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transient low-mass x-ray binaries (lmxbs) that host neutron stars (nss) provide excellent laboratories for probing the dense matter physics present in ns crusts. during accretion outbursts in lmxbs, exothermic reactions may heat the ns crust, disrupting the crust-core equilibrium. when the outburst ceases, the crust cools to restore thermal equilibrium with the core. monitoring this cooling evolution allows us to probe the dense matter physics in the crust. properties of the deeper crustal layers can be probed at later times after the end of the outburst. we report on the unexpected late-time temperature evolution (≳2000 days after the end of their outbursts) of two nss in lmxbs, xte j1701-462 and exo 0748-676. although both these sources exhibited very different outbursts (in terms of duration and the average accretion rate), they exhibit an unusually steep decay of ∼7 ev in the observed effective temperature (occurring in a time span of ∼700 days) around ∼2000 days after the end of their outbursts. furthermore, they both showed an even more unexpected rise of ∼3 ev in temperature (over a time period of ∼500-2000 days) after this steep decay. this rise was significant at the 2.4σ and 8.5σ level for xte j1701-462 and exo 0748-676, respectively. the physical explanation for such behaviour is unknown and cannot be straightforwardly be explained within the cooling hypothesis. in addition, this observed evolution cannot be well explained by low-level accretion either without invoking many assumptions. we investigate the potential pathways in the theoretical heating and cooling models that could reproduce this unusual behaviour, which so far has been observed in two crust-cooling sources. such a temperature increase has not been observed in the other ns crust-cooling sources at similarly late times, although it cannot be excluded that this might be a result of the inadequate sampling obtained at such late times. | unexpected late-time temperature increase observed in the two neutron star crust-cooling sources xte j1701-462 and exo 0748-676 |
the existence of ev-mass sterile neutrinos is not ruled out because of persistent experimental anomalies. upcoming multimessenger detections of neutron-star merger remnants could provide indirect constraints on the existence of these particles. we explore the active-sterile flavor conversion phenomenology in a two-flavor scenario (one active plus one sterile species) as a function of the sterile neutrino mixing parameters, the neutrino emission angle from the accretion torus, and the temporal evolution of the merger remnant. the torus geometry and the neutron richness of the remnant are responsible for the occurrence of multiple resonant active-sterile conversions. the number of resonances strongly depends on the neutrino emission direction above or inside the remnant torus and leads to a large production of sterile neutrinos (and no antineutrinos) in the proximity of the polar axis, as well as more sterile antineutrinos than neutrinos in the equatorial region. as the black-hole torus evolves in time, the shallower baryon density is responsible for more adiabatic flavor conversion, leading to larger regions of the mass-mixing parameter space being affected by flavor mixing. our findings imply that the production of sterile states could have indirect implications on the disk cooling rate, its outflows, and related electromagnetic observables which remain to be assessed. | resonant production of light sterile neutrinos in compact binary merger remnants |
the evolution of magnetic field in isolated neutron stars is one of the most important ingredients in the attempt to build a unified description of these objects. a prediction of field evolution models is the existence of an equilibrium configuration, in which the hall cascade vanishes. recent calculations have explored the field structure in this stage, called the hall attractor. we use x-ray data of near-by, cooling neutron stars to probe this prediction, as these sources are surmised to be close to or at hall attractor phase. we show that the source rx j1856.5-3754 might be closer to the attractor than other sources of its class. our modelling indicates that the properties of surface thermal emission, assuming that the star is in the hall attractor, are in contradiction to the spectral data of rx j1856.5-3754. | probing the surface magnetic field structure in rx j1856.5-3754 |
background: low-lying collective excitations of the inner crust matter in neutron stars are expected to affect observables such as the quasiperiodic oscillation in giant flares or cooling of the inner crust in transient phenomena. the coupling between the anderson-bogoliubov (ab) superfluid phonon in superfluid neutron gas and collective excitations of nuclear clusters is crucially important. purpose: our aim is to describe the characteristics of the low-lying excitation modes of the superfluid inner crust matter, focusing on quadrupole excitations around a spherical nuclear cluster. we studied the effect of the inhomogeneous structure of the inner crust matter on the ab phonon of neutron superfluid and on the possible quadrupole shape vibration of clusters. methods: the coordinate-space hartree-fock-bogoliubov method and the quasiparticle random phase approximation formulated in a spherical wigner-seitz cell were used to describe the neutron superfluidity and low-lying collective excitations. we performed systematic numerical calculations for the quadrupole excitations by varying the neutron chemical potential and the number of protons in the cell. results: the calculated results indicate the appearance of both ab phonon and quadrupole shape vibration of the cluster with small mixing between the two collective modes. the quadruple ab phonon is similar to that in the uniform superfluid, apart from the small admixture of the shape vibration of cluster. the excitation energy and the collectivity of the cluster vibration mode show a strong and oscillatory dependence on the neutron chemical potential (the neutron gas density), resulting in softening and instability under certain conditions. this is due to the resonance shell effect, where unbound, but resonant single-particle states of neutrons are crucially important. conclusions: the ab phonon of the superfluid neutron gas and the quadrupole shape vibration of the nuclear cluster coexist in the inner crust. the coupling between the ab phonon and the quadrupole shape vibration is weak. the collectivity of the quadrupole shape vibration is controlled by the resonance shell effect, which suggests that the appearance of deformed nuclear clusters is possible in any layer of the inner crust. | coexistence of anderson-bogoliubov phonon and quadrupole cluster vibration in the inner crust of neutron stars |
in the present work we used five different versions of the quark-meson coupling (qmc) model to compute astrophysical quantities related to the gw170817 event and to neutron star cooling process. two of the models are based on the original bag potential structure and three versions consider a harmonic oscillator potential to confine the quarks. the bag-like models also incorporate the pasta phase used to describe the inner crust of neutron stars. we show that the pasta phase always play a minor or negligible role in all studies. moreover, while no clear correlation between the models that satisfy the gw170817 constraints and the slope of the symmetry energy is found, a clear correlation is observed between the slope and the fact that the cooling is fast or slow, i.e., fast (slow) cooling is related to higher (lower) values of the slope. we did not find one unique model that can describe, at the same time, gw170817 constraints and give a perfect description of the possible cooling processes. | neutron star cooling and gw170817 constraint within quark-meson coupling models |
our bimetric spacetime model of glitching pulsars is applied to the remnant of gw170817. accordingly, pulsars are born with embryonic incompressible superconducting gluon-quark superfluid cores (susu-matter) that are embedded in minkowski spacetime, whereas the ambient compressible and dissipative media (cdm) are imbedded in curved spacetime. as pulsars cool down, the equilibrium between both spacetime is altered, thereby triggering the well-observed glitch phenomena. based thereon and assuming all neutron stars (nss) to be born with the same initial mass of $m_{ns}(t=0) \approx 1.25\,\mathcal{m}_{\odot},$ we argue that the remnant of gw170817 should be a relatively faint ns with a hypermassive central core made of susu-matter. the effective mass and radius of the remnant are predicted to be $[2.8 \mathcal{m}_{\odot} < \mathcal{m}_{rem} \le 3.351 \mathcal{m}_{\odot}]$ and $r_{rem}=10.764$ km, whereas the mass of the enclosed susu-core is $\mathcal{m}_{core}=1.7 \mathcal{m}_{\odot}.$ here, about $1/2~ \mathcal{m}_{core}$ is an energy enhancement triggered by the phase transition of the gluon-quark-plasma from the microscopic into macroscopic scale. the current compactness of the remnant is $\alpha_c = 0.918,$ but predicted to increase as the cdm and cools down, rendering the remnant an invisible dark energy object, and therefore to an excellent black hole candidate. | the remnant of gw170817: a trapped neutron star with a massive incompressible superfluid core |
we study the ν ν bar -pair synchrotron emission from electrons and protons in a relativistic quantum approach. this process occurs only in the presence of a strong magnetic field, and it is considered to be one of effective processes for neutron star cooling. in this work we calculate the luminosity of the ν ν bar -pairs emitted from neutron-star-matter with a magnetic field of about 1015 g. we find that the energy loss is much larger than that of the modified urca process. the ν ν bar -pair emission processes in strong magnetic fields is expected to contribute significantly to the cooling of the magnetars. | ν ν bar -pair synchrotron emission in neutron-star matter based on a relativistic quantum approach |
we reanalyse the x-ray spectrum of the psr b0833-45 (the vela pulsar) using the data of the chandra space observatory. in contrast to previous works, we consider a wide range of possible masses and radii of the pulsar. the derived surface temperature of the star ts∞=0.66−0.01+0.04mk (1σ level over the entire mass and radius range of our study) is consistent with earlier results. however, the preferable values of vela's mass and radius given by the spectral analysis are different from those used previously; they are consistent with modern equation of state models of neutron star matter. in addition, we evaluate the vela's surface temperature as a function of assumed values of its mass and radius. this allows us to analyse the neutrino cooling rates consistent with the evaluated surface temperatures and explore the additional restrictions that could be set on the vela's mass and radius using different versions of the neutron star cooling theory. | thermal spectrum and neutrino cooling rate of the vela pulsar |
crust cooling of soft x-ray transients has been observed after outbursts, but an additional shallow heating during accretion in outburst is needed to explain the quiescent light curve. however, shallow heating is significantly different between sources and even within one source between different outbursts, and the source of shallow heat is as yet unknown. using the open source code "dstar" which solves the fully general relativistic heat diffusion equation for the crust, we investigate the effect of magnitude and depth of shallow heating on crust cooling and find that some exceptional sources (swift j174805.3-244637, maxi j0556-332 during outburst ii and gro j1750-27) in which shallow heating may be inactive could be explained by a deeper shallow heating mechanism. we compare our results with those from previous works and find that the shallow heating is model dependent. in addition, the effects of mass and radius of a neutron star on shallow heating are studied, and it is shown that the more compact the star, the less shallow heating will be needed to fit the crust cooling light curves. | crust cooling of soft x-ray transients-the uncertainties of shallow heating |
the influence of short-range correlations (src) on the spectral distribution of neutrons is incorporated in the solution of the gap equation for the ${}^3p_2-{}^3f_2$ coupled channel in pure neutron matter at high density. this effect is studied for three different realistic interactions. the gap in this channel is strongly suppressed by these correlations but does not vanish. for a consistent treatment we also include for the first time the effect of long-range correlations (lrc) by incorporating polarization terms in addition to the bare interaction. this allows the neutrons to exchange density and spin fluctuations governed by the strength of landau parameters with values that are consistent with the available literature. while these lrc have an antiscreening tendency, they only slightly increase the gap in the ${}^3p_2-{}^3f_2$ coupled channel for all three realistic interactions as long as src are included. all three interactions generate maximum gaps around 0.1 to 0.2 mev at most with a small dependence on the hardness of the interaction. these results are relevant for the cooling scenarios of neutron stars, in particular the young neutron star in cassiopeia a. | pairing in high-density neutron matter including short- and long-range correlations |
psr j1119-6127 is a radio pulsar that behaved with magnetar-like bursts, and we performed a comprehensive investigation of this pulsar using the archival high-energy observations obtained after its outburst in 2016 july. after the 2016 outburst, specific regions on the neutron star (ns) surface were heated up to >0.3 and >1 kev from ∼0.2 kev. a hard nonthermal spectral component with a photon index <0.5 related to the magnetospheric emission can be resolved from the nustar spectra above 10 kev. we find that the thermal emitting regions did not cool down and gradually shrank by about 20%-35% 4 months after the outburst. hard x-ray pulsations were detected with nustar immediately after the outburst at a 5σ confidence level and with a background-subtracted pulsed fraction of 40% ± 10%. however, the signal became undetectable after a few days. using fermi data, we found that the gamma-ray emission in 0.5-300 gev was suppressed along with the disappearance of the radio pulsations. this is likely caused by a reconfiguration of the magnetic field. we also discovered that the timing noise evolved dramatically, and the spin-down rate significantly increased after the 2016 glitch. we proposed that postoutburst temporal and spectral behaviors from radio to gamma-ray bands were caused by changes of the magnetosphere structure, pair plasma injection, and the shrinking emission sites on the ns. | investigation of the high-energy emission from the magnetar-like pulsar psr j1119-6127 after the 2016 outburst |
we present the results of a 30 ks xmm-newton observation of the supergiant fast x-ray transient (sfxt) sax j1818.6-1703 - the first in-depth soft x-ray study of this source around periastron. integral observations shortly before and after the xmm-newton observation show the source to be in an atypically active state. over the course of the xmm-newton observation, the source shows a dynamic range of ∼100 with a luminosity greater than 1 × 1035 erg s-1 for the majority of the observation. after an ∼6 ks period of low-luminosity (∼1034 erg s-1) emission, sax j1818.6-1703 enters a phase of fast flaring activity, with flares ∼250 s long, separated by ∼2 ks. the source then enters a larger flare event of higher luminosity and ∼8 ks duration. spectral analysis revealed evidence for a significant change in spectral shape during the observation with a photon index varying from γ ∼ 2.5 during the initial low-luminosity emission phase, to γ ∼ 1.9 through the fast flaring activity, and a significant change to γ ∼ 0.3 during the main flare. the intrinsic absorbing column density throughout the observation (nh ∼ 5 × 1023 cm-2) is among the highest measured from an sfxt, and together with the xmm-newton and integral luminosities, consistent with the neutron star encountering an unusually dense wind environment around periastron. although other mechanisms cannot be ruled out, we note that the onset of the brighter flares occurs at 3 × 1035erg s-1, a luminosity consistent with the threshold for the switch from a radiative-dominated to compton cooling regime in the quasi-spherical settling accretion model. | spectral variation in the supergiant fast x-ray transient sax j1818.6-1703 observed by xmm-newton and integral |
