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we discuss the prospects for identifying the nearest isolated black holes (ibhs) in our galaxy. ibhs accreting gas from the interstellar medium likely form magnetically arrested disks (mads). we show that thermal electrons in the mads emit optical signals through the thermal synchrotron process while nonthermal electrons accelerated via magnetic reconnections emit a flat-spectrum synchrotron radiation in the x-ray to mev gamma-ray ranges. the gaia catalog will include at most a thousand ibhs within ≲1 kpc that are distributed on and around the cooling sequence of white dwarfs (wds) in the hertzsprung-russell diagram. these ibh candidates should also be detected by erosita, with which they can be distinguished from isolated wds and neutron stars. follow-up observations with hard x-ray and mev gamma-ray satellites will be useful to unambiguously identify ibhs. | multiwavelength emission from magnetically arrested disks around isolated black holes |
observations of galactic white dwarfs with gaia have allowed for unprecedented modelling of white dwarf cooling, resolving core crystallization, and sedimentary heating from neutron-rich nuclei. these cooling sequences are sensitive to the diffusion coefficients of nuclei in coulomb plasmas which have order 10 per cent uncertainty and are often not valid across coupling regimes. using large-scale molecular dynamics simulations we calculate diffusion coefficients at high resolution in the regime relevant for white dwarf modelling. we present a physically motivated law for diffusion with a semi-empirical correction which is accurate at the percent level. implemented along with linear mixing in stellar evolution codes, this law should reduce the error from diffusion coefficients by an order of magnitude. | precise diffusion coefficients for white dwarf astrophysics |
the onset of s10 proton spin-singlet pairing in neutron-star matter is studied in the framework of the bcs theory including medium polarization effects. the strong three-body coupling of the diproton pairs with the dense neutron environment and the bcs self-energy effects severely reduce the gap magnitude, so to reshape the scenario of the proton superfluid phase inside the star. at low density the vertex corrections due to the medium polarization are attractive in most isospin-asymmetry range and tend to favor that pairing. however quantitative estimates of their effect on the energy gap do not give significant changes. implications of the new scenario on the role of the s10 proton pairing in neutron-star cooling are briefly discussed. | proton-proton s10 pairing in neutron stars |
strongly magnetized isolated neutron stars (nss) are categorized into two families, according mainly to their magnetic field strength. those with a higher magnetic field of 1014-1015 g are called magnetars, characterized by repeated short bursts, and the others are x-ray isolated neutron stars (xinss) with 1013 g. both magnetars and xinss show thermal emission in x-rays, but it has been considered that the thermal spectrum of magnetars can be reproduced with a two-temperature blackbody (2bb), while that of xinss shows only a single-temperature blackbody (1bb) and the temperature is 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. here we report that all seven known xinss show a high-energy component in addition to the 1bb model. analyzing all the xmm-newton data for 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. 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 emissions are the same. the lower temperature in xinss can be explained if xinss are older than magnetars. therefore, these results are an observational indication that supports the standard hypothesis on the classification of highly magnetized nss. | universal detection of high-temperature emission in x-ray isolated neutron stars |
about 10% of the massive main sequence stars have recently been found to host a strong, large scale magnetic field. both, the origin and the evolutionary consequences of these fields are largely unknown. we argue that these fields may be sufficiently strong in the deep interior of the stars to suppress convection near the outer edge of their convective core. we performed parametrised stellar evolution calculations and assumed a reduced size of the convective core for stars in the mass range 16m⊙ to 28m⊙ from the zero age main sequence until core carbon depletion. we find that such models avoid the coolest part of the main sequence band, which is usually filled by evolutionary models that include convective core overshooting. furthermore, our "magnetic" models populate the blue supergiant region during core helium burning, i.e., the post-main sequence gap left by ordinary single star models, and some of them end their life in a position near that of the progenitor of supernova 1987a in the hertzsprung-russell diagram. further effects include a strongly reduced luminosity during the red supergiant stage, and downward shift of the limiting initial mass for white dwarf and neutron star formation. | blue supergiants as descendants of magnetic main sequence stars |
the persistent thermal luminosity of magnetars and their outbursts suggest the existence of some internal heat sources located in their outer crust. the compression of matter accompanying the decay of the magnetic field may trigger exothermic electron captures and, possibly, pycnonuclear fusions of light elements that may have been accreted onto the surface from the fallback of supernova debris, from a disk or from the interstellar medium. this scenario bears some resemblance to deep crustal heating in accreting neutron stars, although the matter composition and the thermodynamic conditions are very different. the maximum possible amount of heat that can be released by each reaction and their locations are determined analytically taking into account the landau–rabi quantization of electron motion. numerical results are also presented using experimental, as well as theoretical nuclear data. whereas the heat deposited is mainly determined by atomic masses, the locations of the sources are found to be very sensitive to the magnetic field strength, thus providing a new way of probing the internal magnetic field of magnetars. most sources are found to be concentrated at densities 1010–1011 g cm‑3 with heat power w∞∼1035–1036 erg/s, as found empirically by comparing cooling simulations with observed thermal luminosity. the change of magnetic field required to trigger the reactions is shown to be consistent with the age of known magnetars. this suggests that electron captures and pycnonuclear fusion reactions may be a viable heating mechanism in magnetars. the present results provide consistent microscopic inputs for neutron star cooling simulations, based on the same model as that underlying the brussels-montreal unified equations of state. | heating in magnetar crusts from electron captures |
evidence has accumulated for an as-yet unaccounted for source of heat located at shallow depths within the accreted neutron star crust. however, the nature of this heat source is unknown. i demonstrate that the inferred depth of carbon ignition in x-ray superbursts can be used as an additional constraint for the magnitude and depth of shallow heating. the inferred shallow heating properties are relatively insensitive to the assumed crust composition and carbon fusion reaction rate. for low accretion rates, the results are weakly dependent on the duration of the accretion outburst, so long as accretion has ensued for enough time to replace the ocean down to the superburst ignition depth. for accretion rates at the eddington rate, results show a stronger dependence on the outburst duration. consistent with earlier work, it is shown that urca cooling does not impact the calculated superburst ignition depth unless there is some proximity in depth between the heating and cooling sources. | constraining accreted neutron star crust shallow heating with the inferred depth of carbon ignition in x-ray superbursts |
it was pointed out in a recent paper that the observed cooling rate of old, cold neutron stars (ns) can provide an upper limit on the transition rate of neutron to mirror neutron (n -n' ). this limit is so stringent that it would preclude any discovery of n →n' oscillation in the current round of terrestrial searches for the process. motivated by this crucially important conclusion, we critically analyze this suggestion and note an interesting new effect present in nearly exact mirror models for n →n' oscillation, which significantly affects this bound. the new element is the β decay n'→p'+e'+ν¯e ' , which creates a cloud of mirror particles n', p', e', and d' inside the ns core. the e' can "rob" the energy generated by the n →n' transition via e -e' scattering enabled by the presence of a (minute) millicharge in mirror particles. this energy is emitted as unobserved mirror photons via fast mirror bremsstrahlung leading to a relaxation of this upper limit. | neutron-mirror-neutron oscillation and neutron star cooling |
modeling of crust heating and cooling across multiple accretion outbursts of the low mass x-ray binary mxb 1659-29 indicates that the neutrino luminosity of the neutron star core is consistent with direct urca (durca) reactions occurring in ~1% of the core volume. we investigate this scenario with neutron star models that include a detailed equation of state parametrized by the slope of the nuclear symmetry energy l, and a range of neutron and proton superfluid gaps. we find that the predicted neutron star mass depends sensitively on l and the assumed gaps. we discuss which combinations of superfluid gaps reproduce the inferred neutrino luminosity. larger values of l ≳ 80 mev require superfluidity to suppress durca reactions in low mass neutron stars, i.e., the proton or neutron gap is sufficiently strong and extends to high enough density. however, the largest gaps give masses near the maximum mass, making it difficult to accommodate colder neutron stars. the heat capacities of our models span the range from fully paired to fully unpaired nucleons meaning that long-term observations of core cooling could distinguish between models. as a route to solutions with a larger emitting volume, which could provide a more natural explanation for the inferred neutrino luminosity, we discuss the possibility of alternative, less efficient, fast cooling processes in exotic cores. to be consistent with the inferred neutrino luminosity, such processes must be within a factor of ~1000 of durca. we discuss the impact of future constraints on neutron star mass, radius, and the density dependence of the symmetry energy. | fast neutrino cooling in the accreting neutron star mxb 1659-29 |
we study a class of dfsz-like models for the qcd axion that can address observed anomalies in stellar cooling. stringent constraints from sn1987a and neutron stars are avoided by suppressed couplings to nucleons, while axion couplings to electrons and photons are sizable. all axion couplings depend on few parameters that also control the extended higgs sector, in particular lepton flavor-violating couplings of the standard model-like higgs boson h. this allows us to correlate axion and higgs phenomenology, and we find that br(h → τe) can be as large as the current experimental bound of 0.22%, while br(h → μμ) can be larger than in the standard model by up to 70%. large parts of the parameter space will be tested by the next generation of axion helioscopes such as the iaxo experiment. | flavor-violating higgs decays and stellar cooling anomalies in axion models |
based on the oscillation between the b phase and a phase of 3p2 neutron superfluid in neutron stars, we propose a new glitch model for young pulsars. according to our model, the glitch is explained as a repeat phenomena from the quasi period 3p2 neutron superfluid b phase, to a phase, then to b phase (many repeated glitches with quasi-period). with the repeating of the phase transition, the vortex quantum number of 3p2 neutron superfluid vortices is gradually reduced, and the heating rate ε (b) in b phase also gets lower and lower. after a number of glitches, the time intervals between successive glitches will gradually become longer, and the glitch amplitude decreases. when the heating rate ε (b) of an old neutron star becomes lower than the cooling rate of the direct urca, which can happen in a super-strongmagnetic field, the b phase of the 3p2 neutron superfluid vortices is no longer returned to the a phase state and the phase oscillation of the system is stopped immediately. the slowing glitch phenomenon of some older pulsars is a natural result of our theory, which means that very old pulsars will no longer present the glitch. our model will be tested in future pulsar glitch observations. | new insight into the physical essence of pulsar glitch |
in this lecture i will briefly review some of the effects of hyperons on the properties of neutron and proto-neutron stars. in particular, i will revise the problem of the strong softening of the eos, and the consequent reduction of the maximum mass, induced by the presence of hyperons, a puzzle which has become more intringuing and difficult to solve due the recent measurements of the unusually high masses of the millisecond pulsars psr j1903+0327 (1.667±0.021m⊙), psr j1614-2230 (1.97±0.04m⊙), and psr j0348+0432 (2.01±0.04m⊙). finally, i will also examine the role of hyperons on the cooling properties of newly born neutron stars and on the so-called r-mode instability. | hyperons and neutron stars |
circulating tumor cells (ctcs) have become an established biomarker for prognosis in patients with various carcinomas. however, current ex vivo ctc isolation technologies rely on small blood volumes from a single venipuncture limiting the number of captured ctcs. this produces statistical variability and inaccurate reflection of tumor cell heterogeneity. here, we describe an in vivo indwelling intravascular aphaeretic ctc isolation system to continuously collect ctcs directly from a peripheral vein. the system returns the remaining blood products after ctc enrichment, permitting interrogation of larger blood volumes than classic phlebotomy specimens over a prolonged period of time. the system is validated in canine models showing capability to screen 1-2% of the entire blood over 2 h. our result shows substantial increase in ctc capture, compared with serial blood draws. this technology could potentially be used to analyze large number of ctcs to facilitate translation of analytical information into future clinical decisions. | a temporary indwelling intravascular aphaeretic system for in vivo enrichment of circulating tumor cells |
