremely metal-poor star lamost j110901.22+075441.8 |
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