we explore spectral properties of a two-component advective flow around a neutron star. we compute the effects of thermal comptonization of soft photons emitted from a keplerian disc and the boundary layer of the neutron star by the post-shock region of a sub-keplerian flow, formed due to the centrifugal barrier. the shock location xs is also the inner edge of the keplerian disc. we compute a series of realistic spectra assuming a set of electron temperatures of the post-shock region tce, the temperature of the normal boundary layer (nbol) tns of the neutron star and the shock location xs. these parameters depend on the disc and halo accretion rates (\dot{m}d and \dot{m}h, respectively) that control the resultant spectra. we find that the spectrum becomes harder when \dot{m}_h is increased. the spectrum is controlled strongly by tns due to its proximity to the comptonizing cloud since photons emitted from the nbol cool down the post-shock region very effectively. we also show the evidence of spectral hardening as the inclination angle of the disc is increased. | monte carlo simulations of thermal comptonization process in a two-component advective flow around a neutron star |
thermal evolution of neutron stars is studied in the f (r )=r +α r2 theory of gravity. we first review the equations of stellar structure and evolution for a spherically symmetric spacetime plus a perfect fluid at rest. we then present numerical results for the structure of neutron stars using four nucleonic dense matter equations of state and a series of gravity theories for α ranging from zero, i.e., general relativity, up to α ≈1016 cm2. we emphasize properties of these neutron star models that are of relevance for their thermal evolution as the threshold masses for enhanced neutrino emission by the direct urca process, the proper volume of the stellar cores where this neutrino emission is allowed, the crust thickness, and the surface gravitational acceleration that directly impact the observable effective temperature. finally, we numerically solve the equations of thermal evolution and explicitly analyze the effects of altering gravity. we find that uncertainties in the dense matter microphysics, such as the core chemical composition and superfluidity/superconductivity properties, as well as the astrophysical uncertainties on the chemical composition of the surface layers, have a much stronger impact than possible modifications of gravity within the studied family of f (r ) theories. we conclude that within this family of gravity theories, conclusions from previous studies of neutron star thermal evolution are not significantly altered by modification of gravity theory. conversely, this implies that neutron star cooling modeling may not be a useful tool to constrain deviations of gravity from einstein theory unless these are much more radical than in the f (r )=r +α r2 framework. | probing strong field f (r ) gravity and ultradense matter with the structure and thermal evolution of neutron stars |
cowan et al. review how roughly half the elements heavier than iron found in the sun are produced by rapid neutron capture and half by slow neutron capture, the r- and s-processes. in the sun, their relative contribution to individual elemental abundances is well understood, except for the lightest and heaviest elements beyond iron. their contributions are especially uncertain for the heaviest nonradioactive element, lead (pb, z = 82). this is constrained by deriving lead abundances in metal-poor stars. for in the most metal-poor halo stars, strontium and heavier elements are found in the solar r-process proportion; s-process elements appear only at metallicities above one-thirtieth solar. in unevolved metal-poor stars of roughly solar heavy-element content, only two uv pb lines are detectable. four such stars have high-resolution spectra of the strongest line, pb ii at 2203.53 å. roederer et al. analyzed this line in one star, deriving a lead-to-iron abundance ratio 10 times solar. this and its blueshifted profile suggested strong s-process production. this work analyzes the uv spectra of all four stars. calculations including a predicted fe i line blueward of the pb ii line, and assuming the lead abundance scales with r-process abundances, match all four profiles extremely well. a scaled s-process contribution might improve the match to the much lower lead abundance found in the unevolved star analyzed previously, but its s-process excess is modest. an fe ii line blends the other lead line, pb i at 2833.05 å, which constrains the lead abundance only in the coolest star. | the abundance of lead in four metal-poor stars |
we study the long-term thermal evolution of axisymmetric rotating neutron stars in full general relativity. to this aim, we develop nscool 2d rot, a major upgrade to the 1d neutron stars thermal evolution code nscool by page. as a first application of our new code, we address the standard cooling of isolated neutron stars with rotation frequencies up to the mass shedding limit. we investigate the effects of the equation of state (eos) by considering different combinations of core and crust eoss. the results indicate complex time-dependent evolution of temperature distribution throughout the whole volume of the star, and in particular, in the crust. we show that most of that complexity can be attributed to the formation of a heat blob in the crust and the latitude dependence of the heat diffusion timescale through the crust. | standard cooling of rapidly rotating isolated neutron stars in 2d |
we apply the model of subsonic settling accretion on to isolated neutron stars accreting from the interstellar medium (ains). we show that in this regime the expected mean x-ray luminosity from ains turns out to be two to three orders of magnitude as small as the maximum possible bondi value, i.e. 1027-1028 erg s-1. the intrinsically unstable character of settling accretion due to long plasma cooling time leads to regular appearance of x-ray flares with a duration of about 1 h and a maximum luminosity of about the bondi value, ∼1031 erg s-1. this feature can be used to distinguish ains from other dim x-ray sources. with the sensitivity of the forthcoming all-sky x-ray surveys the expected number of the potentially detectable ains can be from a few to 10. | settling accretion on to isolated neutron stars from interstellar medium |
the macroscopic magnetic properties and microscopic magnetic structure of rb2mn3(moo4)3(oh) 2 (space group p n m a ) are investigated by magnetization, heat capacity, and single-crystal neutron diffraction measurements. the compound's crystal structure contains bond-alternating [mn3o11] ∞ chains along the b axis, formed by isosceles triangles of mn ions occupying two crystallographically nonequivalent sites (the mn1 site on the base and mn2 site on the vertex). these chains are only weakly linked to each other by nonmagnetic oxyanions. both superconducting quantum interference device magnetometry and neutron diffraction experiments show two successive magnetic transitions as a function of temperature. on cooling, it transitions from a paramagnetic phase into an incommensurate phase below 4.5 k with a magnetic wave vector near k1=(0 ,0.46 ,0 ) . an additional commensurate antiferromagnetically ordered component arises with k2=(0 ,0 ,0 ) , forming a complex magnetic structure below 3.5 k with two different propagation vectors of different stars. on further cooling, the incommensurate wave vector undergoes a lock-in transition below 2.3 k. the experimental results suggest that the magnetic superspace group is p n m a .1'(0 b 0 ) s 0 s s for the single-k incommensurate phase and is p n'm a (0 b 0 ) 00 s for the two-k magnetic phase. we propose a simplified magnetic structure model taking into account the major ordered contributions, where the commensurate k2 defines the ordering of the c -axis component of the mn1 magnetic moment, while the incommensurate k1 describes the ordering of the a b -plane components of both mn1 and mn2 moments into elliptical cycloids. | complex magnetic order in the decorated spin-chain system rb2mn3(moo4)3(oh) 2 |
aquatic habitats are severely threatened by human activities. for anadromous species, managing freshwater habitats to maximize production of more, larger juveniles could improve resilience to threats in marine habitats and enhance population viability. in some juvenile salmonid habitats, complexity created by large substrates provides resources and reduces competitive interactions, thereby promoting juvenile production. in lowland rivers, which lack large substrates, aquatic plants might provide similar complexity and enhance fish productivity. to test the influence of aquatic plants on juvenile atlantic salmon and sympatric brown trout in a lowland river, we directly manipulated the cover of the dominant macrophyte, ranunculus, in nine sites during summer and autumn for two years. we quantified the abundance, site retention and growth of salmon and trout under high, medium or low ranunculus cover. to investigate the effects of ranunculus cover on feeding opportunities and interspecific competition, we quantified available prey biomass and body size, fish diet composition and compared dietary niche overlap. experimentally increased ranunculus cover supported higher salmon abundance in summer and autumn, and higher site retention and growth of salmon in summer. trout abundance and growth were not influenced by ranunculus cover, but trout site retention doubled in high, relative to low, cover sites. despite the weak effects of ranunculus cover on prey availability, salmon and trout inhabiting high cover sites consumed larger prey and a higher biomass of prey. furthermore, dietary niche overlap was lower in high, relative to low, cover sites, suggesting that abundant ranunculus reduced interspecific competition. this field experiment shows that high ranunculus cover can support more and better growing juvenile salmon, and facilitate foraging and co‑existence of sympatric salmonid species. maintaining or enhancing natural macrophyte cover can be achieved through sympathetic in‑river and riparian vegetation management and mitigating pressures on them, such as sediment inputs and low flows, or through planting. further research should test whether macrophyte cover benefits propagate to subsequent life stages, particularly juvenile overwintering associated with high mortality. this knowledge, in combination with our findings, would further clarify whether beneficial juvenile habitat can improve the viability of at‑risk salmonid populations. overall, our findings suggest that the aims of river restoration might be achieved through promotion of in‑stream aquatic vegetation. | high summer macrophyte cover increases abundance, growth, and feeding of juvenile atlantic salmon |
we study the diversities in the properties of the neutron stars arising due to the different choices for the cross coupling between various mesons, which governs the density dependence of the nuclear symmetry energy in the extended relativistic mean-field (rmf) model. for this purpose, we obtain two different families of the extended rmf model corresponding to different nonlinear cross-coupling terms in the isovector part of the effective lagrangian density. the lowest-order contributions for the δ mesons are also included. the different models within the same family yield wide variation in the value of neutron-skin thickness in the 208pb nucleus. these models are employed to compute the neutron-star properties such as core-crust transition density, radius and red shift at canonical mass ( 1.4 m⊙) , tidal polarizability parameter, and threshold mass required for the enhanced cooling through the direct urca process. most of the neutron-star properties considered are significantly different(10-40%) for the different families of models at a smaller neutron-skin thickness (∼0.15 fm ) in the 208pb nucleus. | diversity of neutron star properties at the fixed neutron-skin thickness of 208pb |
the heat capacity of neutron matter is studied over the range of densities and temperatures prevailing in neutron-star crusts, allowing for the transition to a superfluid phase at temperatures below some critical temperature tsf and including the transition to the classical limit. finite-temperature hartree-fock-bogoliubov equations are solved and compared to existing approximate expressions. in particular, the formula given by levenfish and yakovlev is found to reproduce the numerical results with a high degree of accuracy for temperatures t ≤ tsf. in the non-superfluid phase, t ≥ tsf, the linear approximation is valid only at temperature t ≪ tfn (tfn being the fermi temperature of the neutron gas) which is rarely the case in the shallow layers of the neutron-star's crust. a non-perturbative interpolation between the quantal and the classical regimes is proposed here. the heat capacity, conveniently parametrized solely in terms of tsf, tfn, and the neutron number density nn, can be easily implemented in neutron-star cooling simulations. | heat capacity of low-density neutron matter: from quantum to classical regimes |
we analyzed the influence of the density profile on neutron star cooling by neutrino emission, considering four different equations of state. after interpolated density profiles are obtained from discrete data, we calculate numerically, as a function of the radial distance to the center of the star, the following quantities: the neutron and proton number densities, their fermi momenta, the proton fractions and the neutrino emissivities for two models of neutron stars with masses 1.33m⊙ and 1.4m⊙. for a specific equation of state and considering the effects of the density profile, we calculate the neutrino and photon luminosities and the cooling curve for these two models, taking into account two different possibilities for their particle composition. the photon luminosities obtained are consistent with the measurements of the bolometric luminosities for isolated neutron stars with thermal emission presented in the literature. the cooling curves are in good agreement with empirical data for the surface temperatures observed in several neutron stars. | about the influence of the density profile on neutron star cooling by neutrino emission |
fast radio bursts (frbs) at cosmological distances still hold concealed physical origins. previously liu (2018) proposes a scenario that the collision between a neutron star (ns) and a white dwarf (wd) can be one of the progenitors of non-repeating frbs and notices that the repeating frbs can also be explained if a magnetar formed after such ns-wd merger. in this paper, we investigate this channel of magnetar formation in more detail. we propose that the ns-wd post-merger, after cooling and angular momentum redistribution, may collapse to either a black hole or a new ns or even remains as a hybrid wdns, depending on the total mass of the ns and wd. in particular, the newly formed ns can be a magnetar if the core of the wd collapsed into the ns while large quantities of degenerate electrons of the wd compressed to the outer layers of the new ns. a strong magnetic field can be formed by the electrons and positive charges with different angular velocities induced by the differential rotation of the newborn magnetar. such a magnetar can power the repeating frbs by the magnetic reconnections due to the crustal movements or starquakes. | the remnant of neutron star-white dwarf merger and the repeating fast radio bursts |
the process of a non-equilibrium chemical composition formation during cooling due to neutrino energy loss in the shells of hot, formed neutron stars is considered. a model constructed is to explain the presence of a large quantity of nuclear energy accumulated, which can maintain the x-ray luminosity of such compact objects for a long period of time. the study of the numerically obtained final chemical composition dependence on various parameters of the medium has been carried out. | dynamic model of a non-equilibrium chemical composition formation in the shell of single neutron stars |