we study the thermal evolution of neutron stars containing deconfined quark matter in their core. such objects are generally referred to as quark-hybrid stars. the confined hadronic matter in their core is described in the framework of nonlinear relativistic nuclear field theory. for the quark phase we use a nonlocal extension of the su(3) nambu-jona-lasinio model with vector interactions. the gibbs condition is used to model phase equilibrium between confined hadronic matter and deconfined quark matter. our study indicates that high-mass neutron stars may contain between 35 and 40% deconfined quark-hybrid matter in their cores. neutron stars with canonical masses of around 1.4 m⊙ would not contain deconfined quark matter. the central proton fractions of the stars are found to be high, enabling them to cool rapidly. very good agreement with the temperature evolution established for the neutron star in cassiopeia a (cas a) is obtained for one of our models (based on the popular nl3 nuclear parametrization), if the protons in the core of our stellar models are strongly paired, the repulsion among the quarks is mildly repulsive, and the mass of cas a has a canonical value of 1.4 m⊙ . | thermal evolution of hybrid stars within the framework of a nonlocal nambu-jona-lasinio model |
newborn, strongly magnetized neutron stars (so-called magnetars) surrounded by their stellar or merger ejecta are expected to be sources of ultrahigh-energy neutrinos via decay of mesons produced in hadronic interactions of protons which are accelerated to ultrahigh energies by magnetic dissipation of the spindown energy. we show that not only pions and kaons but also charm hadrons, which are typically neglected due to their small production cross sections, can represent dominant contributions to neutrino fluence at ultrahigh energies, because of their short lifetimes, while the ultrahigh-energy neutrino fluence from pion and kaon production is suppressed at early times due to their significant cooling before their decay. we show that the next-generation detectors such as probe of extreme multi-messenger astrophysics (poemma), giant radio array for neutrino detection (grand) and icecube-gen2 have a good chance of observing neutrinos, primarily originating from charm hadrons, from nearby magnetars. we also show that neutrinos from nearby magnetar-driven merger novae could be observed in the time interval between 102 s and 103 s , where the charm hadron contribution is dominant for neutrino energies above 108 gev , of relevance to next generation detectors. we also comment on potential impacts of the charm hadron contribution to the diffuse neutrino flux. | charm contribution to ultrahigh-energy neutrinos from newborn magnetars |
background: superfluidity in the crust is a key ingredient for the cooling properties of proto-neutron stars. present theoretical calculations employ the quasiparticle mean-field hartree-fock-bogoliubov theory with temperature-dependent occupation numbers for the quasiparticle states. purpose: finite temperature stellar matter is characterized by a whole distribution of different nuclear species. we want to assess the importance of this distribution on the calculation of heat capacity in the inner crust. method: following a recent work, the wigner-seitz cell is mapped into a model with cluster degrees of freedom. the finite temperature distribution is then given by a statistical collection of wigner-seitz cells. we additionally introduce pairing correlations in the local density bcs approximation both in the homogeneous unbound neutron component, and in the interface region between clusters and neutrons. results: the heat capacity is calculated in the different baryonic density conditions corresponding to the inner crust, and in a temperature range varying from 100 kev to 2 mev. we show that accounting for the cluster distribution has a small effect at intermediate densities, but it considerably affects the heat capacity both close to the outer crust and close to the core. we additionally show that it is very important to consider the temperature evolution of the proton fraction for a quantitatively reliable estimation of the heat capacity. conclusions: we present the first modelization of stellar matter containing at the same time a statistical distribution of clusters at finite temperature, and pairing correlations in the unbound neutron component. the effect of the nuclear distribution on the superfluid properties can be easily added in future calculations of the neutron star cooling curves. a strong influence of resonance population on the heat capacity at high temperature is observed, which deserves to be further studied within more microscopic calculations. | heat capacity of the neutron star inner crust within an extended nuclear statistical equilibrium model |
the fermi surface depletion of beta-stable nuclear matter is calculated to study its effects on several physical properties that determine the neutron star (ns) thermal evolution. the neutron and proton z factors measuring the corresponding fermi surface depletions are calculated within the brueckner-hartree-fock approach, employing the av18 two-body force supplemented by a microscopic three-body force. neutrino emissivity, heat capacity, and in particular neutron 3pf2 superfluidity, turn out to be reduced, especially at high baryonic density, to such an extent that the cooling rates of young nss are significantly slowed. | role of nucleonic fermi surface depletion in neutron star cooling |
we investigate the spectral and timing properties of the millihertz quasi-periodic oscillations (mhz qpos) in neutron-star low-mass x-ray binary 4u 1636-53 using xmm-newton and rossi x-ray timing explorer (rxte) observations. the mhz qpos in the xmm-newton/rxte observations show significant frequency variation and disappear right before type i x-ray bursts. we find no significant correlation between the mhz qpo frequency and the temperature of the neutron-star surface, which is different from theoretical predictions. for the first time we observed the full lifetime of a mhz qpo lasting 19 ks. besides, we also measure a frequency drift time-scale ∼15 ks, we speculate that this is the cooling time-scale of a layer deeper than the burning depth, possibly heated by the previous burst. moreover, the analysis of all x-ray bursts in this source shows that all type i x-ray bursts associated with the mhz qpos are short, bright and energetic, suggesting a potential connection between mhz qpos and he-rich x-ray bursts. | spectral and timing analysis of the mhz qpos in the neutron-star low-mass x-ray binary 4u 1636-53 |
we use the bayesian approach to write the posterior probability density for the three-dimensional velocity of a pulsar and for its kinematic age. as a prior, we use the bimodal velocity distribution found in a recent article by verbunt, igoshev & cator. when we compare the kinematic ages with spin-down ages we find that in general they agree with each other. in particular, maximum likelihood analysis sets the lower limit for the exponential magnetic field decay time-scale at 8 myr with slight preference of tdec ≈ 12 myr and compatible with no decay at all. one of the objects in the study, pulsar b0950+08 has kinematic and cooling ages ≈2 myr which is in strong contradiction with its spin-down age τ ≈ 17 myr. the 68 per cent credible range for the kinematic age is 1.2-8.0 myr. we conclude that the most probable explanation for this contradiction is a combination of magnetic field decay and long initial period. further timing, uv and x-ray observations of b0950+08 are required to better constrain its origin and evolution. | ages of radio pulsar: long-term magnetic field evolution |
nuclear reactions heat and cool the crust of accreting neutron stars and need to be understood to interpret observations of x-ray bursts and long-term cooling in transiently accreting systems. it was recently suggested that previously ignored neutron transfer reactions may play a significant role in the nuclear processes. we present results from full nuclear network calculations that now include these reactions and determine their impact on crust composition, crust impurity, heating, and cooling. we find that a large number of neutron transfer reactions indeed occur and impact crust models. in particular, we identify a new type of reaction cycle that brings a pair of nuclei across the nuclear chart into equilibrium via alternating neutron capture and neutron release, interspersed with a neutron transfer. while neutron transfer reactions lead to changes in crust model predictions and need to be considered in future studies, previous conclusions concerning heating, cooling, and compositional evolution are remarkably robust. | the impact of neutron transfer reactions on the heating and cooling of accreted neutron star crusts |
the long-period (p = 1091 s) of the recently discovered pulsar gleam-x j162759.5-523504.3 can be attained by neutron stars evolving with fallback discs and magnetic dipole moments of a few 1030 g cm3 at ages greater than ~2 × 105 yr consistently with the observational upper limits to the period derivative, $\dot{p}$, and the x-ray luminosity, lx, of the source. the current upper limits for $\dot{p}$ allow two alternative present states: (1) the disc is still active with ongoing accretion at a low rate such that the accretion luminosity is much less than the neutron star's cooling luminosity, which in turn is below the upper limit for lx. in this scenario, the spin-down will continue at $\dot{p}\sim 10^{-10}$ s s-1 until the disc becomes inactive; the final period will be p ~ a few 103 s. (2) the disc is already inactive, there is no accretion. in this case, the period evolution has levelled off to the observed value in the final period range. the remaining, very weak, dipole torque sustains asymptotic spin-down at $\dot{p}\sim 4 \times 10^{-18}$ s s-1. long periods p ~ a few 103 s were predicted for the final states of soft gamma repeaters and anomalous x-ray pulsars with relatively strong dipole fields in earlier work with the fallback disc model. | evolution of the long-period pulsar gleam-x j162759.5-523504.3 |
in this work, we reinvestigate the electron fraction $y_{e}$ and electron fermi energy $e_{f}(e)$ of neutron stars, based on our previous work of li et al.(2016), in which we firstly deduced a special solution to $e_{f}(e)$, and then obtained several useful analytical formulae for $y_{\rm e}$ and matter density $\rho$ within classical models and the relativistic mean field(rmf) theory using numerically fitting. the advantages of this work include the following aspects:(1) the linear functions are substituted for the nonlinear exponential functions used in the previous work. this method may be more simple, and closer to realistic equation of state\,(eos) of a neutron star(ns), because there are linear or quasi-linear relationships between number fractions of leptons and matter density, which can be seen by solving ns eos; (2)we introduce a dimensionless variable $\varrho$\,($\varrho=\rho/\rho_0$, $\rho_{0}$ is the standard saturated nuclear density), which greatly reduces the scope of the fitting coefficients;(3)we present numerical errors including absolute and relative deviations between the data and fit. by numerically simulating, we have obtained several analytical formulae for $y_{e}$ and $\rho$ for both apr98 and rmf models. combining these analytical formulae with the special solution, we can calculate the value of $e_{\rm f}(e)$ for any given matter density. since $y_e$ and $e_{ f}(e)$ are important in assessing cooling rate of a ns and the possibility of kaon/pion condensation in the ns interior, this study could be useful in the future study on the thermal evolution of a ns. | reinvestigation of the electron fraction and electron fermi energy of neutron star |
valuable information about the neutron star (ns) interior can be obtained by comparing observations of thermal radiation from a cooling ns crust with theoretical models. nuclear burning of lighter elements that diffuse to deeper layers of the envelope can alter the relation between surface and interior temperatures and can change the chemical composition over time. we calculate new temperature relations and consider two effects of diffusive nuclear burning (dnb) for h-c envelopes. first, we consider the effect of a changing envelope composition and find that hydrogen is consumed on short time-scales and our temperature evolution simulations correspond to those of a hydrogen-poor envelope within ∼100 d. the transition from a hydrogen-rich to a hydrogen-poor envelope is potentially observable in accreting ns systems as an additional initial decline in surface temperature at early times after the outburst. second, we find that dnb can produce a non-negligible heat flux, such that the total luminosity can be dominated by dnb in the envelope rather than heat from the deep interior. however, without continual accretion, heating by dnb in h-c envelopes is only relevant for <1-80 d after the end of an accretion outburst, as the amount of light elements is rapidly depleted. comparison to crust cooling data shows that dnb does not remove the need for an additional shallow heating source. we conclude that solving the time-dependent equations of the burning region in the envelope self-consistently in thermal evolution models instead of using static temperature relations would be valuable in future cooling studies. | the effect of diffusive nuclear burning in neutron star envelopes on cooling in accreting systems |
a review of wind accretion in high-mass x-ray binaries is presented. we focus on different regimes of quasi-spherical accretion onto the neutron star (ns): the supersonic (bondi) accretion, which takes place when the captured matter cools down rapidly and falls supersonically towards the ns magnetosphere, and subsonic (settling) accretion which occurs when plasma remains hot until it meets the magnetospheric boundary. these two regimes of accretion are separated by an x-ray luminosity of about 4 × 1036 erg s-1. in the subsonic case, which sets in at lower luminosities, a hot quasi-spherical shell must form around the magnetosphere, and the actual accretion rate onto ns is determined by the ability of the plasma to enter the magnetosphere due to rayleigh-taylor instability. in turn, two regimes of subsonic accretion are possible, depending on plasma cooling mechanism (compton or radiative) near the magnetopshere. the transition from the high-luminosity with compton cooling to the lowluminosity (lx ≲ 3 × 1035 erg s-1) with radiative cooling can be responsible for the onset of the off states repeatedly observed in several low-luminosity slowly accreting pulsars, such as vela x-1, gx 301-2, and 4u 1907+09. the triggering of the transitionmay be due to a switch in the x-ray beam pattern in response to a change in the optical depth in the accretion column with changing luminosity. we also show that in the settling accretion theory, bright x-ray flares (~1038-1040 erg) observed in supergiant fast x-ray transients (sfxt) can be produced by sporadic capture of magnetized stellar wind plasma. at sufficiently low accretion rates, magnetic reconnection can enhance the magnetospheric plasma entry rate, resulting in copious production of x-ray photons, strong compton cooling and ultimately in unstable accretion of the entire shell. a bright flare develops on the free-fall time scale in the shell, and the typical energy released in an sfxt bright flare corresponds to the mass of the shell. | wind accretion: theory and observations |