a detailed description of the baryon direct urca processes a: n\to p+e+{\bar{ν }}e, b: λ \to p+e+{\bar{ν }}e and c: {\xi }-\to λ +e+{\bar{ν }}e related to the neutron star cooling is given in the relativistic mean field approximation. the contributions of the reactions b and c on the neutrino luminosity are calculated by means of the relativistic expressions of the neutrino energy losses. our results show that the total neutrino luminosities of the reactions a, b and c within the mass range (1.603-2.067) m⊙ ((1.515-1.840) m⊙ for tm1 model) for gm1 model are larger than the corresponding values for neutron star without hyperons. furthermore, although the neutrino emissivity of the reaction a is suppressed with the appearance of the proton 1 s 0 superfluid, the contribution of the reactions b and c can still quicken a massive neutron star cooling. in particular, the reaction c in psr j1614-2230 and j0348+0432 is not suppressed by the proton 1 s 0 superfluid due to the higher threshold density of the reaction c, which will further speed up the two pulsars cooling. supported by the national natural science foundation of china under grant nos. 11447165, 11373047, 11404336 and u1731240, youth innovation promotion association, cas under grant no. 2016056, and the development project of science and technology of jilin province under grant no. 20180520077jh | direct urca processes involving proton 1 s 0 superfluidity in neutron star cooling |
the radiative-condensation instability (rci) in self-gravitating strongly coupled dusty plasma (scdp) is investigated considering the effects of dust thermal velocity and polarization force on the massive dust particulates. in particular, the outer core of the dense neutron star which is supposed to be strongly coupled in nature with temperature t∼107 k and number density n∼1.3×1030 cm-3 is analyzed. the modified generalized hydrodynamic (gh) equations and electron temperature perturbation equation with radiative effects are solved using the linear perturbation method. in the classical hydrodynamic limit, the modified condition of jeans instability owing to radiative condensation, polarization force and dust thermal velocity is obtained. in the kinetic limit, velocity of compressional mode also modifies the condition of jeans instability. the dust thermal velocity and viscoelastic effects have stabilizing whereas polarization force and radiative cooling have destabilizing influence on the growth rate of the jeans instability. the radiative effects stabilize the growth rate of unstable radiative modes. in isobaric mode (short wavelength), the basic condition of radiative instability is obtained which is unaffected due to the presence of polarization force and viscoelastic effects. the radiative cooling time in the outer core of neutron star is estimated and compared with the gravitational free fall time, and it is found that the cooling takes place too fast for self-gravity to be important. | radiative-condensation instability in gravitating strongly coupled dusty plasma with polarization force |
gw170817 was the first detection of a binary neutron star merger via gravitational waves. the event was observed over a wide range of the electromagnetic spectrum, revealing a thermal kilonova dominating the optical signal during the first ∼15 days, and a non-thermal synchrotron emission that has continued to rise ∼200 days post-merger, dominating the radio and x-ray emission. at early times, when the kilonova is still dominant, the synchrotron emitting electrons can efficiently cool by up-scattering the kilonova photos through inverse-compton. yet, the cooling frequency is not observed up to the x-ray band. this can only be explained if the source is moving at least at a mildly relativistic velocity. we find a lower limit on the source's bulk lorentz factor of γ > 2.1 at 9 days. this lower limit is model independent and relies directly on the observed quantities, providing additional robust evidence to the relativistic motion in this event at early times. | cooling off with a kilonova - lower limit on the expansion velocity of gw170817 |
we report the results of a long-term spectral and timing study of the first transient magnetar, xte j1810-197 which was discovered in 2003, when its x-ray luminosity increased ≈ 100 fold. we fit x-ray spectra of all archival x-ray observations using a two-component blackbody model, where the cool component is most likely originating from the whole surface of the neutron star and the hot component is from a much smaller hot spot. we investigate the long-term evolution of the surface emission characteristics via tracing its surface temperature, apparent emitting area and pulsed fraction. we evaluate the pulsed fraction in two energy intervals ( < 1.5 kev and > 1.5 kev) and show that the xte j1810-197 exhibits slightly higher pulsed emission at energies above 1.5 kev. we explore the characteristics of an absorption line detected around 1.1 kev. we find that the absorption feature is highly variable and its profile is asymmetric. to accurately represent this feature, we introduced an asymmetric gaussian profile, and quantified the level of asymmetry of the absorption feature. | variable absorption line in xte j1810-197 |
polish doughnuts (pds) are geometrically thick disks that rotate with super-keplerian velocities in their innermost parts, and whose long and narrow funnels along rotation axes collimate the emerging radiation into beams. in this paper we construct an extremal family of pds that maximize both geometrical thickness and radiative efficiency. we then derive upper limits for these quantities and subsequently for the related ability to collimate radiation. pds with such extreme properties may explain the observed properties of ultraluminous x-ray sources without the need for the black hole masses to exceed ~10 m⊙. however, we show that strong advective cooling, which is expected to be one of the dominant cooling mechanisms in accretion flows with super-eddington accretion rates, tends to reduce the geometrical thickness and luminosity of pds substantially. we also show that the beamed radiation emerging from the pd funnels corresponds to isotropic luminosities that obey lcol ≈ 0.1ṁc2 for ṁ ≫ ṁedd, and not the familiar and well-known logarithmic relation, l ~ lnṁ. | limits on thickness and efficiency of polish doughnuts in application to the ulx sources |
thermal emission of neutron stars in soft x-ray transients (sxts) in a quiescent state is believed to be powered by the heat deposited in the stellar crust due to nuclear reactions during accretion (deep crustal heating paradigm). confronting observations of sxts with simulations helps to verify theoretical models of the dense matter in the neutron stars. usually, such simulations were carried out assuming that the free neutrons and nuclei in the inner crust move together. a recently proposed thermodynamically consistent approach allows for independent motion of the free neutrons. we simulate the thermal evolution of the sxts within the thermodynamically consistent approach and compare the results with the traditional approach and with observations. for the latter, we consider a collection of quasi-equilibrium thermal luminosities of the sxts in quiescence and the observed neutron star crust cooling in sxt mxb 1659-29. we test different models of the equation of state and baryon superfluidity and take into account additional heat sources in the shallow layers of neutron-star crust (the shallow heating). we find that the observed quasi-stationary thermal luminosities of the sxts can be equally well fitted using the traditional and thermodynamically consistent models, provided that the shallow heat diffusion into the core is taken into account. the observed crust cooling in mxb 1659-29 can also be fitted in the frames of both models, but the choice of the model affects the derived parameters responsible for the thermal conductivity in the crust and for the shallow heating. | thermal evolution of neutron stars in soft x-ray transients with thermodynamically consistent models of the accreted crust |
young isolated neutron stars (inss) most commonly manifest themselves as rotationally powered pulsars that involve conventional radio pulsars as well as gamma-ray pulsars and rotating radio transients. some other young ins families manifest themselves as anomalous x-ray pulsars and soft gamma-ray repeaters that are commonly accepted as magnetars, i.e. magnetically powered neutron stars with decaying super-strong fields. yet some other young inss are identified as central compact objects and x-ray dim isolated neutron stars that are cooling objects powered by their thermal energy. older pulsars, as a result of a previous long episode of accretion from a companion, manifest themselves as millisecond pulsars and more commonly appear in binary systems. we use dirichlet process gaussian mixture model (dpgmm), an unsupervised machine learning algorithm, for analysing the distribution of these pulsar families in the parameter space of period and period derivative. we compare the average values of the characteristic age, magnetic dipole field strength, surface temperature, and transverse velocity of all discovered clusters. we verify that dpgmm is robust and provide hints for inferring relations between different classes of pulsars. we discuss the implications of our findings for the magnetothermal spin evolution models and fallback discs. | classification of pulsars with dirichlet process gaussian mixture model |
type-ia supernovas are powerful stellar explosions that provide important distance indicators in cosmology. recently, we proposed a new type-ia supernova mechanism that involves a nuclear fission chain-reaction in an isolated white dwarf [phys. rev. lett. 126, 131101 (2021), 10.1103/physrevlett.126.131101]. here we perform novel reaction network simulations of the actinide-rich first solids in a cooling white dwarf. the network includes neutron-capture and fission reactions on a range of u and th isotopes with various possible values for 235u enrichment. we find, for modest 235u enrichments, neutron capture on 238u and 232th can breed additional fissile nuclei so that a significant fraction of all u and th nuclei may fission during the chain reaction. finally, we compute the energy release from the fission chain reaction for various uranium enrichments; a novel result that is a necessary input for thermal diffusion simulations of carbon ignition. | nuclear fission reaction simulations in compact stars |
we performed penning trap mass measurements for 61zn at the national superconducting cyclotron laboratory and nushellx calculations of the 61zn and 62ga structure using the gxpf1a hamiltonian to obtain improved estimates of the 61zn(p ,γ )62ga and 60cu(p ,γ )61zn reaction rates. surveying astrophysical conditions for type-i x-ray bursts with the code mesa, implementing our improved reaction rates, and taking into account updated nuclear masses for 61v and 61cr from the recent literature, we refine the neutrino luminosity from the important mass number a =61 urca cooling source in accreted neutron-star crusts. this improves our understanding of the thermal barrier between deep heating in the crust and the shallow depths where extra heat is needed to explain x-ray superbursts, as well as the expected signature of crust urca neutrino emission in light curves of cooling transients. | improved nuclear physics near a =61 refines urca neutrino luminosities in accreted neutron star crusts |
a novel mathematical and computational model for the formation and evolution of radiolytic gas in aqueous fissile solutions is presented. the model predicts the rate at which bubbles are formed and/or removed from the system using semi-empirical correlations calibrated by means of numerical simulation. the model is able to reliably predict the behaviour of aqueous fissile solutions, including transient effects due to the formation and removal of radiolytic gas. a further extension to the model enables its application to boiling systems. | a semi-empirical model of radiolytic gas bubble formation and evolution during criticality excursions in uranyl nitrate solutions for nuclear criticality safety assessment |
we report detection of the binary companion to the millisecond pulsar j2302+4442 based on the deep observations performed with the gran telescopio canarias. the observations revealed an optical source with r' = 23.33 ± 0.02 and i' = 23.08 ± 0.02, whose position coincides with the pulsar radio position. by comparing the source colour and magnitudes with the white dwarf cooling predictions, we found that it likely represents an he or co-core white dwarf and estimated its mass of 0.52^{+0.25}_{-0.19} m⊙ and effective temperature of 6300^{+1000}_{-800} k. combining our results with the radio timing measurements, we set constraints on the binary system inclination angle i = 73^{+6}_{-5} deg and the pulsar mass m_ p = 3.1^{+2.7}_{-2.0} m⊙. | optical identification of the binary companion to the millisecond psr j2302+4442 with the gran telescopio canarias |
accreting neutron stars (ns) can exhibit high frequency modulations in their light curves during thermonuclear x-ray bursts, known as burst oscillations. the frequencies can be offset from the spin frequency of the ns by several hz, and can drift by 1-3 hz. one possible explanation is a mode in the bursting ocean, the frequency of which would decrease (in the rotating frame) as the burst cools, hence explaining the drifts. most burst oscillations have been observed during the h/he-triggered bursts; however there has been one observation of oscillations during a superburst; hours long type i x-ray bursts caused by unstable carbon burning deeper in the ocean. this paper calculates the frequency evolution of an oceanic r mode during a superburst. the rotating frame frequency varies during the burst from 4-14 hz and is sensitive to the background parameters, in particular the temperature of the ocean and ignition depth. this calculation is compared to the superburst oscillations observed on 4u-1636-536. the predicted mode frequencies (∼10 hz) would require a spin frequency of ∼592 hz to match observations; 6 hz higher than the spin inferred from an oceanic r-mode model for the h/he-triggered burst oscillations. this model also overpredicts the frequency drift during the superburst by 90 per cent. | superburst oscillations: ocean and crustal modes excited by carbon-triggered type i x-ray bursts |
we investigate the cooling of neutron stars with relativistic and non-relativistic 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 $2 m_\odot$ neutron stars psr j1614-2230 and psr j0343+0432 has triggered the revival of stiff nuclear equation of state at high densities. in the mean time, 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 evenlope 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 |
investigation of the long-term evolution of rotating radio transients (rrats) is important to understand the evolutionary connections between the isolated neutron star populations in a single picture. the x-ray luminosities of rrats (except one source) are not known. in the fallback disc model, we have developed a method to estimate the dipole field strengths of rrats without x-ray information. we have found that rrats could have dipole field strengths, b0, at the poles ranging from ∼7 × 109 to ∼6 × 1011 g which fill the gap between the b0 ranges of central compact objects (ccos) and dim isolated neutron stars (xdins) estimated in the same model. in our model, most of rrats are evolving at ages (∼2-6) × 105 yr, much smaller than their characteristic ages, such that, cooling luminosities of a large fraction of relatively nearby rrats could be detected by the erosita all-sky survey. many rrats are located above the upper border of the pulsar death valley with the fields inferred from the dipole-torque formula, while they do not show strong, continuous radio pulses. the b0 values estimated in our model, place all rrats either into the death valley or below the death line. we have tentatively proposed that rrats could be the sources below their individual death points, and their short radio bursts could be ignited by the disc-field interaction occasionally enhancing the flux of open field lines through the magnetic poles. we have also discussed the evolutionary links between ccos, rrats, and xdins. | on the long-term evolution of rotating radio transients |