the application of standard accretion theory to observations of x-ray binaries provides valuable insights into neutron star (ns) properties, such as their spin period and magnetic field. however, most studies concentrate on relatively old systems, where the ns is in its late propeller, accretor, or nearly spin equilibrium phase. here, we use an analytic model from standard accretion theory to illustrate the evolution of high-mass x-ray binaries (hmxbs) early in their life. we show that a young ns is unlikely to be an accretor because of the long duration of ejector and propeller phases. we apply the model to the recently discovered ∼4000 yr old hmxb xmmu j051342.6-672412 and find that the system's ns, with a tentative spin period of 4.4 s, cannot be in the accretor phase and has a magnetic field b > a few × 1013 g, which is comparable to the magnetic field of many older hmxbs and is much higher than the spin equilibrium inferred value of a few × 1011 g. the observed x-ray luminosity could be the result of thermal emission from a young cooling magnetic ns or a small amount of accretion that can occur in the propeller phase. | early neutron star evolution in high-mass x-ray binaries |
we report that the rs cvn-type star gt mus (hr 4492, hd 101379+hd 101380) was the most active star in the x-ray sky in the last decade in terms of the scale of recurrent energetic flares. we detected 11 flares from gt mus in 8 yr of observations with the monitor of all-sky x-ray image (maxi) from 2009 august to 2017 august. the detected flare peak luminosities were 1-4 × 1033 erg s-1 in the 2.0-20.0 kev band for its distance of 109.6 pc. our timing analysis showed long durations (τr + τd) of 2-6 days with long decay times (τd) of 1-4 days. the released energies during the decay phases of the flares in the 0.1-100 kev band were in the range of 1-11 × 1038 erg, which are at the upper end of the observed stellar flare. the released energies during the whole duration were in the range of 2-13 × 1038 erg in the same band. we carried out x-ray follow-up observations for one of the 11 flares with the neutron star interior composition explorer (nicer) on 2017 july 18 and found that the flare cooled quasi-statically. on the basis of a quasi-static cooling model, the flare loop length is derived to be 4 × 1012 cm (or 60 r⊙). the electron density is derived to be 1 × 1010 cm-3, which is consistent with the typical value of solar and stellar flares (1010-13 cm-3). the ratio of the cooling timescales between radiative (τrad) and conductive (τcond) cooling is estimated to be τrad ∼ 0.1 τcond from the temperature; thus, radiative cooling was dominant in this flare. | the rs cvn-type star gt mus shows most energetic x-ray flares throughout the 2010s |
we discuss the effect of convection driven by chemical separation at the ocean-crust boundary of accreting neutron stars. we extend the steady-state results of medin & cumming to transient accretors, by considering the time-dependent cases of heating during accretion outbursts and cooling during quiescence. during accretion outbursts, inward heat transport has only a small effect on the temperature profile in the outer layers until the ocean is strongly enriched in light elements, a process that takes hundreds of years to complete. during quiescence, however, inward heat transport rapidly cools the outer layers of the ocean while keeping the inner layers hot. we find that this leads to a sharp drop in surface emission at around a week followed by a gradual recovery as cooling becomes dominated by the crust. such a dip should be observable in the light curves of these neutron star transients, if enough data is taken at a few days to a month after the end of accretion. if such a dip is definitively observed, it will provide strong constraints on the chemical composition of the ocean and outer crust. | time-dependent, compositionally driven convection in the oceans of accreting neutron stars |
accreting neutron stars (ns) can exhibit high frequency modulations in their lightcurves during thermonuclear x-ray bursts, known as burst oscillations. these frequencies can be offset from the ns spin frequency by several hz (where known independently) and can drift by 1-3 hz. one plausible explanation is that a wave is present in the bursting ocean, the rotating frame frequency of which is the offset. the frequency of the wave should decrease (in the rotating frame) as the burst cools hence explaining the drift. a strong candidate is a buoyant r-mode. to date, models that calculated the frequency of this mode taking into account the radial structure neglected relativistic effects and predicted rotating frame frequencies of ~4 hz and frequency drifts of >5 hz; too large to be consistent with observations. we present a calculation that includes frame-dragging and gravitational redshift that reduces the rotating frame frequency by up to $30 \, {\rm per\, cent}$ and frequency drift by up to $20 \, {\rm per\, cent}$. updating previous models for the ocean cooling in the aftermath of the burst to a model more representative of detailed calculations of thermonuclear x-ray bursts reduces the frequency of the mode still further. this model, combined with relativistic effects, can reduce the rotating frequency of the mode to ~2 hz and frequency drift to ~2 hz, which is closer to the observed values. | relativistic ocean r-modes during type-i x-ray bursts |
designs for future gravitational wave detection facilities feature silicon test masses at cryogenic temperatures to reduce thermal noise and thermally induced aberrations. designers call for operation at 123 k or close to 18 k to exploit the vanishing thermal expansion of crystalline silicon. the amount of absorbed heat that can be radiatively removed from the test masses is limited at these temperatures, forcing complex cooling scenarios to be considered, including conduction through suspension wires. this is particularly relevant for the kilohertz class of detectors that aim for extremely high circulating power, i.e., roughly a factor 20 more than the world record at the time of writing, to reduce quantum noise. we explore the impact of raising the test mass temperature and show that a dedicated kilohertz-band cryogenic instrument can do so without significant sensitivity penalty, thereby boosting the radiative cooling rate and allowing higher power operation with simpler suspensions. we also explore the implications of operating cryogenic broadband detectors at elevated temperatures. the work presented here was instrumental in the development of the neutron star extreme matter observatory kilohertz-band gravitational wave detector design concept. | practical test mass and suspension configuration for a cryogenic kilohertz gravitational wave detector |
previous optical and radio observations of the binary millisecond pulsar psr j1640+2224 have come to inconsistent conclusions about the identity of its companion, with some observations suggesting that the companion is a low-mass helium-core (he-core) white dwarf (wd), while others indicate that it is most likely a high-mass carbon-oxygen (co) wd. binary evolution models predict psr j1640+2224 most likely formed in a low-mass x-ray binary based on the pulsar’s short spin period and long-period, low-eccentricity orbit, in which case its companion should be a he-core wd with mass about 0.35-0.39 m ⊙, depending on metallicity. if instead it is a co wd, it would suggest that the system has an unusual formation history. in this paper we present the first astrometric parallax measurement for this system from observations made with the very long baseline array (vlba), from which we determine the distance to be {1520}-150+170 {pc}. we use this distance and a reanalysis of archival optical observations originally taken in 1995 with the wide field planetary camera 2 on the hubble space telescope (hst) to measure the wd’s mass. we also incorporate improvements in calibration, extinction model, and wd cooling models. we find that the existing observations are not sufficient to tightly constrain the companion mass, but we conclude the wd mass is >0.4 m ⊙ with >90% confidence. the limiting factor in our analysis is the low signal-to-noise ratio of the original hst observations. | reconciling optical and radio observations of the binary millisecond pulsar psr j1640+2224 |
we study the effects of heat blanketing envelopes of neutron stars on their cooling. to this aim, we perform cooling simulations using newly constructed models of the envelopes composed of binary ion mixtures (h-he, he-c, c-fe) varying the mass of lighter ions (h, he or c) in the envelope. the results are compared with those calculated using the standard models of the envelopes which contain the layers of lighter (accreted) elements (h, he and c) on top of the fe layer, varying the mass of accreted elements. the main effect is that the chemical composition of the envelopes influences their thermal conductivity and, hence, thermal insulation of the star. for illustration, we apply these results to estimate the internal temperature of the vela pulsar and to study the cooling of neutron stars of ages of 105-106 yr at the photon cooling stage. the uncertainties of the cooling models associated with our poor knowledge of chemical composition of the heat insulating envelopes strongly complicate theoretical reconstruction of the internal structure of cooling neutron stars from observations of their thermal surface emission. | cooling of neutron stars with diffusive envelopes |
context. an increasing number of low-mass (m⋆/m⊙ ≲ 0.45) and extremely low-mass (elm, m⋆/m⊙ ≲ 0.18-0.20) white-dwarf stars are being discovered in the field of the milky way. some of these stars exhibit long-period g-mode pulsations, and are called elmv variable stars. also, some low-mass pre-white dwarf stars show short-period p-mode (and likely radial-mode) photometric variations, and are designated as pre-elmv variable stars. the existence of these new classes of pulsating white dwarfs and pre-white dwarfs opens the prospect of exploring the binary formation channels of these low-mass white dwarfs through asteroseismology.aims: we aim to present a theoretical assessment of the expected temporal rates of change of periods (\dot{π}) for such stars, based on fully evolutionary low-mass he-core white dwarf and pre-white dwarf models.methods: our analysis is based on a large set of adiabatic periods of radial and nonradial pulsation modes computed on a suite of low-mass he-core white dwarf and pre-white dwarf models with masses ranging from 0.1554 to 0.4352 m⊙, which were derived by computing the non-conservative evolution of a binary system consisting of an initially 1 m⊙ zams star and a 1.4 m⊙ neutron star companion.results: we computed the secular rates of period change of radial (ℓ = 0) and nonradial (ℓ = 1,2) g and p modes for stellar models representative of elmv and pre-elmv stars, as well as for stellar objects that are evolving just before the occurrence of cno flashes at the early cooling branches. we find that the theoretically expected magnitude of \dot{π} of g modes for pre-elmvs is by far larger than for elmvs. in turn, \dot{π} of g modes for models evolving before the occurrence of cno flashes are larger than the maximum values of the rates of period change predicted for pre-elmv stars. regarding p and radial modes, we find that the larger absolute values of \dot{π} correspond to pre-elmv models.conclusions: we conclude that any eventual measurement of a rate of period change for a given pulsating low-mass pre-white dwarf or white dwarf star could shed light about its evolutionary status. also, in view of the systematic difficulties in the spectroscopic classification of stars of the elm survey, an eventual measurement of \dot{π} could help to confirm that a given pulsating star is an authentic low-mass white dwarf and not a star from another stellar population. | pulsating low-mass white dwarfs in the frame of new evolutionary sequences. iv. the secular rate of period change |
we report the discovery of the isolated neutron star (ins) candidates erassu j065715.3+260428 and erassu j131716.9−402647 from the spectrum roentgen gamma (srg) erosita all-sky survey. selected for their soft x-ray emission and absence of catalogued counterparts, both objects were recently targeted with the large binocular telescope and the southern african large telescope. the absence of counterparts down to deep optical limits (25 mag, 5σ) and, as a result, large x-ray-to-optical flux ratios in both cases strongly suggest an ins nature. the x-ray spectra of both sources are well described by a simple absorbed blackbody, whereas other thermal and non-thermal models (e.g., a hot-plasma emission spectrum or power law) are disfavoured by the spectral analysis. within the current observational limits, and as expected for cooling inss, no significant variation (> 2σ) has been identified over the first two-year time span of the survey. upcoming dedicated follow-up observations will help us to confirm the candidates' nature. | discovery of two promising isolated neutron star candidates in the srg/erosita all-sky survey |