in this paper, we studied thermal evolution of isolated neutron stars (nss) including the pion condensation core, with an emphasis on the stiffness of equation of state (eos). many temperature observations can be explained by the minimal cooling scenario which excludes the fast neutrino cooling process. however, several nss are cold enough to require it. the most crucial problem for ns cooling theory is whether the nucleon direct urca (du) process is open. the du process is forbidden if the nucleon symmetry energy is significantly low. hence, another fast cooling process is required in such an eos. as the candidate to solve this problem, we consider the pion condensation. we show that the low-symmetry energy model can account for most cooling observations including cold nss, with strong neutron superfluidity. simultaneously, it holds the 2m⊙ observations even if the pion condensation core exists. thus, we propose the possibility of pion condensation, as an exotic state to solve the problem in low-symmetry energy eoss. we examined the consistency of our eoss with other various observations as well. | cooling of isolated neutron stars with pion condensation: possible fast cooling in a low-symmetry energy model |
superfluid neutron matter is a key ingredient in the composition of neutron stars. the physics of the inner crust are largely dependent on those of its s-wave neutron superfluid, which has made its presence known through pulsar glitches and modifications in neutron star cooling. moreover, with recent gravitational-wave observations of neutron star mergers, the need for an equation of state for the matter of these compact stars is further accentuated and a model-independent treatment of neutron superfluidity is important. ab initio techniques developed for finite systems can be guided to perform extrapolations to the thermodynamic limit and attain this model-independent extraction of various quantities of infinite superfluid neutron matter. to inform such an extrapolation scheme, we performed calculations of the neutron 1s0 pairing gap using model-independent odd–even staggering in the context of the particle-conserving, projected bardeen–cooper–schrieffer (bcs) theory under twisted boundary conditions. while the practice of twisted boundary conditions is standard in solid-state physics and has been used repeatedly in the past to reduce finite-size effects, this is the first time that it has been employed in the context of pairing. we find that a twist-averaging approach results in a substantial reduction of the finite-size effects, bringing systems with n⪆50 within a 2% error margin from the infinite system. this can significantly reduce extrapolation-related errors in the extraction of superfluid neutron matter quantities. | superfluid neutron matter with a twist |
evolution of a large part of low-mass x-ray binaries (lmxbs) leads to the formation of rapidly rotating pulsars with a helium white dwarf (he wd) companion. observations indicate that some he wds in binary pulsar systems are ultracool (with the effective temperatures teff ≲ 4000 k). it is hard to cool down a he wd to such low temperatures within the hubble time because a thick hydrogen envelope was left behind around the he core after the mass transfer process. a possible mechanism that can accelerate the wd cooling is the evaporative wind mass-loss from the he wd driven by the high-energy radiation from the recycled pulsar. in this paper, we evolve a large number of lmxbs and investigate the influence of the pulsar's high-energy radiation on the wd cooling with different input parameters, including the neutron star's spin-down luminosity, the evaporation efficiency, and the metallicity of the companion star. by comparing our results with observations we note that, for relatively hot he wds (with teff > 7000 k), standard wd cooling without evaporation considered is able to reproduce their temperatures, while evaporation is probably required for the he wds with relatively low temperatures (teff < 5000 k). | on the effect of pulsar evaporation on the cooling of white dwarfs |
this paper concerns numerical comparisons between five mathematical models capable of modelling the stochastic behaviour of neutrons in low extraneous (extrinsic or fixed) neutron source applications. these models include analog monte-carlo (amc), forward probability balance equations (fpb), generating function form of the forward probability balance equations (fgf), generating function form of the backward probability balance equations (pál-bell), and an itô calculus model using both an explicit and implicit euler-maruyama discretization scheme. results such as the survival probability, extinction probability, neutron population mean and standard deviation, and neutron population cumulative distribution function have all been compared. the least computationally demanding mathematical model has been found to be the use of the pál-bell equations which on average take four orders of magnitude less time to compute than the other methods in this study. the accuracy of the amc and fpb models have been found to be strongly linked to the computational efficiency of the models. the computational efficiency of the models decrease significantly as the maximum allowable neutron population is approached. the itô calculus methods, utilising explicit and implicit euler-maruyama discretization schemes, have been found to be unsuitable for modelling very low neutron populations. however, improved results, using the itô calculus methods, have been achieved for systems containing a greater number of neutrons. | numerical comparison of mathematical and computational models for the simulation of stochastic neutron kinetics problems |
the many-body theory of asymmetric nuclear matter is developed beyond the brueckner-hartree-fock approximation to incorporate the medium polarization effects. the extension is performed within the babu-brown induced interaction theory. after deriving the particle-hole interaction in the form of landau-migdal parameters, the effects of the induced component on the symmetry energy are investigated along with the screening of 1 s 0 proton-proton and 3 pf 2 neutron-neutron pairing, which are relevant for the neutron-star cooling. the crossover from repulsive (screening) to attractive (anti-screening) interaction going from pure neutron matter to symmetric nuclear matter is discussed. | medium polarization and pairing in asymmetric nuclear matter |
we have observed the neutron star low-mass x-ray binary sax j1810.8-2609 in quiescence with xmm-newton. sax j1810.8-2609 is one of the faintest non-pulsing neutron star low-mass x-ray binaries in quiescence and previously only had upper limits on its quiescent thermal emission. we found sax j1810.8-2609 at the same 0.5-10 kev, unabsorbed luminosity as the previous quiescent observation in 2003, {l}{{x}}=1.5× {10}32 erg s-1. we show that the spectrum requires both thermal and nonthermal components, each contributing approximately half the total emission. the low neutron star luminosity suggests a time-averaged outburst accretion rate of \dot{m}≈ {10}-12 m ⊙ yr-1, in conflict with its observed outburst activity corresponding to a mass accretion rate that is an order of magnitude larger (\dot{m}≈ {10}-11 m ⊙ yr-1). our observation designates sax j1810.8-2609 more firmly as a member of a population of faint quiescent neutron star lmxbs whose quiescent thermal luminosity is not aligned with standard cooling models. | thermal emission in the quiescent neutron star sax j1810.8-2609 |
based on new chandra x-ray telescope data, we present empirical evidence of plasma compton cooling during a flare in the non-pulsating massive x-ray binary 4u1700-37. this behaviour might be explained by quasi-spherical accretion on to a slowly rotating magnetized neutron star (ns). in quiescence, the ns in 4u1700-37 is surrounded by a hot radiatively cooling shell. its presence is supported by the detection of mhz quasi-periodic oscillations likely produced by its convection cells. the high plasma temperature and the relatively low x-ray luminosity observed during the quiescence, point to a small emitting area ∼1 km, compatible with a hotspot on an ns surface. the sudden transition from a radiative to a significantly more efficient compton cooling regime triggers an episode of enhanced accretion resulting in a flare. during the flare, the plasma temperature drops quickly. the predicted luminosity for such transitions, ∼3 × 1035 erg s-1, is very close to the luminosity of 4u1700-37 during quiescence. the transition may be caused by the accretion of a clump in the stellar wind of the donor star. thus, a magnetized ns nature of the compact object is strongly favoured. | evidence of compton cooling during an x-ray flare supports a neutron star nature of the compact object in 4u1700-37 |
context. 4u 1538-52, an absorbed high mass x-ray binary with an orbital period of ~3.73 days, shows moderate orbital intensity modulations with a low level of counts during the eclipse. several models have been proposed to explain the accretion at different orbital phases by a spherically symmetric stellar wind from the companion.aims: the aim of this work is to study both the light curve and orbital phase spectroscopy of this source in the long term. in particular, we study the folded light curve and the changes in the spectral parameters with orbital phase to analyse the stellar wind of qv nor, the mass donor of this binary system.methods: we used all the observations made from the gas slit camera on board maxi of 4u 1538-52 covering many orbits continuously. we obtained the good interval times for all orbital phase ranges, which were the input for extracting our data. we estimated the orbital period of the system and then folded the light curves, and we fitted the x-ray spectra with the same model for every orbital phase spectrum. we also extracted the averaged spectrum of all the maxi data available.results: the maxi spectra in the 2-20 kev energy range were fitted with an absorbed comptonisation of cool photons on hot electrons. we found a strong orbital dependence of the absorption column density but neither the fluorescence iron emission line nor low energy excess were needed to fit the maxi spectra. the variation in the spectral parameters over the binary orbit were used to examine the mode of accretion onto the neutron star in 4u 1538-52. we deduce a best value of ṁ/v∞ = 0.65 × 10-9m⊙ yr-1/ (km s-1) for qv nor. | orbital phase-resolved spectroscopy of 4u 1538-52 with maxi |
we present here the solutions of magnetized accretion flow on to a compact object with hard surface such as neutron stars. the magnetic field of the central star is assumed dipolar and the magnetic axis is assumed to be aligned with the rotation axis of the star. we have used an equation of state for the accreting fluid in which the adiabatic index is dependent on temperature and composition of the flow. we have also included cooling processes like bremsstrahlung and cyclotron processes in the accretion flow. we found all possible accretion solutions. all accretion solutions terminate with a shock very near to the star surface and the height of this primary shock does not vary much with either the spin period or the bernoulli parameter of the flow, although the strength of the shock may vary with the period. for moderately rotating central star, there is possible formation of multiple sonic points in the flow and therefore, a second shock far away from the star surface may also form. however, the second shock is much weaker than the primary one near the surface. we found that if rotation period is below a certain value (p*), then multiple critical points or multiple shocks are not possible and p* depends upon the composition of the flow. we also found that cooling effect dominates after the shock and that the cyclotron and the bremsstrahlung cooling processes should be considered to obtain a consistent accretion solution. | study of magnetized accretion flow with variable γ equation of state |
the cooling of type-i x-ray bursts can be used to probe the nuclear burning conditions in neutron star envelopes. the flux decay of the bursts has been traditionally modelled with an exponential, even if theoretical considerations predict power-law-like decays. we have analysed a total of 540 type-i x-ray bursts from five low-mass x-ray binaries observed with the rossi x-ray timing explorer. we grouped the bursts according to the source spectral state during which they were observed (hard or soft), flagging those bursts that showed signs of photospheric radius expansion (pre). the decay phase of all the bursts were then fitted with a dynamic power-law index method. this method provides a new way of probing the chemical composition of the accreted material. our results show that in the hydrogen-rich sources the power-law decay index is variable during the burst tails and that simple cooling models qualitatively describe the cooling of presumably helium-rich sources 4u 1728-34 and 3a 1820-303. the cooling in the hydrogen-rich sources 4u 1608-52, 4u 1636-536, and gs 1826-24, instead, is clearly different and depends on the spectral states and whether pre occurred or not. especially the hard state bursts behave differently than the models predict, exhibiting a peculiar rise in the cooling index at low burst fluxes, which suggests that the cooling in the tail is much faster than expected. our results indicate that the drivers of the bursting behaviour are not only the accretion rate and chemical composition of the accreted material, but also the cooling that is somehow linked to the spectral states. the latter suggests that the properties of the burning layers deep in the neutron star envelope might be impacted differently depending on the spectral state. | flux decay during thermonuclear x-ray bursts analysed with the dynamic power-law index method |
exploiting a stiff equation of state of the relativistic mean-field model mkvorh ϕ with σ -scaled hadron effective masses and couplings, including hyperons, we demonstrate that the existing neutron-star cooling data can be appropriately described within "the nuclear medium cooling scenario" under the assumption that different sources have different masses. | on cooling of neutron stars with a stiff equation of state including hyperons |
we investigate the thermal stability of optically thin, two-temperature, radiative cooling-dominated accretion disks. our linear analysis shows that the disk is thermally unstable without magnetic fields, which agrees with previous stability analysis on the shapiro-lightman-eardley disk. by taking into account the effects of magnetic fields, however, we find that the disk can be, or partly be, thermally stable. our results may be helpful to understand the outflows in optically thin flows. moreover, such radiative cooling-dominated disks may provide a new explanation of the different behaviors between black hole and neutron star x-ray binaries on the radio/x-ray correlation. | thermal stability of magnetized, optically thin, radiative cooling-dominated accretion disks |