the structure and composition of the crust of neutron stars plays an important role in their thermal and magnetic evolution, hence in setting their observational properties. one way to study the properties of the crust of a neutron star, is to measure how it cools after it has been heated during an accretion outburst in a low-mass x-ray binary (lmxb). such studies have shown that there is a tantalizing source of heat, of currently unknown origin, that is located in the outer layers of the crust and has a strength that varies between different sources and different outbursts. with the aim of understanding the mechanism behind this `shallow heating', we present chandra and swift observations of the neutron star lmxb aql x-1, obtained after its bright 2016 outburst. we find that the neutron star temperature was initially much lower, and started to decrease at much later time, than observed after the 2013 outburst of the source, despite the fact that the properties of the two outbursts were very similar. comparing our data to thermal evolution simulations, we infer that the depth and magnitude of shallow heating must have been much larger during the 2016 outburst than during the 2013 one. this implies that basic neutron star parameters that remain unchanged between outbursts do not play a strong role in shallow heating. furthermore, it suggests that outbursts with a similar accretion morphology can give rise to very different shallow heating. we also discuss alternative explanations for the observed difference in quiescent evolution after the 2016 outburst. | crust cooling of the neutron star in aql x-1: different depth and magnitude of shallow heating during similar accretion outbursts |
in this work, we shortly review the role and properties of hyperons in finite and infinite nuclear systems such as hypernuclei and neutron stars. particularly, we describe different production mechanisms of hypernuclei, discuss some aspects of their γ-ray spectroscopy and their weak decay modes, and give a few strokes on their theoretical description. we reexamine also the role played by hyperons on the properties of neutron and proto-neutron stars with a special emphasis on the well-known "hyperon puzzle", of which we discuss some of the solutions that have been proposed to tackle this problem. finally, we review the role of hyperons on the cooling properties of newly born neutron stars and on the so-called r-mode instability. | hyperons in finite and infinite nuclear systems |
we perform 2d axisymmetric radiative relativistic mhd simulations of radiation pressure supported neutron star accretion columns in split-monopole magnetic fields. the accretion columns exhibit quasi-periodic oscillations, which manifest in the luminosity power spectrum as 2-10 khz peaks, together with broader extensions to somewhat higher frequencies. the peak frequency decreases for wider columns or higher mass accretion rates. in contrast to the case of shorter columns in uniform magnetic fields, pdv work contributes substantially to maintaining the radiation pressure inside the column against sideways radiative cooling. this is in part due to the compression associated with accretion along the converging magnetic field lines towards the stellar surface. propagating entropy waves which are associated with the slow-diffusion photon bubble instability form in all our simulations. radial advection of radiation from the oscillation itself as well as the entropy waves is also important in maintaining radiation pressure inside the column. the time-averaged profile of our fiducial simulation accretion is approximately consistent with the classical 1d stationary model provided one incorporates the correct column shape. we also quantify the porosity in all our accretion column simulations so that this may also in principle be used to improve 1d models. | dynamics of neutron star accretion columns in split-monopole magnetic fields |
disk accretion at a high rate onto a white dwarf (wd) or a neutron star has been suggested to result in the formation of a spreading layer (sl)—a belt-like structure on the object's surface, in which the accreted matter steadily spreads in the poleward (meridional) direction while spinning down. to assess its basic characteristics, we perform two-dimensional hydrodynamic simulations of supersonic sls in the relevant morphology with a simple prescription for cooling. we demonstrate that supersonic shear naturally present at the base of the sl inevitably drives sonic instability that gives rise to large-scale acoustic modes governing the evolution of the sl. these modes dominate the transport of momentum and energy, which is intrinsically global and cannot be characterized via some form of local effective viscosity (e.g., α-viscosity). the global nature of the wave-driven transport should have important implications for triggering type i x-ray bursts in low-mass x-ray binaries. the nonlinear evolution of waves into a system of shocks drives effective rearrangement (sensitively depending on thermodynamical properties of the flow) and deceleration of the sl, which ultimately becomes transonic and susceptible to regular kelvin-helmholtz instability. we interpret this evolution in terms of the global structure of the sl and suggest that mixing of the sl material with the underlying stellar fluid should become effective only at intermediate latitudes on the accreting object's surface, where the flow has decelerated appreciably. in the near-equatorial regions the transport is dominated by acoustic waves and mixing is less efficient. we speculate that this latitudinal nonuniformity of mixing in accreting wds may be linked to the observed bipolar morphology of classical nova ejecta. | spreading layers in accreting objects: role of acoustic waves for angular momentum transport, mixing, and thermodynamics |
based on the linear-mixing approach, we calculate the latent heat for crystallizing fully ionized 12c/16o and 16o/20ne mixtures in white dwarf (wd) cores for two different parametrizations of the corrections to the linear-mixing energies and with account of ion quantum effects. we report noticeable composition-dependent deviations of the excess entropy in both directions from the standard value of 0.77 per ion. within the same framework, we evaluate the excess entropy and released or absorbed heat accompanying the exsolution process in solidified wd layers. the inclusion of this effect is shown to be important for reliable interpretation of wd cooling data. we also analyse the latent heat of crystallization for the eutectic 12c/22ne mixture, where we find a qualitative dependence of both the phase diagram and the latent heat behaviour on ion quantum effects. this may be important for the model with 22ne distillation in cooling c/o/22ne wd proposed as a solution for the ultramassive wd multi-gyr cooling anomaly. astrophysical implications of our findings for crystallizing wd are discussed. | on excess entropy and latent heat in crystallizing white dwarfs |
new shell-model hamiltonians which can successfully describe spin responses in nuclei are used to evaluate nuclear weak rates in stellar environments. the e-capture and β-decay rates for the nuclear pair with 31mg-31al, in the island of inversion, which have been pointed out to be important for the cooling of neutron star crusts, are studied by shell-model calculations with the effective interaction in sd-pf shell obtained by the extended kuo-krenciglowa (ekk) method. the weak rates induced by the gamow-teller transitions between the low-lying states in the nuclear pair lead to a nuclear urca process. the spin-dipole strengths and e-capture rates for 78ni are evaluated by shell-model with full pf-sdg shells including up to 5p-5h excitations outside filling configurations of 78ni. the e-capture rates obtained are compared with rpa calculations and the effective rate formula. weak rates for the second-forbidden transition in 20ne are evaluated by the multipole expansion method of walecka as well as the behrens-bühring method within sd-shell. difference in the rates between the two methods is found to be rather small as far as the conserved-vector-current (cvc) relation is satisfied. possible important contributions of the forbidden transition to the heating of the onemg core by double e-captures on 20ne in a late stage of the evolution of the core and implications on the final fate of the core, whether core-collapse or thermonuclear explosion, are discussed. | shell-model study of nuclear weak rates relevant to astrophysical processes in stars |
we model the quiescent luminosity of accreting neutron stars with several equation of states (eoss), including the effect of pion condensation and superfluidity. as a consequence of comparison with the observations, we show that the results with togashi eos (the strong direct urca process is forbidden) and tm1e eos (mass at direct urca process is 2.06 m⊙) can explain the observations well by considering pion condensation and the effect of superfluidity, while ls220 eos and tm1 eos can explain the observations well by considering the baryon direct urca process and the effect of superfluidity. besides, we compare the results with the observations of a neutron star rx j0812.4-3114 which has the low average mass accretion rate (⟨m ˙ ⟩∼(4 - 15 )×10-12 m⊙ yr-1 ) but high thermal luminosity (lq∞∼(0.6 - 3 )×1033 erg s-1 ), and we suggest that a low-mass neutron star (<1 m⊙ ) with minimum cooling can explain the lower limit of the observation of thermal luminosity of rx j0812.4-3114, which is qualitatively consistent with the previous work [zhao et al., soft excess in the quiescent be/x-ray pulsar rx j0812.4-3114, mon. not. r. astron. soc. 488, 4427 (2019), 10.1093/mnras/stz1946]. however, to explain its upper limit, some other heating mechanisms besides standard deep crustal heating may be needed. | quiescent luminosities of accreting neutron stars with different equation of states |
the nuclear symmetry energy and its behavior with density has been recently evaluated with enhanced value by prex-2 experiment. this new values enables direct urca neutrino emission process to be functioning in the dense matter inside neutron stars. with this new outlook we study the cooling rate of canonical mass neutron stars and compare with available observational cooling data. we find most of the isolated neutron star thermal profile is compatible with the cooling of canonical mass star including superfluidity suppression. | fast neutron star cooling in light of the prex-2 experiment |
the presence of strong magnetic fields in neutron stars, such as in magnetars, may significantly affect their crust-core transition properties and the crust size. this knowledge is crucial in the correct interpretation of astrophysical phenomena involving magnetars, such as glitches in observed rotation frequencies, cooling, bursts and possibly tidal polarizabilities. a recently developed meta-modelling technique allows exploring the model dependence of density functional theory equation of state calculations. in this work, we extend this metamodel to investigate the effect of strong magnetic fields on spinodal instabilities of neutron star matter and the associated crust-core properties. both tolman-oppenheimer-volkov and a full self-consistent numerical calculations are performed for the neutron star structure, the results being quantitatively different for strong magnetic fields. | estimating magnetar radii with an empirical meta-model |
in this work, we revisit the thermal relaxation process for neutron stars. such a process is associated with the thermal coupling between the core and the crust of neutron stars. the thermal relaxation, which takes place at around 10-100 years, is manifested as a sudden drop in the star's surface temperature. such a drop is smooth for slowly cooling objects and very sharp for fast-cooling ones. in our study, we focused particularly on the cooling of neutron stars whose mass is slightly greater than the value above which the direct urca (du) process sets in. considering different mechanisms for neutrino production in each region of the star, and working with equations of state with different properties, we solved the thermal evolution equation and calculated the thermal relaxation time for an ample range of neutron star masses. by performing a comprehensive study of neutron stars just above the onset of the du process, we show that stars under these conditions exhibit a peculiar thermal relaxation behavior. we demonstrate that such stars exhibit an abnormally late relaxation time, characterized by a second drop in its surface temperature taking place a later age. we qualified such behavior by showing that it is associated with limited spatial distribution of the du process in such stars. we show that as the star's mass increases, the du region also grows, and the star exhibits the expected behavior of fast-cooling stars. finally, we show that one can expect high relaxation times for stars in which the du process takes place in a radius no larger than 3 km. | revisiting the thermal relaxation of neutron stars |
the be/x-ray transient gro j1750-27 exhibited a type-ii (giant) outburst in 2015. after the source transited to quiescence, we triggered our multi-year chandra monitoring programme to study its quiescent behaviour. the programme was designed to follow the cooling of a potentially heated neutron-star crust due to accretion of matter during the preceding outburst, similar to what we potentially have observed before in two other be/x-ray transients, namely 4u 0115+63 and v 0332+53. however, unlike for these other two systems, we do not find any strong evidence that the neutron-star crust in gro j1750-27 was indeed heated during the accretion phase. we detected the source at a rather low x-ray luminosity (∼1033 erg s-1) during only three of our five observations. when the source was not detected it had very low-luminosity upper limits (< 1032 erg s-1; depending on assumed spectral model). we interpret these detections and the variability observed as emission likely due to very low-level accretion onto the neutron star. we also discuss why the neutron-star crust in gro j1750-27 might not have been heated while the ones in 4u 0115+63 and v 0332+53 possibly were. | quiescent x-ray variability in the neutron star be/x-ray transient gro j1750-27 |
we report on the results of a ∼40-d multi-wavelength monitoring of the be x-ray binary system igr j05007-7047 (lxp 38.55). during that period the system was monitored in the x-rays using the swift telescope and in the optical with multiple instruments. when the x-ray luminosity exceeded 1036 erg s-1 we triggered an xmm-newton too observation. timing analysis of the photon events collected during the xmm-newton observation reveals coherent x-ray pulsations with a period of 38.551(3) s (1σ), making it the 17th known high-mass x-ray binary pulsar in the lmc. during the outburst, the x-ray spectrum is fitted best with a model composed of an absorbed power law (γ = 0.63) plus a high-temperature blackbody (kt ∼2 kev) component. by analysing ∼12 yr of available ogle optical data we derived a 30.776(5) d optical period, confirming the previously reported x-ray period of the system as its orbital period. during our x-ray monitoring the system showed limited optical variability while its ir flux varied in phase with the x-ray luminosity, which implies the presence of a disc-like component adding cooler light to the spectral energy distribution of the system. | multi-wavelength properties of igr j05007-7047 (lxp 38.55) and identification as a be x-ray binary pulsar in the lmc |
the code harm\_cool, a conservative scheme for relativistic magnetohydrodynamics, is being developed in our group and works with a tabulated equation of state of dense matter. this eos can be chosen and used during dynamical simulation, instead of the simple ideal gas one. in this case, the inversion scheme between the conserved and primitive variables is not a trivial task. in principle, the code needs to solve numerically five coupled non-linear equations at every time-step. the 5-d recovery schemes were originally implemented in harm and worked accurately for a simple polytropic eos which has an analytic form. our current simulations support the composition-dependent eos, formulated in terms of rest-mass density, temperature and electron fraction. in this proceeding, i discuss and compare several recovery schemes that have been included in our code. i also present and discuss their convergence tests. finally, i show set of preliminary results of a numerical simulation, addressed to the post-merger system formed after the binary neutron stars (bns) coalescence. | neutrino cooled disk in post-merger system studied via numerical gr mhd simulation with a composition-dependent equation of state |
we investigate the gravitational waves (gws) at low frequencies produced by neutrinos that are emitted anisotropically from the protoneutron star (pns) during its cooling phase that lasts for about a minute. we are particularly interested in the deci-hz range, in which some satellite-borne detectors are expected to have good sensitivities. we first give a formulation based on the spherical-harmonic expansion of the neutrino luminosity to obtain the gravitational waveform as well as the characteristic strain. in the absence of multidimensional simulations of pns cooling, from which we can extract reliable data on the neutrino luminosities as a function of a solid angle, we construct them by hand. in the first model, the time evolution is approximated by piecewise exponential functions. in the second model we employ the time profile obtained in a 1d cooling simulation for all harmonic components for simplicity. in both cases, we consider not only axisymmetric components but also nonaxisymmetric ones. in the third model, we consider axisymmetric neutrino emissions, the axis of which is misaligned with the rotation axis and, as a result, rotates with the pns. we find from the first model that the decay times in piecewise exponential function at late phases can be inferred from the positions of bumps and dips in the characteristic strain of the gw in the case of a slow cooling, whereas they may be obtained by identifying the positions of slope change in the case of rapid cooling, which may be induced by convection in pns. we confirm the former result also in the second model. the results of the third model show that the gravitational waves emitted by the neutrinos contain circularly-polarized components in contrast to the first two models, in which only linear polarizations occur, and give oscillatory features in the waveform with the frequencies of integral multiples of the rotation frequency, which are also clearly reflected in the characteristic strain. finally, we compare the gw signals thus obtained with the sensitivity curves of some planned gw detectors that are expected to be sensitive to gws in the deci-hz range. if these admittedly crude models are any guide, these detectors, decigo in particular, will have a fair chance of detecting the gw signals as we consider them in this paper if they are emitted from within our galaxy. | gravitational wave signals in the deci-hz range from neutrinos during the protoneutron star cooling phase |
the gravitational instability, responsible for the formation of the structure of the universe, occurs below energy thresholds and above spatial scales of a self-gravitating expanding region, when thermal energy can no longer counterbalance self-gravity. i argue that at sufficiently-large scales, dark energy may restore thermal stability. this stability re-entrance of an isothermal sphere defines a turnaround radius, which dictates the maximum allowed size of any structure generated by gravitational instability. on the opposite limit of high energies and small scales, i will show that an ideal, quantum or classical, self-gravitating gas is subject to a high-energy relativistic gravothermal instability. it occurs at sufficiently-high energy and small radii, when thermal energy cannot support its own gravitational attraction. applications of the phenomenon include neutron stars and core-collapse supernovae. i also extend the original oppenheimer–volkov calculation of the maximum mass limit of ideal neutron cores to the non-zero temperature regime, relevant to the whole cooling stage from a hot proto-neutron star down to the final cold state. | the gravothermal instability at all scales: from turnaround radius to supernovae |
accreting neutron stars (nss) can exhibit high-frequency modulations, known as burst oscillations, in their light curves during thermonuclear x-ray bursts. their frequencies can be offset from the spin frequency of the ns (known independently) by several hz, and can drift by 1-3 hz. one plausible explanation for this phenomenon is that a wave is present in the bursting ocean that decreases in frequency (in the rotating frame) as the burst cools. the strongest candidate is the buoyant r-mode; however, models for the burning ocean background used in previous studies over-predict frequency drifts by several hz. using new background models (which include shallow heating, and burning in the tail of the burst) the evolution of the buoyant r-mode is calculated. the resulting frequency drifts are smaller, in line with observations. this illustrates the importance of accounting for the detailed nuclear physics in these bursts. | burning in the tail: implications for a burst oscillation model |
late activity of the central engine is often invoked in order to explain the flares observed in the early x-ray afterglow of gamma-ray bursts, either in the form of an active neutron star remnant or (fall-back) accretion on to a black hole. however, these scenarios are not always plausible, in particular when flares are delayed to very late times after the burst. recently, a new scenario was proposed that suggests x-ray flares can be the result of the passing of a long-lived reverse shock through a stratified ejecta, with the advantage that it does not require late-time engine activity. in this work, we numerically demonstrate this scenario to be physically plausible, by performing one-dimensional simulations of ejecta dynamics and emission using our novel moving-mesh relativistic hydrodynamics code. improved efficiency and precision over previous work enables the exploration of a broader range of set-ups. we can introduce a more physically realistic description of the circumburst medium mass density. we can also locally trace the cooling of electrons when computing the broad-band emission from these set-ups. we show that the synchrotron cooling time-scale can dominate the flare decay time if the stratification in the ejecta is constrained to a localized angular region inside the jet, with size corresponding to the relativistic causal connection angle, and that it corresponds to values reported in observations. we demonstrate that this scenario can produce a large range of observed flare times, suggesting a connection between flares and initial ejection dynamics rather than with late-time remnant activity. | late x-ray flares from the interaction of a reverse shock with a stratified ejecta in grb afterglows: simulations on a moving mesh |
we study properties of an accretion ring in a steady mass flow from a companion star to a compact object in an x-ray binary. the accretion ring is a place where matter inflowing from a companion star sojourns for a while to bifurcate to accretion and excretion flows due to angular momentum transfer. the matter in the accretion ring rotates along the keplerian circular orbit determined by the intrinsic specific angular momentum of the inflowing matter and forms a thick ring-envelope. two internal flows are expected to appear in the thick envelope. one is a mass-spreading flow bifurcating to a thick accretion flow and a thick excretion flow, as a result of the angular momentum transfer within the ring-envelope. the other is a cooling flow toward the envelope center governed by radiative cooling under the effect of x-ray irradiation. this cooling flow eventually forms a core in the torus, from which a thin accretion disk and a thin excretion disk spread out, again as a result of the angular momentum transfer there. evaluating and comparing the timescales for the two internal flows, the accretion ring is shown to generally originate a two-layer accretion flow in which a thin accretion disk is sandwiched by a thick accretion flow, unless the accretion rate is very low. properties of the thin excretion disk and the thick excretion flow are also investigated. the thin excretion disk is expected to terminate at a distance of four times the accretion ring radius and to form another ring there, unless tidal effects from the companion star exist. the thick excretion flow is, on the other hand, likely to turn into a supersonic wind flow reaching infinity. | properties of the accretion ring in an x-ray binary, and accretion and excretion two-layer flows from it |
superbursts are hours-long x-ray flares attributed to the thermonuclear runaway burning of carbon-rich material in the envelope of accreting neutron stars. by studying the details of the x-ray light curve, properties of carbon combustion can be determined. in particular, we show that the shape of the rise of the light curve is set by the slope of the temperature profile left behind by the carbon flame. we analyse rossi x-ray timing explorer/proportional counter array observations of 4u 1636-536 and separate the direct neutron star emission from evolving photoionized reflection and persistent spectral components. this procedure results in the highest quality light curve ever produced for the superburst rise and peak, and interesting behaviour is found in the tail. the rising light curve between 100 and 1000 s is inconsistent with the idea that the fuel burned locally and instantaneously everywhere, as assumed in some previous models. by fitting improved cooling models, we measure for the first time the radial temperature profile of the superbursting layer. we find d ln t/d ln p ≈ 1/4. furthermore, 20 per cent of the fuel may be left unburned. this gives a new constraint on models of carbon burning and propagation in superbursts. | the imprint of carbon combustion on a superburst from the accreting neutron star 4u 1636-536 |
sax j1810.8-2609 is a faint x-ray transient, mostly known for its low quiescent thermal luminosity, which disagrees with slow cooling in the core. it is also one of a small sample of stars with a mass and radius that has been estimated using spectral modeling of one of its thermonuclear bursts. here we report the discovery of millisecond oscillation in a type i thermonuclear x-ray burst from sax j1810.8-2609 observed by rossi x-ray timing explorer (rxte) during the 2007 outburst. a strong signal (probability of false detection corresponding to 5.75σ of the normal distribution) was present at 531.8 hz during the decay of one out of six bursts observed. an oscillation was detected for about 6 s, during which its frequency increased from 531.4 to 531.9 hz in a manner similar to other burst oscillation sources. the millisecond oscillation establishes the spin frequency of the neutron star (ns), which is important for the spectral modeling, associated mass-radius inference, and the evolutionary status and cooling behavior of the star. the source goes into outburst semi-regularly (most recently in 2018 april), providing an opportunity to acquire new material for the burst oscillation searches. | a millisecond oscillation in the bursting x-ray flux of sax j1810.8-2609 |
coherent oscillations and the evolution of the x-ray spectrum during thermonuclear x-ray bursts in accreting neutron-star x-ray binaries have been studied intensively but separately. we analysed all the x-ray bursts of the source 4u 1728-34 with the rossi x-ray timing explorer. we found that the presence of burst oscillations can be used to predict the behaviour of the blackbody radius during the cooling phase of the bursts. if a burst shows oscillations, during the cooling phase the blackbody radius remains more or less constant for ∼2- ∼ 8 s, whereas in bursts that do not show oscillations the blackbody radius either remains constant for more than ∼2- ∼ 8 s or it shows a rapid (faster than ∼2 s) decrease and increase. both the presence of burst oscillations and the time-dependent spectral behaviour of the bursts are affected by accretion rate. we also found that the rise time and convexity of the bursts' light curve are different in bursts with and without oscillations in 4u 1728-34. bursts with oscillations have a short rise time (∼0.5 s) and show both positive and negative convexity, whereas bursts without oscillations have a long rise time (∼1 s) and mostly positive convexity. this is consistent with the idea that burst oscillations are associated with off-equator ignition. | the link between coherent burst oscillations, burst spectral evolution and accretion state in 4u 1728-34 |
a rapidly spinning, strongly magnetized neutron star (ns) is invoked as the central engine for some gamma-ray bursts (grbs), especially, the `internal plateau' feature of x-ray afterglow. however, for these `internal plateau' grbs, how to produce their prompt emission remains an open question. two different physical processes have been proposed in the literature, (1) a new-born ns is surrounded by a hyper-accreting and neutrino cooling disc, the grb jet can be powered by neutrino annihilation aligning the spin axis; (2) a differentially rotating millisecond pulsar was formed due to different angular velocity between the interior core and outer shell parts of the ns, which can power an episodic grb jet. in this paper, by analysing the data of one peculiar grb 070110 (with internal plateau), we try to test which model is being favoured. by deriving the physical parameters of magnetar with observational data, the parameter regime for initial period (p0) and surface polar cap magnetic field (bp) of the central ns are 0.96 ∼ 1.2 ms and (2.4 ∼ 3.7) × 1014 g, respectively. the radiative efficiency of prompt emission is about ηγ ∼ 6 per cent. however, the radiative efficiency of internal plateau (ηx) is larger than 31 per cent assuming the mns ∼ 1.4 m⊙ and p0∼ 1.2 ms. the clear difference between the radiation efficiencies of prompt emission and internal plateau implies that they maybe originated from different components (e.g. prompt emission from the relativistic jet powered by neutrino annihilation, while the internal plateau from the magnetic outflow wind). | the radiative efficiency of relativistic jet and wind: a case study of grb 070110 |