context. during the third all-sky survey (erass3), erosita, the soft x-ray instrument aboard spectrum-roentgen-gamma, detected a new hard x-ray transient, erasst j040515.6 − 745202, in the direction of the magellanic bridge.aims: we arranged follow-up observations and searched for archival data to reveal the nature of the transient.methods: using x-ray observations with xmm-newton, nicer, and swift, we investigated the temporal and spectral behaviour of the source for over about 10 days.results: the x-ray light curve obtained from the xmm-newton observation with an ∼28 ks exposure revealed a type-i x-ray burst with a peak bolometric luminosity of at least 1.4 × 1037 erg s−1. the burst energetics are consistent with a location of the burster at the distance of the magellanic bridge. the relatively long exponential decay time of the burst of ∼70 s indicates that it ignited in a h-rich environment. the non-detection of the source during the other erosita surveys, twelve and six months before and six months after erass3, suggests that the burst was discovered during a moderate outburst which reached 2.6 × 1036 erg s−1 in persistent emission. during the nicer observations, the source showed alternating flux states with the high level at a similar brightness as during the xmm-newton observation. this behaviour is likely caused by dips as also seen during the last hour of the xmm-newton observation. evidence for a recurrence of the dips with a period of ∼21.8 h suggests erasst j040515.6 − 745202 is a low-mass x-ray binary (lmxb) system with an accretion disk seen nearly edge on. we identify a multi-wavelength counterpart to the x-ray source in uvw1 and g, r, i, and z images obtained by the optical/uv monitor on xmm-newton and the dark energy camera at the cerro tololo inter-american observatory. the spectral energy distribution is consistent with radiation from an accretion disk which dominates the uv and from a cool late-type star detected in the optical to infrared wavelengths.conclusions: after the discovery of x-ray bursts in m 31, the magellanic bridge is only the second location outside of the milky way where an x-ray burster was found. the burst uniquely identifies erasst j040515.6 − 745202 as an lmxb system with a neutron star. its location in the magellanic bridge confirms the existence of an older stellar population which is expected if the bridge was formed by tidal interactions between the magellanic clouds, which stripped gas and stars from the clouds. | erasst j040515.6 − 745202, an x-ray burster in the magellanic bridge |
we study the thermal evolution of neutron stars in the presence of hyperons or kaons in the core. our results indicate that the nucleon and hyperon direct urca processes play crucial roles for the cooling of neutron stars. the presence of hyperons drives fast cooling mechanisms in two ways: 1) it allows the hyperon direct urca prior to the nucleon direct urca, 2) and it makes the nucleon direct urca more feasible by reducing the neutron fermi momentum. we found that the neutron star equation of state (eos) with hyperons can be consistent with both mass and temperature observations. we also found that the neutron star eos with kaon condensation can be consistent with observations, even though the cooling behavior is seldom useful to identify or isolate the effect of kaon condensation. | strangeness in neutron star cooling |
in 2017, the be/x-ray transient 4u 0115+63 exhibited a new type ii outburst that was two times fainter than its 2015 giant outburst (in the swift/bat count rates). despite this difference between the two bright events, the source displayed similar x-ray behaviour after these periods. once the outbursts ceased, the source did not transit towards quiescence directly, but was detected about a factor of 10 above its known quiescent level. it eventually decayed back to quiescence over timescales of months. in this paper, we present the results of our swift monitoring campaign, and an xmm-newton observation of 4u 0115+63 during the decay of the 2017 type ii outburst and its subsequent low-luminosity behaviour. we discuss the possible origin of the decaying source emission at this low-level luminosity, which has now been shown as a recurrent phenomenon, in the framework of the two proposed scenarios to explain this faint state: cooling from an accretion-heated neutron star crust or continuous low-level accretion. in addition, we compare the outcome of our study with the results we obtained from the 2015/2016 monitoring campaign on this source. | recurrent low-level luminosity behaviour after a giant outburst in the be/x-ray transient 4u 0115+63 |
we suggest the generalization of the anomalous magneto-hydro-dynamics (amhd) in the chiral plasma of a neutron star (ns) accounting for the mean spin in the ultrarelativistic degenerate electron gas within the magnetized ns core as a continuing source of the chiral magnetic effect. using the mean field dynamo model generalized in amhd, one can obtain the growth of a seed magnetic field up to 1018 g for an old non-superfluid ns at its neutrino cooling era t < 106 yr, while neglecting any matter turbulence within its core and assuming the rigid ns rotation. the application of the suggested approach to the evolution of magnetic fields observed in magnetars, b~ 1015 g, should be self-consistent with all approximations used in the suggested laminar dynamo, at least, up to the jumps of growing fields. | permanent mean spin source of the chiral magnetic effect in neutron stars |
we model a gyroscope that exploits quantum effects in an atomic bose-einstein condensate to gain a tunable enhancement in precision. current inertial navigation systems rely on the sagnac effect using unentangled photons in fibre-optic systems and there are proposals for improving how the precision scales with the number of particles by using entanglement. here we exploit a different route based on sharp resonances associated with quantum phase transitions. by adjusting the interaction between the particles and/or the shape of their trapping potential we are able to tune the width of the resonance and hence the precision of the measurement. here we show how we can use this method to increase the overall sensitivity of a gyroscope by adjusting the system parameters as the measurement proceeds and our knowledge of the rotation improves. we illustrate this with an example where the precision is enhanced by a factor of more than 20 over the case without tuning, after 100 repetitions. metrology schemes with tunable precision based on quantum phase transitions could offer an important complementary method to other quantum-enhanced measurement and sensing schemes. | quantum-enhanced atomic gyroscope with tunable precision |
the outpouring of radiation during an x-ray burst can affect the properties of accretion discs around neutron stars: the corona can cool and collapse, the inner regions can be bled away due to enhanced accretion, and the additional heating will lead to changes in the disc height. in this paper, we investigate whether radiation from bursts can cause the disc to distort through a warping instability. working in the limit of isotropic viscosity and linear growth, we find that bursts are more likely to drive disc warps when they have larger luminosities and longer durations. therefore, warps will be most probable during intermediate-duration bursts (imdbs) and superbursts with evidence for photospheric radius expansion. further, the development of warps depends on the disc viscosity with larger values of α increasing the likelihood of warp growth. we perform time-dependent evolution calculations of the development of warps during type i bursts and imdbs. depending on the initial warp prior to the burst, we find that the burst produces warps at r ≲ 50rg that rapidly grow and decay on second-long time-scales, or ones that grow more slowly and cover a large fraction of the disc. the pulsations of warp at small radii appear to have the properties needed to explain the achromatic fluctuations that have been observed during the tails of some imdbs. the large-scale, slowly growing warps could account for the large reflection strengths and absorbing column densities inferred late in the 4u 1820-30 and 4u 1636-53 superbursts. | radiation-driven warping of accretion discs due to x-ray bursts |
detection of delayed sub-tev photons from gamma-ray bursts (grbs) by magic and hess has proven the promising future of grb afterglow studies with the cherenkov telescope array (cta), the next-generation gamma-ray observatory. with the unprecedented sensitivity of cta, afterglow detection rates are expected to increase dramatically. in this paper, we explore the multidimensional afterglow parameter space to see the detectability of sub-tev photons by cta. we use a one-zone electron synchrotron and synchrotron self-compton model to obtain the spectral energy distribution. we consider bursts going off in a medium of homogenous density. the blast wave is assumed to be radiatively inefficient and evolving adiabatically. considering that the electron acceleration is not efficient if the acceleration time-scale exceeds the radiative cooling time-scale, we find that the sub-tev emission is always due to the self-compton process. we find that jets with high kinetic energy or large bulk lorentz factor decelerating into a dense ambient medium offer better detection prospects for cta. for relatively lower values of the downstream magnetic field, electrons are slow-cooling, and the emitted radiation is positively correlated with the magnetic field. for larger magnetic fields, the electron population enters the fast-cooling phase where the radiated flux is inversely proportional to the magnetic field. we apply our results in the context of bright tev afterglows detected in recent years. our results indicate that cosmological short grbs have only moderate prospects of detection by cta while local neutron star merger counterparts can be detected if the jet is launched towards the observer. | probing gamma-ray burst afterglows with the cherenkov telescope array |
a recent study of a small sample of x-ray binaries (xrbs) suggests a significant softening of spectra of neutron star (ns) binaries as compared to black hole (bh) binaries in the luminosity range 1034-1037 erg s-1. this softening is quantified as an anticorrelation between the spectral index and the 0.5-10 kev x-ray luminosity. we extend the study to significantly lower luminosities (i.e., ∼a few × 1030 erg s-1) for a larger sample of xrbs. we find evidence for a significant anticorrelation between the spectral index and the luminosity for a group of ns binaries in the luminosity range 1032-1033 erg s-1. our analysis suggests a steep slope for the correlation i.e., -2.12 ± 0.63. in contrast, bh binaries do not exhibit the same behavior. we examine the possible dichotomy between ns and bh binaries in terms of a comptonization model that assumes a feedback mechanism between an optically thin hot corona and an optically thick cool source of soft photons. we gauge the ns-bh dichotomy by comparing the extracted corona temperatures, compton-y parameters, and the comptonization amplification factors: the mean temperature of the ns group is found to be significantly lower than the equivalent temperature for the bh group. the extracted compton-y parameters and the amplification factors follow the theoretically predicted relation with the spectral index. | a study of low-mass x-ray binaries in the low-luminosity regime |
the neutrino cooling and gamma heating rates are considered as an important input needed to study the final phases of the evolution of high-mass stars. the weak-interaction mediated processes, namely the β -decay and electron capture, significantly change the lepton to baryon ratio and accelerate the contraction of the core. the emission of resulting neutrinos/antineutrinos tend to cool the stellar core. on the other hand gamma rays are produced because of electron capture and β -decay to excited states in daughter nuclei. these gamma rays heat the core and contribute to increase of entropy which may cause convection to occur. in the present work, the weak-interaction heating and cooling rates on a chain of twenty two isotopes of vanadium having mass in the range 43-64 have been estimated using the proton-neutron quasiparticle random phase approximation theory. the rates have been computed for the temperature ranging from (107-3 ×1010) k and for the density range (10 -1011)g/cm3. our calculated neutrino energy loss rates have also been compared with the previously reported rates calculated using other theoretical models. at high stellar temperatures, our rates are larger by 1-2 orders of magnitude as compared to previous results. | energy rates due to weak decay rates of vanadium isotopes in stellar environment |
nanostructured tas2 with different concentrations of intercalated mn ions has been investigated using ac and dc magnetization measurements. the nanostructures are composed of tapes that are less than a micron wide, which themselves are comprised of tubular structures with an average diameter of about 100 nm. the sample with a concentration x=0.15 of mn exhibited a transition from paramagnetism to a cluster glass state at approximately 8 k. the dynamics of this cluster glass state was better described by the vogel-fulcher-tammann law than by critical slowing down, which suggests that the transition is not a continuous one. the cluster glass state exhibited aging-related memory, which is typically observed in canonical spin glasses and superspin glasses. the sample with x=0.23 underwent a paramagnetic to ferromagnetic transition at 85 k. nonlinear susceptibility measurements suggest that there is another transition close to but somewhat lower than 85 k. with a small measuring field, both zero-field-cooled and field-cooled magnetizations exhibit peaks and dramatic downturns at low temperatures. the field-cooled magnetization also exhibits temperature hysteresis. the unusual behavior of the field-cooled magnetization is due to relaxation and cooling-rate effects. the overall behavior of nanostructured mnxtas2 possesses the hallmarks of competition between clustered magnetic ground states in a disordered magnetic system. | magnetic states in nanostructured manganese-intercalated tas2 |
this study investigates the relativistic neutrino emissivity of the nucleonic and hyperonic direct urca processes in the degenerate baryon matter of neutron stars, within the framework of relativistic mean field theory. in particular, we study the influence of the isovector scalar interaction on the nucleonic and hyperonic direct urca processes by exchanging δ mesons. the results indicate that δ mesons lead to obvious enhancement of the total neutrino emissivity, which must result in a more rapid cooling rate of neutron star matter. supported by natural science foundation of china (11447165, 11373047, 11303063) | effects of δ mesons on baryonic direct urca processes in neutron star matter |
the observations combined with theory of neutron star (ns) cooling play a crucial role in achieving the intriguing information of the stellar interior, such as the equation of state, composition, and superfluidity of dense matter. the traditional ns cooling theory is based on the assumption that the stellar structure does not change with time. the validity of such a static description has not yet been confirmed. we generalize the theory to a dynamic treatment; that is, continuous change of the ns structure (rearrangement of the stellar density distribution with the total baryon number fixed) as the decrease of temperature during the thermal evolution, is taken into account. it is found that the practical thermal energy used for the cooling is slightly lower than that estimated in a static situation, and hence the cooling of nss is accelerated correspondingly but the effect is rather weak. therefore, the static treatment is a good approximation in the calculations of ns cooling. | neutron star cooling with a dynamic stellar structure |
an extension of the standard model (sm) is studied, in which two new vector bosons are introduced, a first boson z' coupled to the sm by the usual minimal coupling, producing an enlarged gauge sector in the sm. the second boson a' field, in the dark sector of the model, remains massless and originates a dark photon γ'. a hybrid mixing scenario is considered based on a combined higgs and stueckelberg mechanisms. in a compton-like process, a photon scattered by a weakly interacting massive particles (wimp) is converted into a dark photon. this process is studied, in an astrophysical application obtaining an estimate of the impact on stellar cooling of white dwarfs and neutron stars. | astrophysical aspects of milli-charged dark matter in a higgs-stueckelberg model |