type i x-ray bursts on the surface of a neutron star are a unique probe into accretion in x-ray binary systems. however, we know little about the feedback of the burst emission on accretion. hard x-ray shortages and enhancements of the persistent emission at soft x-rays have been observed. to put these findings in context with the aim of understanding the possible mechanism underneath, we investigated 68 bursts seen by the rossi x-ray timing explorer from the clocked burster gs 1826-238. we diagnosed jointly the burst influence of both soft and hard x-rays, and we found that the observations can be described by the comptt model with variable normalization, electron temperature, and optical depth. putting these results in a scenario of coronal compton cooling via the burst emission would lead to a shortage of cooling power, which may suggest that additional considerations, like the influence of the burst on corona formation, should be accounted for as well. | diagnosing the burst influence on accretion in the clocked burster gs 1826-238 |
the be/x-ray transient 4u 0115+63 exhibited a giant, type-ii outburst in 2015 october. the source did not decay to its quiescent state but settled in a meta-stable plateau state (a factor ∼ 10 brighter than quiescence) in which its luminosity slowly decayed. we used xmm-newton to observe the system during this phase and we found that its spectrum can be well described using a blackbody model with a small emitting radius. this suggests emission from hotspots on the surface, which is confirmed by the detection of pulsations. in addition, we obtained a relatively long (∼7.9 ksec) swift/x-ray telescope observation ∼35 d after our xmm-newton one. we found that the source luminosity was significantly higher, and although the spectrum could be fitted with a blackbody model, the temperature was higher and the emitting radius smaller. several weeks later the system started a sequence of type-i accretion outbursts. in between those outbursts, the source was marginally detected with a luminosity consistent with its quiescent level. we discuss our results in the context of the three proposed scenarios (accretion down to the magnestospheric boundary, direct accretion on to neutron star magnetic poles or cooling of the neutron star crust) to explain the plateau phase. | the low-luminosity behaviour of the 4u 0115+63 be/x-ray transient |
although accretion disc coronae appear to be common in many accreting systems, their fundamental properties remain insufficiently understood. recent work suggests that type i x-ray bursts from accreting neutron stars provide an opportunity to probe the characteristics of coronae. several studies have observed hard x-ray shortages from the accretion disc during an x-ray burst implying strong coronal cooling by burst photons. here, we use the plasma emission code eqpair to study the impact of x-ray bursts on coronae, and how the coronal and burst properties affect the coronal electron temperatures and emitted spectra. assuming a constant accretion rate during the burst, our simulations show that soft photons can cool coronal electrons by a factor of ≳ 10 and cause a reduction of emission in the 30-50 kev band to $\lesssim 1{{\ \rm per\ cent}}$ of the pre-burst emission. this hard x-ray drop is intensified when the coronal optical depth and aspect ratio is increased. in contrast, depending on the properties of the burst and corona, the emission in the 8-24 kev band can either increase, by a factor of ≳ 20, or decrease, down to $\lesssim 1{{\ \rm per\ cent}}$ of the pre-burst emission. an increasing accretion rate during the x-ray burst reduces the coronal cooling effects and the electron temperature drop can be mitigated by $\gtrsim 60{{\ \rm per\ cent}}$ . these results indicate that changes of the hard x-ray flux during an x-ray burst probe the geometrical properties of the corona. | cooling of accretion disc coronae by type i x-ray bursts |
the magnetic field of a rotating pulsar might be so strong that the equation of state (eos) of neutron star (ns) matter is significantly affected by the spin polarization of baryons. in the present work, the eos of the spin-polarized nuclear matter is investigated in the nonrelativistic hartree-fock formalism, using a realistic density-dependent nucleon-nucleon interaction with its spin- and spin-isospin dependence accurately adjusted to the brueckner-hartree-fock results for spin-polarized nuclear matter. the nuclear symmetry energy and proton fraction are found to increase significantly with increasing spin polarization of baryons, leading to a larger probability of the direct urca process in the cooling of magnetar. the eos of the β -stable np e μ matter obtained at different spin polarizations of baryons is used as the input for the tolman-oppenheimer-volkoff equations to determine the hydrostatic configuration of ns. based on the gw170817 constraint on the radius r1.4 of ns with m ≈1.4 m⊙ , our mean-field results show that up to 60 % of baryons in the ns merger might be spin-polarized. this result supports the magnetar origin of the "blue" kilonova ejecta of gw170817 suggested by metzger et al., and the spin polarization of baryons needs, therefore, to be properly treated in the many-body calculation of the eos of ns matter before comparing the calculated ns mass and radius with those constrained by the multimessenger gw170817 observation. | spin-polarized β -stable neutron star matter: the nuclear symmetry energy and gw170817 constraint |
extremely compact objects trap gravitational waves or neutrinos, assumed to move along null geodesics in the trapping regions. the trapping of neutrinos was extensively studied for spherically symmetric extremely compact objects constructed under the simplest approximation of the uniform energy density distribution, with radius located under the photosphere of the external spacetime; in addition, uniform emissivity distribution of neutrinos was assumed in these studies. here we extend the studies of the neutrino trapping for the case of the extremely compact tolman vii objects representing the simplest generalization of the internal schwarzschild solution with uniform distribution of the energy density, and the correspondingly related distribution of the neutrino emissivity that is thus again proportional to the energy density; radius of such extremely compact objects can overcome the photosphere of the external schwarzschild spacetime. in dependence on the parameters of the tolman vii spacetimes, we determine the "local" and "global" coefficients of efficiency of the trapping and demonstrate that the role of the trapping is significantly stronger than in the internal schwarzschild spacetimes. our results indicate possible influence of the neutrino trapping in cooling of neutron stars. | neutrino trapping in extremely compact tolman vii spacetimes |
we study heat diffusion after an energy release in a deep spherical layer of the outer neutron star crust (107 ≲ ρ ≲ 4 × 1011 g cm-3). we demonstrate that this layer possesses specific heat-accumulating properties, absorbing heat and directing it mostly inside the star. it can absorb up to ∼1043-1044 erg due to its high heat capacity, until its temperature exceeds t ∼ 3 × 109 k and triggers a rapid neutrino cooling. a warm layer (t ∼ 108-3 × 109 k) can serve as a good heat reservoir, which is thermally decoupled from the inner crust and the stellar core for a few months. we present a toy model to explore the heat diffusion within the heat-accumulating layer, and we test this model using numerical simulations. we formulate some generic features of the heat propagation that can be useful, for instance, for the interpretation of superbursts in accreting neutron stars. we present a self-similar analysis of late afterglow after such superbursts, which can be helpful to estimate properties of bursting stars. | model of heat diffusion in the outer crust of bursting neutron stars |
high-velocity neutron stars (hvnss) that were kicked out from their birth location can be potentially identified with their large proper motions, and possibly with large parallax, when they come across the solar neighborhood. in this paper, we study the feasibility of hunting isolated hvnss in wide-area optical surveys by modelling the evolution of ns luminosity taking into account spin-down and thermal radiation. assuming the upcoming 10-yr vera c. rubin observatory's lsst observation, our model calculations predict that about 10 hvnss mainly consisting of pulsars with ages of 104-$10^5\, \rm yr$ and thermally emitting nss with 105-$10^6\, \rm yr$ are detectable. we find that a few nss with effective temperature $\lt 5 \times 10^5\, \rm k$, which are likely missed in the current and future x-ray surveys, are also detectable, and the detectability highly depends on ns cooling models. in particular, if considering dark matter heating in ns cores, we find that such cold nss would account for $70{{\ \rm per\ cent}}$ of the whole detectable sample, and they are typically older than $10^6\, \rm yr$ and tend to have slow transverse velocities, $\lesssim 100\, \rm km\, s^{-1}$, compared to already known pulsars. thus, the future optical observation will give a unique ns sample, which can provide essential constraints on the ns cooling and heating mechanisms. moreover, we suggest that providing hvns samples with optical surveys is helpful for understanding the intrinsic kick-velocity distribution of nss. | hunting wide-area optical surveys for high proper motion isolated neutron stars |
the aim of this study was to perform sensitivity analysis, investigating how different fluidisation and sedimentation characteristics of three-phase wetted uo2 powder beds, may affect a transient nuclear criticality excursion initiated through the addition of water into a fissile powder bed. this type of postulated nuclear criticality accident scenario may occur in nuclear fuel fabrication facilities when a fire is fought through the use of water, supplied via an automatic or manual fire-suppression system. a similar scenario may also develop as a result of water leaks or flooding of the process area housing uo2 powder. the article introduces a model for gas-bubble induced fluidisation of a uo2 powder bed and examines how this phenomenon may affect the neutron kinetic response of a three-phase fluidised fissile powder system. empirical analysis has shown that fissile suspensions form agglomerated structures when suspended in water, at agglomerate sizes that range from 18 μm to 40 μm . simulation results indicate that both the critical gas velocity and rate of fluidisation may significantly affect transient nuclear criticality excursion dynamics. the re-distribution of fissile mass into a highly dispersed suspension generally reduces the reactivity of the system, however, depending on the h/u ratio, a positive reactivity may be added to the system. low péclet numbers in the suspension suggest that gas-bubble induced motion of the suspension causes a highly dispersive flow field. an oscillatory power response is predicted for low critical gas velocities where the reactivity of the system is predominantly governed by the re-distribution of fissile mass within the system. the frequency of these oscillations is greater for a higher hindered settling rate of powder particles. at a higher critical gas velocity, the transient nuclear criticality excursion is governed by the voidage reactivity feedback, making the response quite independent of fluidisation. in all cases, large volumes of uo2 powder may leave the domain due to overflowing of the suspension. transient nuclear criticality excursions in uo2 powder beds with a low critical gas velocity are terminated once the bed becomes fully saturated. | transient nuclear criticality excursion analysis of highly dispersed particulate three-phase fluidised systems |
we report a 72 ks xmm-newton observation of the be/x-ray pulsar (bexrp) rx j0812.4-3114 in quiescence (l_x ≈ 1.6× 10^{33} erg s^{-1}). intriguingly, we find a two-component spectrum, with a hard power-law (γ ≈ 1.5) and a soft blackbody-like excess below ≈1 kev. the blackbody component is consistent in kt with a prior quiescent chandra observation reported by tsygankov et al. and has an inferred blackbody radius of ≈10 km, consistent with emission from the entire neutron star (ns) surface. there is also mild evidence for an absorption line at ≈ 1 and/or ≈ 1.4 kev. the hard component shows pulsations at p ≈ 31.908 s (pulsed fraction 0.84 ± 0.10), agreeing with the pulse period seen previously in outbursts, but no pulsations were found in the soft excess (pulsed fraction ≲ 31 per cent). we conclude that the pulsed hard component suggests low-level accretion on to the ns poles, while the soft excess seems to originate from the entire ns surface. we speculate that, in quiescence, the source switches between a soft thermal-dominated state (when the propeller effect is at work) and a relatively hard state with low-level accretion, and use the propeller cut-off to estimate the magnetic field of the system to be ≲ 8.4× 10^{11} g. we compare the quiescent thermal lx predicted by the standard deep crustal heating model to our observations and find that rx j0812.4-3114 has a high thermal lx, at or above the prediction for minimum cooling mechanisms. this suggests that rx j0812.4-3114 either contains a relatively low-mass ns with minimum cooling, or that the system may be young enough that the ns has not fully cooled from the supernova explosion. | soft excess in the quiescent be/x-ray pulsar rx j0812.4-3114 |
recent observational data on transiently-accreting neutron stars has unequivocally shown fast-cooling sources, such as in the case of neutron star mxb 1659-29. previous calculations have estimated its total neutrino luminosity and heat capacity, as well as suggested that direct urca reactions take place in $1 \%$ of the volume of the core. in this paper, we reproduce the inferred luminosity of this source with detailed models of equations of state (eos) and nuclear pairing gaps. we show that three superfluidity gap models are inconsistent with data for all eos and another three are disfavoured because of fine tuning arguments. we also calculate the total heat capacity for all constructed stars and show that independent observations of mass and luminosity could set constraints on the core superfluidity of a source as well as the density slope of the symmetry energy, l. this is an important step towards defining a universal equation of state for neutron stars and therefore, towards a better understanding of the phase diagram of asymmetric matter at high densities. | probing dense matter physics with transiently-accreting neutron stars: the case of source mxb 1659-29 |