the be x-ray pulsar (bexrp) sxp 59 underwent a giant outburst in 2017 with a peak x-ray luminosity of 1.1 × 1038 erg s-1. we report on the x-ray behaviour of sxp 59 with the xmm-newton and nustar observations collected at the outburst peak, decay, and the low luminosity states. the pulse profiles are energy dependent, the pulse fraction increases with the photon energy and saturates at 65 per cent above 10 kev. it is difficult to constrain the change in the geometry of emitting region with the limited data. nevertheless, because the pulse shape generally has a double-peaked profile at high luminosity and a single peak profile at low luminosity, we prefer the scenario that the source transited from the super-critical state to the sub-critical regime. this result would further imply that the neutron star (ns) in sxp 59 has a typical magnetic field. we confirm that the soft excess revealed below 2 kev is dominated by a cool thermal component. on the other hand, the nustar spectra can be described as a combination of the non-thermal component from the accretion column, a hot blackbody emission, and an iron emission line. the temperature of the hot thermal component decreases with time, while its size remains constant (r ∼ 0.6 km). the existence of the hot blackbody at high luminosity cannot be explained with the present accretion theories for bexrps. it means that either more sophisticated spectral models are required to describe the x-ray spectra of luminous bexrps, or there is non-dipole magnetic field close to the ns surface. | nustar and xmm-newton observations of sxp 59 during its 2017 giant outburst |
the 1s0 nucleonic superfluids are investigated within the relativistic mean-field model and bardeen-cooper-schrieffer theory in hyperonic neutron stars. the 1s0 pairing gaps of neutrons and protons are calculated based on the reid soft-core interaction as the nucleon-nucleon interaction. in particular, we have studied the influence of degrees of freedom for hyperons on the 1s0 nucleonic pairing gap in neutron star matter. it is found that the appearance of hyperons has little impact on the baryonic density range and the size of the 1s0 neutronic pairing gap; the 1s0 protonic pairing gap also decreases slightly in this region where ρb = 0.0-0.393 fm-3. however, if baryonic density becomes greater than 0.393 fm-3, the 1s0 protonic pairing gap obviously increases. in addition, the possible range for a protonic superfluid is obviously enlarged due to the presence of hyperons. in our results, the hyperons change the 1s0 protonic pairing gap, which must change the cooling properties of neutron stars. supported by the national natural science foundation of china. | singlet pairing gaps of neutrons and protons in hyperonic neutron stars |
the loss of magnetic pressure accompanying the decay of the magnetic field in a magnetar may trigger exothermic electron captures by nuclei in the shallow layers of the stellar crust. very accurate analytical formulas are obtained for the threshold density and pressure, as well as for the maximum amount of heat that can be possibly released, taking into account the landau–rabi quantization of electron motion. these formulas are valid for arbitrary magnetic field strengths, from the weakly quantizing regime to the most extreme situation in which electrons are all confined to the lowest level. numerical results are also presented based on experimental nuclear data supplemented with predictions from the brussels-montreal model hfb-24. this same nuclear model has been already employed to calculate the equation of state in all regions of magnetars. | onset of electron captures and shallow heating in magnetars |
the mutual correlation between the nuclear equation of state (eos) and the bulk properties of neutron stars (ns) is crucial in probing both of them. here, we use eoss of hot n p e (n p e μ ) nuclear matter, based on the density-dependent cdm3y-paris nucleon-nucleon interaction in the nonrelativistic hartree-fock scheme, to investigate the temperature dependence of the core-crust transition properties under β equilibrium, at the inner edge of ns. we use four eoss that provide symmetric nuclear matter saturation incompressibility of 218 and 252 mev, with two parametrizations of the density dependence of the isovector part of the m3y force. we found that the softer eos estimates larger proton fraction in the ns matter and indicates a wider range for direct urca (du) cooling process within the core center of nss. increasing the temperature decreases the density corresponding to the threshold proton fraction for du process, increasing the region for the du process inside nss. the muons decrease the isospin asymmetry of the n p e μ ns matter at its core center, its thermal pressure, and the du threshold density. the muon fraction slightly changes with temperature. a value of about half the proton fraction is inferred for the β -stable muon fraction of hot n p e μ matter, around the core center. based on the four considered eoss, the liquid core-solid crust transition density, pressure, and proton fraction are estimated to increase from (0.54 ±0.02 ) ρ0 , 0.36 ±0.12 mev fm−3 , and 0.03, respectively at t =0 mev , to (0.85 ±0.04 ) ρ0 , 7.36 ±0.52 mev fm−3 , and 0.14, respectively at t =50 mev . | investigation of the inner edge of neutron star crusts: temperature dependence and related effects |
neutron stars offer the opportunity to study the behaviour of matter at densities and temperatures inaccessible to terrestrial experiments. gravitational-wave observations of binary neutron star coalescences can constrain the neutron-star equation of state before and after merger. after the neutron star binary merges, hyperons can form in the remnant, changing the behaviour of the neutron-star equation of state. in this study, we use finite-entropy equations of state to show that a post-merger remnant can spin up due to cooling. the magnitude of the spin-up depends on the neutron-star equation of state. if hyperons are present, the post-merger spin-up changes the peak gravitational-wave frequency by $\sim 540$ hz, when the entropy per baryon drops from $s=2$ $k_b$ to $s=0$ $k_b$. if hyperons are not present, the post-merger spin-up changes by $\sim 360$ hz, providing a gravitational-wave signature for exotic matter. we expect the same qualitative behaviour whenever temperature dependent phase transitions are triggered. | temperature dependent appearance of exotic matter makes nascent neutron stars spin faster |
recent advances in the field of ultracold quantum gases have played an important role in expanding our understanding of strongly correlated quantum matter. these gases are isolated, clean and fully controllable systems, allowing bottom-up engineering of idealized condensed matter models. interacting fermi gases are particularly interesting because of their relevance to understanding systems ranging from high-temperature superconductors to neutron stars. in this thesis, i describe the development of a quantum gas microscope for studying fermi gases of lithium-6 in two dimensions. with this tool, we can probe 2d systems containing over a thousand fermions and measure the spin or density on each site as well as n-point correlations of these quantities. the design of our microscope introduces several new simplifying features, including a novel raman cooling scheme for imaging that does not require confining the atoms in the lamb-dicke regime in all directions. i report on two experiments we have performed using this instrument. first i present an exploration of attractive spin-imbalanced gases in two dimensions. we observe in-trap phase separation characterized by the appearance of a spin-balanced core surrounded by a polarized gas. in addition, we observe pair condensation in momentum-space measurements even for large polarizations where phase separation vanishes, indicating the presence of a polarized pair condensate. in a second experiment, we explore fermions in an optical lattice, described by the fermi-hubbard model. compared to the repulsive model, the attractive model has received less experimental attention despite its rich phase diagram, including a possible fflo phase in the polarized system. using the microscope, we directly image charge density wave correlations in our system and use them to put a lower bound on pairing correlations. we also demonstrate that these correlations constitute a sensitive thermometer that might be useful in the development of future cooling schemes. these initial explorations with our fermion quantum gas microscope set the stage for future work that might shed insights on a wide variety of condensed matter problems, ranging from the microscopic mechanisms for pairing in high-temperature superconductors to cooper pairing at non-zero momentum in large magnetic fields. | exploring attractively interacting fermions in 2d using a quantum gas microscope |
glitches, sudden spin-up of pulsars with subsequent recovery, provide us with a unique opportunity to investigate various physical processes, including the crust-core coupling, distribution of reservoir angular momentum within different internal layers, spin-up in neutral and charged superfluids and constraining the equation of state of the neutron star (ns) matter. in this work, depending on the dynamic interaction between the vortex lines and the nuclei in the inner crust, and between the vortex lines and the magnetic flux tubes in the outer core, various types of relaxation behavior are obtained and confronted with the observations. it is shown that the glitches have strong potential to deduce information about the cooling behavior and interior magnetic field configuration of nss. some implications of the relative importance of the external spin-down torques and the superfluid internal torques for recently observed unusual glitches are also discussed. | glitches as probes of neutron star internal structure and dynamics: effects of the superfluid-superconducting core |
for the first time, we present a bayesian time-resolved spectral study of the x-ray afterglow datasets of gw170817/grb17017a observed by the chandra x-ray observatory. these include all 12 public datasets, from the earliest observation taken at t ∼ 9 d to the newest observation at ∼359 d post-merger. while our results are consistent with the other works using cash statistic within uncertainty, the bayesian analysis we performed in this work have yielded gaussian-like parameter distributions. we also obtained the parameter uncertainties directly from their posterior probability distributions. we are able to confirm that the power-law photon index has remained constant of γ ∼ 1.6 throughout the entire year-long observing period, except for the first dataset observed at t = 8.9 d when γ = 1.04 ± 0.44 is marginally harder. we also found that the unabsorbed x-ray flux peaked at t ∼ 155 d, temporally consistent with the x-ray flare model suggested recently by piro et al. (2019). the x-ray flux has been fading since ∼160 days after the merger and has returned to the level as first discovered after one year. our result shows that the x-ray spectrum of gw170817/grb170817a is well-described by a simple power-law originated from non-thermal slow-cooling synchrotron radiation. | bayesian analysis on the x-ray spectra of the binary neutron star merger gw170817 |
we discuss the effect of compression on urca shells in the ocean and crust of accreting neutron stars, especially in superbursting sources. we find that urca shells may be deviated from chemical equilibrium in neutron stars which accrete at several tenths of the local eddington accretion rate. the deviation depends on the energy threshold of the parent and daughter nuclei, the transition strength, the temperature, and the local accretion rate. in a typical crust model of accreting neutron stars, the chemical departures range from a few tenths of kbt to tens of kbt for various urca pairs. if the urca shell can exist in crusts of accreting neutron stars, compression may enhance the net neutrino cooling rate by a factor of about 1 ∼2 relative to the neutrino emissivity in chemical equilibrium. for some cases, such as urca pairs with small energy thresholds and/or weak transition strength, the large chemical departure may result in net heating rather than cooling, although the released heat can be small. strong urca pairs in the deep crust are hard to be deviated even in neutron stars accreting at the local eddington accretion rate. | net reaction rate and neutrino emissivity for the urca process in departure from chemical equilibrium |
strongly magnetized isolated neutron stars (nss) are categorized into two families according, mainly, to their magnetic field strength. the one with a higher magnetic field of 1014–1015 gauss is called "magnetar," and the other is the x‑ray isolated neutron star (xins) with 1013 gauss. both magnetars and xinss show thermal emission in x‑rays, whose spectra are different. the soft x‑ray spectrum (below 10 kev) of a magnetar is reproduced with a two‑temperature blackbody (2bb), whereas that of an xins shows only a single‑temperature blackbody (1bb), and its temperature is even lower than that of magnetars. on the basis of the magnetic field and temperature, it is often speculated that xinss may be old and cooled magnetars. however, no other strong observational evidence has yet been reported to support the speculation. here, we report that all the seven known xinss show high‑temperature emission, which should have a similar origin to that of magnetars. analyzing all the xmm‑newton data of the xinss with the highest statistics ever achieved, we find that their x‑ray spectra are all reproduced with a 2bb model, similar to magnetars, as opposed to the traditional 1bb model. their emission radii and temperature ratios are also similar to those of magnetars except for two xinss, which show significantly smaller radii than the others. the remarkable similarity in the x‑ray spectra between xinss and magnetars suggests that the origins of their emitting regions are also the same. the lower temperature in xinss can be explained if xinss are older than magnetars. therefore, this result is another observational indication that supports the standard hypothesis of classification of highly magnetized nss. this article is based on our paper yoneyama et al. (2019; accepted to pasj). | unification of strongly magnetized neutron stars with regard to x‑ray emission from hot spots |
we present the results of our extensive search using the bayesian block method for long tails following short bursts from a magnetar, sgr j1550-5418, over all rxte observations of the source. we identified four bursts with extended tails, most of which occurred during its 2009 burst active episode. the durations of tails range between ∼13 s and over 3 ks, which are much longer than the typical duration of bursts. we performed detailed spectral and temporal analyses of the burst tails. we find that the spectra of three tails show a thermal nature with a trend of cooling throughout the tail. we compare the results of our investigations with the properties of four other extended tails detected from sgr 1900+14 and sgr 1806-20 and suggest a scenario for the origin of the tail in the framework of the magnetar model. | burst tails from sgr j1550-5418 observed with the rossi x-ray timing explorer |
the defining trait of magnetars, the most strongly magnetized neutron stars (nss), is their transient activity in the x/γ-bands. in particular, many of them undergo phases of enhanced emission, the so-called outbursts, during which the luminosity rises by a factor ~10-1000 in a few hours to then decay over months/years. outbursts often exhibit a thermal spectrum, associated with the appearance of hotter regions on the surface of the star, which subsequently change in shape and cool down. here we simulate the unfolding of a sudden, localized heat injection in the external crust of an ns with a 3d magnetothermal evolution code, finding that this can reproduce the main features of magnetar outbursts. a full 3d treatment allows us to study for the first time the inherently asymmetric hot spots that appear on the surface of the star as the result of the injection and to follow the evolution of their temperature and shape. we investigate the effects produced by different physical conditions in the heated region, highlighting in particular how the geometry of the magnetic field plays a key role in determining the properties of the event. | three-dimensional magnetothermal simulations of magnetar outbursts |
background: neutrino-pair bremsstrahlung processes from nucleon-nucleon scattering n n ν ν ¯ (n n ν ν ¯ ,p p ν ν ¯ , and n p ν ν ¯ ) have recently attracted attention in studies of neutrino emission in neutron stars, because of the implications for the neutron star cooling. the calculated n n ν ν ¯ emissivities within the neutron star environment are relatively insensitive to the two-nucleon dynamical model used in the calculations, but differ significantly from those obtained using an one-pion-exchange (ope) model. purpose: we investigate the free n n ν ν ¯ cross sections using a realistic nucleon-nucleon scattering amplitude, comparing the relative sizes of the cross sections for the three processes n n ν ν ¯ ,p p ν ν ¯ , and n p ν ν ¯ . method: we employ a realistic one-boson-exchange (robe) model for n n scattering and combine those strong scattering amplitudes with the well-known nucleon weak interaction vertices to construct weak bremsstrahlung amplitudes. using the resulting n n ν ν ¯ amplitudes we investigate the relative importance of the vector (γvμ) , axial vector (γaμ) , and tensor (γtμ) terms. the robe model bremsstrahlung amplitudes are also used as a two-nucleon dynamical model with which we calculate the cross sections d/σ d ω for n n ν ν ¯ ,p p ν ν ¯ , and n p ν ν ¯ . results: the three free n n ν ν ¯ cross sections d/σ d ω are of similar order of magnitude. each increases with increasing neutrino-pair energy ω . for the neutrino-pair energy of ω =1 mev our n n ν ν ¯ results are in quantitative agreement with those previously reported by timmermans et al. [phys. rev. c 65, 064007 (2002), 10.1103/physrevc.65.064007], who used the leading-order term of the soft-neutrino-pair bremsstrahlung amplitude to calculate the cross sections. differences between the n n ν ν ¯ and p p ν ν ¯ cross section are not discernible over the nucleon-nucleon incident energy region considered, due to the complete dominance of the axial vector component of the weak interaction nucleon vertex function γμ as demonstrated analytically in appendix a. the n p ν ν ¯ cross section is smaller than either the n n ν ν ¯ or the p p ν ν ¯ cross section for low to moderate values of ω ; this characteristic only changes at larger neutrino-pair energies around ω ∼50 mev , which is above the low energy region characterized in appendix b. conclusions: the free n n ν ν ¯ cross sections, calculated using a realistic nucleon-nucleon amplitude model, are new except for the n n ν ν ¯ cross section at ω =1 mev that was first reported by timmermans et al., and at ω =0.5 ,1 ,2 mev by li et al. [phys. rev. c 80, 035505 (2009), 10.1103/physrevc.80.035505]. the n n ν ν ¯ and p p ν ν ¯ cross sections are virtually identical in magnitude. all three n n ν ν ¯ processes are dominated by the axial vector component of the vertex function γμ, with only slight deviations from this behavior being seen in the n p ν ν ¯ process at large neutrino-pair energies. | neutrino-pair bremsstrahlung from nucleon-nucleon scattering |
this paper reviews the properties of neutron stars based on the recent multi-messenger observations including electromagnetic waves from the low-mass x-ray binaries and gravitational waves from the merger of neutron star binaries. based on these observations, we investigate theoretical models for dense nuclear matter and discuss their implications to the neutron star observations such as mass, radius, cooling, and tidal deformability. we also discuss the uncertainties in the neutron star cooling, neutron star properties with bayesian approaches, and an expansion scheme applied to the nuclear energy density functional theory. | neutron star equations of state and their applications |
in the relativistic mean field theory and cooling theories, relativistic correction on neutrino emission from neutron stars in four typical nuclear parameter sets, gm1, gl85, gps250 and gps300 is studied. results show that relativistic effect makes the neutrino emissivity, neutrino luminosity and cooling rate lower, compared with the non-relativistic case. and the influence of relativistic effect grows with the mass of the neutron star. gps300 set leads to the biggest fall in neutrino emissivity, whereas gm1 set leads to the largest disparity in cooling rate caused by relativistic effect. supported in part by national natural science foundation of china under grant nos. 11265009, 11175077, 11271055, and general project of liaoning provincial department of education under grant no. l2015005 | relativistic correction on neutrino emission from neutron stars in various parameter sets |
numerical simulations of radiative two-temperature hot accretion flows (hafs) around neutron stars (nss) are performed. we assume that all of the energy carried by the haf around a ns will be thermalized and radiated out at the surface of the ns. the thermal photons will propagate outwards radially and cool the haf via comptonization. we define $\dot{m}$ as the mass accretion rate at the surface of the central object in unit of eddington accretion rate ( ${\dot{m}}_{\mathrm{edd}}=10{l}_{\mathrm{edd}}/{c}^{2}$ , with ledd and c being eddington luminosity and speed of light, respectively). when $\dot{m}$ is lower than ∼10-4, the cooling of the haf is not important and outflows are very strong. when $\dot{m}\gt \sim {10}^{-3}$ , cooling becomes important and outflows are significantly weak. in the range ${10}^{-4}\lt \dot{m}\lt {10}^{-3}$ , the hafs transients from a strong outflow phase to a very weak outflow phase with increase of $\dot{m}$ . the properties of the haf around a ns are also compared with those of the haf around a bh. we find that with a similar $\dot{m}$ , the dynamical properties of the haf around a ns are quite similar as those of the haf around a bh. however, the emitted spectrum of a haf around a ns can be quite different from that of a haf around a bh due to the presence of a thermal soft x-ray component coming from the surface of the ns. | two-temperature radiative hot accretion flow around neutron stars |
we present our broadband (2-250 kev) time-averaged spectral analysis of 388 bursts from sgr j1550-5418, sgr 1900+14, and sgr 1806-20 detected with the rossi x-ray timing explorer (rxte) here and as a database in a companion web-catalog. we find that two blackbody functions (bb+bb), the sum of two modified blackbody functions (lb+lb), the sum of a blackbody function and a power-law function (bb+po), and a power law with a high-energy exponential cutoff (compt) all provide acceptable fits at similar levels. we performed numerical simulations to constrain the best fitting model for each burst spectrum and found that 67.6% of burst spectra with well-constrained parameters are better described by the comptonized model. we also found that 64.7% of these burst spectra are better described with the lb+lb model, which is employed in the spectral analysis of a soft gamma repeater (sgr) for the first time here, than with the bb+bb and bb+po models. we found a significant positive lower bound trend on photon index, suggesting a decreasing upper bound on hardness, with respect to total flux and fluence. we compare this result with bursts observed from sgr and axp (anomalous x-ray pulsar) sources and suggest that the relationship is a distinctive characteristic between the two. we confirm a significant anticorrelation between burst emission area and blackbody temperature, and find that it varies between the hot and cool blackbody temperatures differently than previously discussed. we expand on the interpretation of our results in the framework of a strongly magnetized neutron star. | broadband spectral investigations of magnetar bursts |
we study the evolution of close binary systems in order to account for the existence of the recently observed binary system containing the most massive millisecond pulsar ever detected, psr j0740+6620, and its ultra-cool helium white dwarf companion. in order to find a progenitor for this object we compute the evolution of several binary systems composed by a neutron star and a normal donor star employing our stellar code. we assume conservative mass transfer. we also explore the effects of irradiation feedback on the system. we find that irradiated models also provide adequate models for the millisecond pulsar and its companion, so both irradiated and non irradiated systems are good progenitors for psr j0740+6620. finally, we obtain a binary system that evolves and accounts for the observational data of the system composed by psr j0740+6620 (i.e. orbital period, mass, effective temperature and inferred metallicity of the companion, and mass of the neutron star) in a time scale smaller than the age of the universe. in order to reach an effective temperature as low as observed, the donor star should have an helium envelope as demanded by observations. | binary evolution leading to the formation of the very massive neutron star in the j0740+6620 binary system |
we briefly review the role played by hyperons in determining the properties of neutron and proto-neutron stars. in particular, we review the so-called "hyperon puzzle", i.e., the problem of strong softening of the equation of state (eos) of dense matter due to the presence of hyperons which leads to maximum masses of compact stars that are not compatible with the recent observations of ∼2m⊙ millisecond pulsars. we discuss some of the solutions that were proposed to tackle this problem. finally, we re-examine the influence of hyperons on the cooling of newly born neutron stars as well as on the development of the r-mode instability. | do hyperons exist in the neutron star interior? |
isolated pulsars are ideal sources for polarisation studies since the radiation from pulsars generally covers a wide range of wavelengths from radio to very-high-energy γ-rays and the pulsed emission is closely tied to the local magnetic field direction. although radio polarisation data exist for many pulsars, polarisation measurements at higher energies exist for only six sources. all six of these have optical polarisation measurements while only the crab pulsar has measured polarisation in the x-ray band. one member of a small group of pulsars only visible at optical and x-ray wavelengths, thought to be powered by thermal emission from the cooling neutron star surface, has detected optical polarisation. this chapter will review the current multi-wavelength polarisation observations and the theoretical modelling by which we can probe the pulsar magnetosphere structure and emission mechanisms. | multi-wavelength polarimetry of isolated pulsars |
analytical expressions for the specific heat of superfluid fermi liquids (sfls) with anisotropic spin-triplet p-wave pairing of the 3he-a type have been obtained based on a generalised fermi-liquid approach both at the low temperatures of 0 < t ≪tc0 (n) and near the temperature of the phase transition [tc0 (n)] from a normal to a superfluid phase in the absence of a magnetic field. apart from liquid 3he-a, we also study dense superfluid neutron matter (snm) with anisotropic spin-triplet p-wave pairing (similar to 3he-a) at subnuclear (n < n0, where n0 = 0.17 fm-3 is the nuclear density) and supernuclear densities (n > n0) in view of generalised skyrme forces (with additional terms depending on the density n). at 0 < t ≪tc0 (n), we have obtained an asymptotic expansion for the specific heat csnm(sfl)(t,n), in which, apart from the main term ∼ t 3 (known for 3he-a in the limit of t → 0) there is an additional correction term (∼ t5), which may reach several percent of the contribution from the main term to the decomposition of the specific heat of snm (or sfls). an analytical formula has also been derived for calculating the specific heat csnm(sfl)(t,n) at temperatures near tc0(n). the expressions obtained for the csnm(t,n) functions (valid for an arbitrary parametrisation of the effective skyrme interaction in neutron matter) have been defined for snm with the generalised bsk21 parametrisation of skyrme forces. in addition, dependency graphs were plotted for the specific heat cbsk21(t, y) in the reduced temperature range of 0 < t ≡ t/tc0(n) ≪ 1 and at t ≲ 1 for dense snm (at 0.1 ≤ y ≡ n/n0 ≤ 1.7). these results may be of interest for neutron star physics in connection with neutron star cooling (in the presence of neutron superfluidity with anisotropic spin-triplet p-wave cooper pairing in the outer part of dense liquid neutron star cores). | analytical derivation of expressions for the specific heat of superfluid fermi liquids with anisotropic spin-triplet p-wave pairing at finite temperatures |
within the "nuclear medium cooling" scenario of neutron stars all reliably known temperature - age data, including those of the central compact objects in the supernova remnants of cassiopeia a and xmmu-j1732, can be comfortably explained by a set of cooling curves obtained by variation of the star mass within the range of typical observed masses. the recent measurements of the high masses of the pulsars psr j1614-2230 and psr j0348-0432 on the one hand, and of the low masses for psr j0737-3039b and the companion of psr j1756-2251 on the other, provide independent proof for the existence of neutron stars with masses in a broad range from $\sim 1.2$ to 2 $m_\odot$. the values $m>2 m_{\odot}$ call for sufficiently stiff equations of state for neutron star matter. we investigate the response of the set of neutron star cooling curves to a stiffening of the nuclear equation of state so that maximum masses of about $2.4 m_\odot$ would be accessible and to a deconfinement phase transition from such stiff nuclear matter in the outer core to color superconducting quark matter in the inner core. without readjustment of cooling inputs the mass range required to cover all cooling data for the stiff dd2 equation of state should include masses of $2.426 m_\odot$ for describing the fast cooling of casa while the existence of a quark matter core accelerates the cooling so that casa cooling data are described with a hybrid star of mass $1.674 m_\odot$. | cooling of neutron stars and hybrid stars with a stiff hadronic eos |
in accreting neutron star x-ray transients, the neutron star crust can be substantially heated out of thermal equilibrium with the core during an accretion outburst. the observed subsequent cooling in quiescence (when accretion has halted) offers a unique opportunity to study the structure and thermal properties of the crust. initially crust cooling modelling studies focussed on transient x-ray binaries with prolonged accretion outbursts (> 1 year) such that the crust would be significantly heated for the cooling to be detectable. here we present the results of applying a theoretical model to the observed cooling curve after a short accretion outburst of only ~10 weeks. in our study we use the 2010 outburst of the transiently accreting 11 hz x-ray pulsar in the globular cluster terzan 5. observationally it was found that the crust in this source was still hot more than 4 years after the end of its short accretion outburst. from our modelling we found that such a long-lived hot crust implies some unusual crustal properties such as a very low thermal conductivity (> 10 times lower than determined for the other crust cooling sources). in addition, we present our preliminary results of the modelling of the ongoing cooling of the neutron star in mxb 1659-298. this transient x-ray source went back into quiescence in march 2017 after an accretion phase of ~1.8 years. we compare our predictions for the cooling curve after this outburst with the cooling curve of the same source obtained after its previous outburst which ended in 2001. | a window into the neutron star: modelling the cooling of accretion heated neutron star crusts |