we analyse and determine the effects of modest progenitor rotation in the context of core-collapse supernovae by comparing two separate long-duration three-dimensional simulations of 9 m$_{\odot}$ progenitors, one rotating with an initial spin period of $\sim$60 seconds and the other non-rotating. we determine that both models explode early, though the rotating model explodes a bit earlier. despite this difference, the asymptotic explosion energies ($\sim$10$^{50}$ ergs) and residual neutron star baryon masses ($\sim$1.3 m$_{\odot}$) are similar. we find that the proto-neutron star (pns) core can deleptonize and cool significantly more quickly. soon into the evolution of the rotating model, we witness more vigorous and extended pns core convection that early in its evolution envelopes the entire inner sphere, not just a shell. moreover, we see a corresponding excursion in both the $\nu_e$ luminosity and gravitational-wave strain that may be diagnostic of this observed dramatic phenomenon. in addition, after bounce the innermost region of the rotating model seems to execute meridional circulation. the rotationally-induced growth of the convective pns region may facilitate the growth of core b-fields by the dynamo mechanism by facilitating the achievement of the critical rossby number condition for substantial growth of a dipole field, obviating the need for rapid rotation rates to create dipole fields of significance. the next step is to explore the progenitor-mass and spin dependencies across the progenitor continuum of the supernova explosion, dynamics, and evolution of pns convection and its potential role in the generation of magnetar and pulsar magnetic fields. | the physical effects of progenitor rotation: comparing two long-duration 3d core-collapse supernova simulations |
astrophysicists have found the first direct evidence for the fastest neutrino-emission mechanism by which neutron stars can cool. | a rapidly cooling neutron star |
the accretion behaviour in low-mass x-ray binaries (lmxbs) at low luminosities, especially at <1034 erg s-1, is not well known. this is an important regime to study to obtain a complete understanding of the accretion process in lmxbs, and to determine if systems that host neutron stars with accretion-heated crusts can be used probe the physics of dense matter (which requires their quiescent thermal emission to be uncontaminated by residual accretion). here, we examine ultraviolet (uv) and x-ray data obtained when exo 0748-676, a crust-cooling source, was in quiescence. our hubble space telescope spectroscopy observations do not detect the far-uv continuum emission, but do reveal one strong emission line, c iv. the line is relatively broad (≳3500 km s-1), which could indicate that it results from an outflow such as a pulsar wind. by studying several epochs of x-ray and near-uv data obtained with xmm-newton, we find no clear indication that the emission in the two wavebands is connected. moreover, the luminosity ratio of lx/luv ≳ 100 is much higher than that observed from neutron star lmxbs that exhibit low-level accretion in quiescence. taken together, this suggests that the uv and x-ray emission of exo 0748-676 may have different origins, and that thermal emission from crust-cooling of the neutron star, rather than ongoing low-level accretion, may be dominating the observed quiescent x-ray flux evolution of this lmxb. | uv and x-ray observations of the neutron star lmxb exo 0748-676 in its quiescent state |
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 the 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 quarks. the bag-like models also incorporate the pasta phase used to describe the inner crust of neutron stars. with a simple method studied in the present work, we show that the pasta phase does not play a significant role. moreover, the qmc model that satisfies the gw170817 constraints with the lowest slope of the symmetry energy exhibits a cooling profile compatible with observational data. * this work is a part of the project inct-fna proc. (464898/2014-5), was partially supported by cnpq (brazil) (301155.2017-8)(d.p.m.), 310242/2017-7, 406958/2018-1 (o.l.), 308486/2015-3 (t.f.), 433369/2018-3 (m.d.), by capes-pnpd program (c.v.f.), and by fundação de amparo à pesquisa do estado de são paulo (fapesp) under the thematic projects 2013/26258-4 (o.l, t.f.) and 2017/05660-0 (o.l.,m.d.,t.f.). r.n. also acknowledges that this project was partly funded by faperj, under grant e-26/203.299/2017. m.b.acknowledge the support from fapesp project no. 2017/05660-0, and fostect project no. fostect.2019b.04. pds acknowledges support from the uk stfc under project number st/p005314/1 | neutron star cooling and gw170817 constraint within quark-meson coupling models |
axial anomalous contributions into neutrino pbf losses due to triplet pairing of neutrons are still ignored in modeling the evolution of neutron stars. in this paper, the influence of the anomalous axial contributions onto the rate of neutron stars cooling is estimated. | on the pbf neutrino losses in superfluid cores of neutron stars |
an efficient panchromatic planar perovskite solar cell is developed based on highly uniform, lead-reduced ch3nh3sn0.5pb0.5i3 perovskite films with full film-coverage on the substrates. we demonstrate here that full-coverage of the ch3nh3sn0.5pb0.5i3 films can be developed by a facile chlorobenzene-assisted spin-coating method. a power conversion efficiency of ~7% is achieved using low-temperature processes, which is among the best-reported performance for panchromatic planar perovskite solar cells with a light-absorption over 1,000 nm. | highly compact and uniform ch3nh3sn0.5pb0.5i3 films for efficient panchromatic planar perovskite solar cells |
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 the hyperon direct urca processes play crucial roles for macroscopic cooling behavior 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, and 2) it makes the nucleon direct urca more feasible by reducing the neutron fermi momentum. all of the equations of state (eoss) used in this work satisfy the maximum mass constraints and the allowed areas of mass and radii of neutron stars. we found that the neutron star eos with hyperons can still be consistent with both mass and temperature observations if the original neutron star's eos without hyperons is stiff enough. on the other hand, we found that the neutron star's eos with kaons can be consistent only with mass observations, but can hardly explain the cooling behavior if we take into account the statistical distribution of observed neutron star masses. | strangeness in neutron star cooling |
we construct the quiescent neutron star models in the evolutionary calculations. the x-ray luminosities have been derived in terms of the time-averaged mass accretion rate for various neutron star masses and surface compositions. we compare the quiescent luminosities observed from x-ray transients in low mass x-ray binaries, where the stellar evolutionary calculations of accreting neutron stars include neutrino cooling due to strong pion condensations. our results based on the evolutionary calculations suggest that stronger cooling process would be necessary to be consistent with observations. | quiescent luminosities of accreting neutron stars-possibility of neutrino losses due to strong pion condensations |
after an introduction to the qcd phase diagram, the nuclear equations of state, and neutron star mergers, i discuss three projects related to transport and nuclear matter in neutron star mergers. the first is the nature of beta equilibrium in the portion of a merger that is transparent to neutrinos. we calculate the weak interaction (urca) rates and find that the beta equilibrium condition needs to be modified by adding an additional chemical potential, which changes slightly the particle content in neutrino-transparent beta equilibrium. secondly, we calculate the bulk viscosity in neutrino-transparent nuclear matter in conditions encountered in neutron star mergers. bulk viscosity arises from a phase lag between the pressure and density in the nuclear matter, which is due to the finite rate of beta equilibration. when bulk viscosity is sufficiently strong, which happens when the equilibration rate nearly matches the frequency of the density oscillation, it can noticeably dampen the oscillation. we find that in certain thermodynamic conditions likely encountered in mergers, oscillations in nuclear matter can be damped on timescales on the order of 10 milliseconds, so we conclude that bulk viscosity should be included in merger simulations. finally, we study thermal transport due to axions in neutron star mergers. we conclude that axions are never trapped in mergers, but instead escape, carrying energy away from the merger. we calculate the cooling time due to the energy carried away by axions and find that within current constraints on the axion-nucleon coupling, axions could cool fluid elements in mergers on timescales which could affect the dynamics of the merger. | transport in neutron star mergers |
we find that the proton separation energy, s (p ) , of 73rb is -640 (40 ) kev, deduced from the observation of β -delayed ground-state protons following the decay of 73sr. this lower-limit determination of the proton separation energy of 73rb coupled with previous upper limits from nonobservation, provides a full constraint on the mass excess with δ m (73rb)=-46.01 ±0.04 mev. with this new mass excess and the excitation energy of the jπ=5 /2- isobaric-analog state (t =3 /2 ) in 73rb, an improved constraint can be put on the mass excess of 73sr using the isobaric-multiplet mass equation (imme), and we find δ m (73sr)=-31.98 ±0.37 mev. these new data were then used to study the composition of ashes on accreting neutron stars following type i x-ray bursts. counterintuitively, we find that there should be an enhanced fraction of a >102 nuclei with more negative proton separation energies at the 72kr rp-process waiting point. larger impurities of heavier nuclei in the ashes of accreting neutron stars will impact the cooling models for such astrophysical scenarios. | influence of 73rb on the ashes of accreting neutron stars |
we develop a multicomponent hydrodynamic model based on moments of the born-bogolyubov-green-kirkwood-yvon hierarchy equations for physical conditions relevant to astrophysical plasmas. these equations incorporate strong correlations through a density functional theory closure, while transport enters through a relaxation approximation. this approach enables the introduction of coulomb coupling correction terms into the standard burgers equations. the diffusive currents for these strongly coupled plasmas is self-consistently derived. the settling of impurities and its impact on cooling can be greatly affected by strong coulomb coupling, which we show can be quantified using the direct correlation function. | a dynamic density functional theory approach to diffusion in white dwarfs and neutron star envelopes |
in this work, we explore different aspects in which strong magnetic fields play a role in the composition, structure and evolution of neutron stars. more specifically, we discuss (i) how strong magnetic fields change the equation of state of dense matter, alter its composition, and create anisotropies, (ii) how they change the structure of neutron stars (such mass and radius) and the formalism necessary to calculate those changes, and (iii) how they can affect neutron stars' evolution. in particular, we focus on how a time-dependent magnetic field modifies the cooling of a special group known as x-ray dim neutron stars. | many aspects of magnetic fields in neutron stars |
magnetars are neutron stars having extreme magnetic field strengths. study of their emission properties in quiescent state can help understand effects of a strong magnetic field on neutron stars. sgr 0501+4516 is a magnetar that was discovered in 2008 during an outburst, which has recently returned to quiescence. we report its spectral and timing properties measured with new and archival observations from the chandra x-ray observatory, xmm-newton, and suzaku. we found that the quiescent spectrum is best fit by a power-law plus two blackbody model, with temperatures of kt low ∼ 0.26 kev and kt high ∼ 0.62 kev. we interpret these two blackbody components as emission from a hotspot and the entire surface. the hotspot radius shrunk from 1.4 km to 0.49 km since the outburst, and there was a significant correlation between its area and the x-ray luminosity, which agrees well with the prediction by the twisted magnetosphere model. we applied the two-temperature spectral model to all magnetars in quiescence and found that it could be a common feature among the population. moreover, the temperature of the cooler blackbody shows a general trend with the magnetar field strength, which supports the simple scenario of heating by magnetic field decay. | x-ray observations of magnetar sgr 0501+4516 from outburst to quiescence |
we present x-ray and radio monitoring observations of the gamma-ray binary psr j2032+4127/mt91 213 during its periastron passage in late 2017. dedicated chandra, xmm-newton, nustar x-ray observations, and very large array radio observations of this long orbit (50 yr), 143 ms pulsar/be star system clearly revealed flux and spectral variability during the passage. the x-ray spectrum hardened near periastron, with a significant decrease in the power-law (pl) photon index from γ ≈ 2 to 1.2 and evidence of an increased absorption column density. we identified a possible spectral break at a few kev in the spectrum that suggests synchrotron cooling. a coincident radio and x-ray flare occurred one week after periastron, which is possibly the result of the pulsar wind interacting with the be stellar disk and generating synchrotron radiation. however, a multiwavelength comparison indicates that the x-ray and radio spectra cannot be simply connected by a single pl component. hence, the emission in these two energy bands must originate from different particle populations. | x-ray and radio variabilities of psr j2032+4127 near periastron |