we perform for the first time hydrodynamic simulations to study the properties of hot accretion flow (haf) around a neutron star (ns). the energy carried by the haf will eventually be radiated out at the surface of the ns. the emitted photons can propagate inside the haf and cool the haf via comptonization. we find that the compton cooling can affect the properties of the haf around an ns significantly. we define the eddington accretion rate as {\dot{m}}edd}=10{l}edd}/{c}2, with l edd and c being the eddington luminosity and the speed of light, respectively. we define \dot{m} as the mass accretion rate at the ns surface in units of {\dot{m}}edd}. when \dot{m}> {10}-4, compton cooling can effectively cool the haf and suppress wind. therefore, the mass accretion rate is almost a constant with radius. the density profile is ρ ∝ r -1.4. when \dot{m}< {10}-4, the compton cooling effects become weaker, wind becomes stronger, and accretion rate is proportional to r 0.3-0.5. consequently, the density profile becomes flatter, ρ \propto {r}-1∼ -0.8. when \dot{m}< {10}-6, the compton cooling effects can be neglected. we find that with the same accretion rate, the temperature of the haf around an ns is significantly lower than that of the haf around a black hole (bh). also, the compton y-parameter of the haf around an ns is significantly smaller than that of the haf around a bh. this result predicts that the haf around an ns will produce a softer spectrum compared to the haf around a bh, which is consistent with observations. | hot accretion flow around neutron stars |
we present two recent parametrizations of the equation of state (fsu2r and fsu2h models) that reproduce the properties of nuclear matter and finite nuclei, fulfill constraints on high-density matter stemming from heavy-ion collisions, produce 2$m_{\odot}$ neutron stars, and generate neutron star radii below 13 km. making use of these equations of state, cooling simulations for isolated neutron stars are performed. we find that two of the models studied, fsu2r (with nucleons) and, in particular, fsu2h (with nucleons and hyperons), show very good agreement with cooling observations, even without including nucleon pairing. this indicates that cooling observations are compatible with an equation of state that produces a soft nuclear symmetry energy and, thus, generates small neutron star radii. nevertheless, both schemes produce cold isolated neutron stars with masses above $1.8 m_{\odot}$. | the equation of state and cooling of hyperonic neutron stars |
the large magellanic cloud supernova remnant j0453-6655 in the h ii region n4 has been observed with xmm-newton and with chandra. almost all of the diffuse x-ray emission is from within a 3.‧5 × 2.‧3 region of bright optical filaments, and x-ray results from xmm-newton and chandra are very similar. spectra indicate that the plasma is close to equilibrium and that the remnant is likely the result of a type ii supernova powered by the collapse of a <12 m ⊙ star. composition, density, and mass of the x-ray-emitting plasma are derived, and the age of the remnant is estimated to be 60-80 kyr. there is an apparent blowout of energy into an adjoining cavity. because cooling neutron stars are objects of interest, the surrounding sky is searched for possible compact remnants of the core. there are six point-like sources within the remnant, all rather faint. two are probably background active galactic nuclei. the others have soft spectral components and no ir counterparts, but, with the present data, none can be proved to be a neutron star. they do represent upper limits to any neutron star luminosity. | j0453-6655, the supernova remnant in n4d |
several phenomena occurring in neutron stars are affected by the elementary excitations that characterize the stellar matter. in particular, low-energy excitations can play a major role in the emission and propagation of neutrinos, neutron star cooling, and transport processes. in this paper, we consider the elementary modes in the star region where both proton and neutron components are superfluid. we study the overall spectral functions of protons, neutrons, and electrons on the basis of the coulomb and nuclear interactions. this paper is performed in the framework of the random-phase approximation, generalized to superfluid systems. the formalism we use ensures that the generalized ward's identities are satisfied. we focus on the coupling between neutrons and protons. on one hand, this coupling results in collective modes that involve simultaneously neutrons and protons; on the other hand, it produces a damping of the excitations. both effects are especially visible in the spectral functions of the different components of the matter. at high densities while the neutrons and protons tend to develop independent excitations as indicated by the spectral functions, the neutron-proton coupling still produces a strong damping of the modes. | coupling between superfluid neutrons and superfluid protons in the elementary excitations of neutron star matter |
i consider differential rotation, associated with radiation-driven chandrasekhar-friedman-schutz (cfs) instability, and respective observational manifestations. i focus on the evolution of the apparent spin frequency, which is typically associated with the motion of a specific point on the stellar surface (e.g. polar cap). i start from long-term evolution (on the time-scale when instability significantly changes the spin frequency). for this case, i reduce the evolution equations to one differential equation and i demonstrate that it can be directly derived from energy conservation law. this equation governs the evolution rate through a sequence of thermally equilibrium states and it provides linear coupling for the cooling power and rotation energy losses via gravitational wave emission. in particular, it shows that differential rotation does not affect long-term spin-down. in contrast, on short time-scales, differential rotation can significantly modify the apparent spin-down, if we examine a strongly unstable star with a very small initial amplitude for the unstable mode. this statement is confirmed by considering a newtonian non-magnetized perfect fluid and dissipative stellar models as well as a magnetized stellar model. for example, despite the fact that the widely applied evolution equations predict effective spin to be constant in the absence of dissipation, the cfs-unstable star should be observed as spinning-down. however, the effects of differential rotation on apparent spin-down are negligible for realistic models of neutron star recycling, unless the neutron star is non-magnetized, the r-mode amplitude is modulated faster than the shear viscosity dissipation time-scale, and the amplitude is large enough that spin-down can be measured on a modulation time-scale. | long-term evolution of cfs-unstable neutron stars and the role of differential rotation on short time-scales |
the neutron star ocean is a plasma of ions and electrons that extends from the base of the neutron star’s envelope to a depth where the plasma crystallizes into a solid crust. during an accretion outburst in an x-ray transient, material accumulates in the envelope of the neutron star primary. this accumulation compresses the neutron star’s outer layers and induces nuclear reactions in the ocean and crust. accretion-driven heating raises the ocean’s temperature and increases the frequencies of g-modes in the ocean; when accretion halts, the ocean cools and ocean g-mode frequencies decrease. if the observed low-frequency quasi-periodic oscillations on accreting neutron stars are g-modes in the ocean, the observed quasi-periodic oscillation frequencies will increase during the outburst—reaching a maximum when the ocean temperature reaches steady state—and subsequently decrease during quiescence. for time-averaged accretion rates during outbursts between < \dot{m}> =0.1{--}1.0 {\dot{m}}{edd} the predicted g-mode fundamental n = 1 l = 2 frequency is between ≈3-7 hz for slowly rotating neutron stars. accreting neutron stars that require extra shallow heating, such as the z-sources maxi j0556-332, mxb 1659-29, and xte j1701-462, have predicted g-mode fundamental frequencies between ≈3-16 hz. therefore, observations of low-frequency quasi-periodic oscillations between ≈ 8{--}16 {hz} in these sources, or in other transients that require shallow heating, will support a g-mode origin for the observed quasi-periodic oscillations. | ocean g-modes on transient neutron stars |
psr b0656+14 is a middle-aged rotation-powered pulsar and a member of the “three musketeers,” which also includes geminga and psr b1055-52. these pulsars show complex, multi-component pulsed x-ray emission best fit with two thermal spectra plus a power law. psr b0656+14 has been extensively observed by nicer since its launch in 2017, obtaining precise dependence of spectral features with pulse phase. the nicer data show three distinct hot spots that cover different energy bands and rotational phases - a cool thermal radiation component from the entire neutron star surface, a smaller hot spot presumably from polar cap heating, and a more mysterious "spot" of intermediate temperature. we see that the x-ray emission peaks from these hot spots occur at different rotation phases, that are also different from the phases of the radio and gamma-ray peaks. the complex variation of temperature across the surface possibly suggests evolution of multipolar magnetic field structure. | nicer observations of the thermal emission from psr b0656+14 |
probing quark matter is one of the important tasks in the studies of neutron stars (ns). some works explicitly consider the existence of quark matter in the appearance of hybrid star (hs) or pure quark star (qs). in the present work, we study the roto-chemical heating with accretion in hs and qs, and compare their chemical evolution and cooling features with pure ns. different from hs and ns, there are two jumps in the chemical evolution of qs, which results from the fast direct urca (durca) reaction causing the fast recovery to chemical balance. however, the sudden change in the chemical evolution doesn't provide an obvious heating effect in the thermal evolution. differently, the roto-chemical heating effect appears both in the accretion phase and spin-down phase of the hs, and the heating platform in the accretion phase relies on the accretion rate. larger accretion rate results in larger chemical deviation, higher and longer heating platform, and earlier appearance of the heating effect. interestingly, with the disappearance of the heating effect in the accretion phase, the surface temperature drops fast, which is another possibility of the rapid cooling trend of the ns in cas a. furthermore, the surface temperature of the qs is obviously lower than the hs and ns, which is a latent candidate for the explanation of the old classical pulsar j2144-3933 with the lowest known surface temperature. | roto-chemical heating with fall-back disk accretion in the neutron stars containing quark matter |
in newtonian physics, higher temperature leads to higher thermal pressure, which provides stronger support against the gravitational contraction of stars. however, in the temperature range of tens of mev involved in the evolution of a proto-neutron star or a higher massive neutron star, the effects of temperature are richer. we showed that, for a high temperature neutron star (htns) constructed with a realistic equation of state (eos), the htns may expand or contract during cooling, the central density may increase or decrease, the quasi-normal mode oscillation frequencies may increase or decrease, and in particular, (i) independent of the eos, for a htns of a given mass, there exists a maximum temperature t_{max} that it could ever attend at birth (with the value of t_{max} different for different eos), and (ii) for the hempel eos and the shen eos, there is a range of mass that the htns may gravitationally collapse after a period of radiative cooling; however, for the lattimer-swesty eos and banik eos, no delayed collapse is possible. our study, which describes the cooling of htnss with simple quasi-stationary tov sequences, provides an understanding of the effects of the thermal energy/pressure at high temperature, and a demonstration that different eoss can lead to qualitatively different evolution paths. | effects of temperature on the structure of neutron stars at high temperature |
we study in this work the evolution of the magnetic field in `redback-black widow' pulsars. evolutionary calculations of these `spider' systems suggest that first the accretion operates in the redback stage, and later the companion star ablates matter due to winds from the recycled pulsar. it is generally believed that mass accretion by the pulsar results in a rapid decay of the magnetic field when compared to the rate of an isolated neutron star. we study the evolution of the magnetic field in black widow pulsars by solving numerically the induction equation using the modified crank-nicolson method with intermittent episodes of mass accretion on to the neutron star. our results show that the magnetic field does not fall below a minimum value (`bottom field') in spite of the long evolution time of the black widow systems, extending the previous conclusions for much younger low-mass x-ray binary systems. we find that in this scenario, the magnetic field decay is dominated by the accretion rate, and that the existence of a bottom field is likely related to the fact that the surface temperature of the pulsar does not decay as predicted by the current cooling models. we also observe that the impurity of the pulsar crust is not a dominant factor in the decay of magnetic field for the long evolution time of black widow systems. | magnetic field decay in black widow pulsars |
we discuss the structure of neutron stars by modelling the homogeneous nuclear matter of the core by a suitable microscopic equation of state, based on the brueckner-hartree-fock many-body theory, and the crust, including the pasta phase, by the bcpm energy density functional which is based on the same equation of state. this allows for a uni ed description of the neutron star matter over a wide density range. a comparison with other uni ed approaches is discussed. with the same equation of state, which features strong direct urca processes and using consistent nuclear pairing gaps as well as effective masses, we model neutron star cooling, in particular the current rapid cooldown of the neutron star cas a. we nd that several scenarios are possible to explain the features of cas a, but only large and extended proton 1 s 0 gaps and small neutron 3 pf 2 gaps can accommodate also the major part of the complete current cooling data. | a unified equation of state on a microscopic basis : implications for neutron stars structure and cooling |
we briefly review a simple method to analyse observations of cooling neutron stars (developed in our previous works) and discuss its applications to xmmu j173203.3-244518, rx j1856.5-3754 and the vela pulsar. | cooling status of three neutron stars |
the 1s 0 nucleon superfluidity in neutron star matter was investigated in the framework of relativistic σ-ω-π-ρ model with the tensor couplings of ω and ρ mesons using the relativistic hartree-fock (rhf) approximation. it was found that the tensor couplings of ω and ρ mesons lead to a clear growth of the 1s 0 neutron pairing gap in the density range where there exists 1s 0 neutron superfluidity. the 1s 0 pairing gap of proton with the tensor couplings of ω and ρ mesons in the density range of ρb = 0.0-0.079fm-3 is lower and then in the density range of ρb = 0.079-0.383fm-3 higher than the corresponding value without the tensor couplings of ω and ρ mesons. our results provide a basic to understand the influence of the tensor couplings of ω and ρ mesons on the cooling properties of neutron star. | effects of tensor couplings of ω and ρ mesons on 1s0 nucleon superfluidity in neutron star matter |
we investigate structure and cooling properties of neutron stars making use of microscopic descriptions of nuclear matter based on realistic nucleon-nucleon interactions. the equation of state is obtained from solutions of the brueckner-hartree-fock equation for interacting nucleons at zero temperature. the structure aspects are described by the tolman- oppenheimer-volkoff equation for hydrostatic equilibrium of non-rotating stars. furthermore we have also solved bardeen-cooper-schrieffer (bcs) pairing gap equations to identify superfluid states in neutronic matter. we obtain mass-radius behavior and cooling curves consistent with current understanding. | structure and cooling of neutron stars: nuclear pairing and superfluid effects |
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