we present a new mechanism for deep crustal heating in accreting neutron stars. charged pions (π+) are produced in nuclear collisions on the neutron star surface during active accretion and upon decay they provide a flux of neutrinos into the neutron star crust. for massive and/or compact neutron stars, neutrinos deposit ≈1 -2 mev of heat per accreted nucleon into the inner crust. the strength of neutrino heating is comparable to the previously known sources of deep crustal heating, such as from pycnonuclear fusion reactions, and is relevant for studies of cooling neutron stars. we model the thermal evolution of a transient neutron star in a low-mass x-ray binary, and in the particular case of the neutron star mxb 1659-29 we show that additional deep crustal heating requires a higher thermal conductivity for the neutron star inner crust. a better knowledge of pion-production cross sections near threshold would improve the accuracy of our predictions. | deep crustal heating by neutrinos from the surface of accreting neutron stars |
we investigate whether the intrinsic spin of neutron stars (nss) leaves an observable imprint on the spectral properties of x-ray binaries. to evaluate this, we consider a sample of nine nss for which the spins have been measured that are not accreting pulsars (for which the accretion geometry will be different). for each source, we perform spectroscopy on a majority of rxte hard-state observations. our sample of sources and observations spans the range of the eddington ratios lx/ledd ∼ 0.005-0.100. we find a clear trend between the key comptonization properties and the ns spin for a given accretion rate. specifically, at a given l/ledd, for more rapidly rotating nss we find lower seed photon temperatures and a general increase in comptonization strength, as parametrized by the comptonization y parameter and amplification factor a. this is in good agreement with the theoretical scenario whereby less energy is liberated in a boundary layer for more rapidly spinning nss, resulting in a lower seed photon luminosity and, consequently, less compton cooling in the corona. this effect in extremis results in the hard states of the most rapidly spinning sources encroaching upon the regime of comptonization properties occupied by black holes. | the impact of neutron star spin on x-ray spectra |
we report on the long-term flux relaxation of the magnetar sgr 1627-41 after its 2008 outburst, and evidence for hard x-ray excess measured with nustar. we use new observations made with chandra and xmm-newton, and an archival nustar observation, which add flux measurements at ∼2000 days into quiescence after the 2008 outburst. we find that the source flux has further declined since the last measurement made in 2011, ∼1000 days after the outburst in 2008. this trend is similar to the relaxation after the source’s 1998 outburst. we use crustal cooling models to reproduce the flux relaxation; if the whole surface of the star is heated in the outbursts, the modeling suggests that the 2008 outburst of sgr 1627-41 deposited energy into the inner crust and that the core temperature of sgr 1627-41 is low (tc≲ 108 k), as previously suggested. on the other hand, if only a small fraction of the surface is heated or the temperature in the crust reached the melting temperature, relaxation at early times requires another emission mechanism. finally, we report on evidence for hard x-ray emission in sgr 1627-41 that follows the observational correlation suggested by kaspi & boydstun in magnetars. | flux relaxation after two outbursts of the magnetar sgr 1627-41 and possible hard x-ray emission |
we carry out the first robust numerical simulation of accretion flows on a weakly magnetized neutron star using smoothed particle hydrodynamics (sph). we follow the two-component advective flow (tcaf) paradigm for black holes and focus only on the advective component for the case of a neutron star. this low-viscosity sub-keplerian flow will create a normal boundary layer (nbol) right on the star surface in addition to the centrifugal pressure supported boundary layer (cenbol) present in a black hole accretion. these density jumps could give rise to standing or oscillating shock fronts. during a hard spectral state, the incoming flow has a negligible viscosity, causing a larger sub-keplerian component as compared to the keplerian disk component. we show that our simulation of flows with cooling and a negligible viscosity produces precisely two shocks and strong supersonic winds from these boundary layers. we find that the specific angular momentum of matter dictates the locations and the nature of oscillations of these shocks. for flows with low angular momentum, the radial oscillation appears to be preferred. for flows with higher angular momentum, the vertical oscillation appears to become dominant. in all cases, asymmetries with respect to the z = 0 plane are seen, and instabilities set in due to the interaction of the inflow and outgoing strong winds. our results capture both the low- and high-frequency quasi-periodic oscillations without invoking magnetic fields or any precession mechanism. most importantly, these solutions directly corroborate observed features of wind-dominated high-mass x-ray binaries, such as cir x-1. | timing properties of shocked accretion flows around neutron stars in the presence of cooling |
context. transient neutron star low-mass x-ray binaries undergo episodes of accretion, alternated with quiescent periods. during an accretion outburst, the neutron star heats up due to exothermic accretion-induced processes taking place in the crust. besides the long-known deep crustal heating of nuclear origin, a likely non-nuclear source of heat, dubbed "shallow heating", is present at lower densities. most of the accretion-induced heat slowly diffuses into the core on a timescale of years. over many outburst cycles, a state of equilibrium is reached when the core temperature is high enough that the heating and cooling (photon and neutrino emission) processes are in balance.aims: we investigate how stellar characteristics and outburst properties affect the long-term temperature evolution of a transiently accreting neutron star. for the first time the effects of crustal properties are considered, particularly that of shallow heating.methods: using our code nscool, we tracked the thermal evolution of a neutron star undergoing outbursts over a period of 105 yr. the outburst sequence is based on the regular outbursts observed from the neutron star transient aql x-1. for each model we calculated the timescale over which equilibrium was reached and we present these timescales along with the temperature and luminosity parameters of the equilibrium state.results: we performed several simulations with scaled outburst accretion rates, to vary the amount of heating over the outburst cycles. the results of these models show that the equilibrium core temperature follows a logarithmic decay function with the equilibrium timescale. secondly, we find that shallow heating significantly contributes to the equilibrium state. increasing its strength raises the equilibrium core temperature. we find that if deep crustal heating is replaced by shallow heating alone, the core would still heat up, reaching only a 2% lower equilibrium core temperature. deep crustal heating may therefore not be vital to the heating of the core. additionally, shallow heating can increase the quiescent luminosity to values higher than previously expected. the thermal conductivity in the envelope and crust, including the potentially low-conductivity pasta layer at the bottom of the crust, is unable to significantly alter the long-term internal temperature evolution. stellar compactness and nucleon pairing in the core change the specific heat and the total neutrino emission rate as a function of temperature, with the consequences for the properties of the equilibrium state depending on the exact details of the assumed pairing models. the presence of direct urca emission leads to the lowest equilibrium core temperature and the shortest equilibrium timescale. | long-term temperature evolution of neutron stars undergoing episodic accretion outbursts |
we study the cooling of isolated neutron stars with particular regard to the importance of nuclear pairing gaps. a microscopic nuclear equation of state derived in the brueckner-hartree-fock approach is used together with compatible neutron and proton pairing gaps. we then study the effect of modifying the gaps on the final deduced neutron star mass distributions. we find that a consistent description of all current cooling data can be achieved and a reasonable neutron star mass distribution can be predicted employing the (slightly reduced by about 40%) proton 1s0 bardeen-cooper-schrieffer (bcs) gaps and no neutron 3p2 pairing. | nuclear pairing gaps and neutron star cooling |
summaryeffective biosecurity and pest management are fundamental to sustainable development. invasive ants pose significant risks to the environment and economy, which are well‑managed by biosecurity agencies in developed countries. however, a comprehensive view of the potential impacts of invasive ants in developing pacific island countries and territories (picts) is lacking. we quantified the potential costs of the red imported fire ant (solenopsis invicta buren) across multiple sectors using an extrapolation analysis. overall, we estimated that the impacts of red imported fire ant on developing picts could amount to over usd 329 million annually, corresponding to approximately 0.7% of combined gdp. over half of the costs were predicted to result from impacts on the agriculture sector, a major source of employment and subsistence. we found that over 350 highly threatened species could be at risk from red imported fire ant. we would expect countries with least developed country status and relatively low gdp to be least able to respond to an incursion of these ants, and as a result the costs could be higher than we have extrapolated. red imported fire ant could therefore potentially have considerable impact on the on‑going development of the region. | predicting socio‑economic and biodiversity impacts of invasive species: red imported fire ant in the developing western pacific |
we study the quiescent luminosities of accreting neutron stars by a new mechanism as neutrino heating for additional deep crustal heating, where the neutrino heating is produced by charged pions decay from the nuclear collisions on the surface of neutron star during its active accretion. for low mass neutron star (≲1.4 m⊙), as the neutrino heating is little (≲1 mev per accreted nucleon) or there would be no neutrino heating, the quiescent luminosity will be not affected or slightly affected. while for massive neutron star (≳2 m⊙), the quiescent luminosity will be enhanced more obviously with neutrino heating in the range 2-6 mev per accreted nucleon. the observations on cold neutron stars such as 1h 19605 +00 , sax j1808.4-3658 can be explained with neutrino heating if a fast cooling and heavy elements surface are considered. the observations on a hot neutron star such as rx j0812.4-3114 can be explained with neutrino heating if the direct urca process is forbidden for a massive star with light elements surface, which is different from the previous work that the hot observations should be explained with small mass neutron star and the effect of superfluidity. | quiescent luminosities of transiently accreting neutron stars with neutrino heating due to charged pion decay |
the fundamental difference between accretion around black holes and neutron stars is the inner boundary condition, which affects the behavior of matter very close to the compact objects. this leads to formation of additional shocks and boundary layers for neutron stars. previous studies on the formation of such boundary layers focused on keplerian flows that reached the surface of the star, either directly or through the formation of a transition layer. however, behavior of sub-keplerian matter near the surface of a neutron star has not been studied in detail. here, we study the effect of viscosity, in presence of cooling, on the sub-keplerian flows around neutron stars, using smoothed particle hydrodynamics. our time-dependent study shows that multiple shocks, transition and boundary layers form in such type of accretion, when viscosity is significant, and one or more layers are absent when the viscosity is moderate. these flows are particularly of interest for the wind dominated systems such as cir x-1. we also report the formation of a generalized flow configuration, two-component advective flow, for the first time. | two-component advective flows around neutron stars |
starting from a separable pairing force of finite range of nucleons, a new set of the pairing strengths for the 1s0 hyperon-hyperon pairing based on the quark model is proposed and the 1s0 hyperon superfluidity in neutron stars with the new hyperon pairing strengths is investigated. the reliable λ λ pairing strength is 2/3 times n n pairing strength. the upper limit of σ σ pairing strength is 2/3 times n n pairing strength. the lower limit of the ratio of ξ ξ pairing strength to n n pairing strength is 1. with these pairing strengths, the maximum λ λ pairing gap is several tenths of a mev which can be compared with the results calculated with the widely used pairing force esc00; because of a low fraction of σ− hyperon, the σ superfluidity is very weak or even absent in neutron stars; the ξ superfluidity is stronger than the results calculated with esc08c and the ξ−ξ− pairing may potentially affect the cooling properties of massive neutron stars. | 1s0 hyperon superfluidity in neutron stars from a separable pairing force of finite range |
binary neutron star mergers are believed to eject significant masses with a diverse range of velocities. once these ejected materials begin to be decelerated by a homogeneous medium, relativistic electrons are mainly cooled down by synchrotron radiation, generating a multiwavelength long-lived afterglow. analytic and numerical methods illustrate that the outermost matter, the merger shock-breakout material, can be parametrized by power-law velocity distributions $\propto \left(\beta_{\rm c}\gamma \right)^{-\alpha_s}$. considering that the shock-breakout material is moving on-axis towards the observer and the relativistic jet off-axis, we compute the light curves during the relativistic and the lateral expansion phase. as a particular case, we successfully describe the x-ray, optical and radio light curves alongside the spectral energy distribution from the recently discovered gravitational-wave transient gw170817, when the merger shock-breakout material moves with mildly relativistic velocities near-newtonian phase and the jet with relativistic velocities. future electromagnetic counterpart observations of this binary system could be able to evaluate different properties of these light curves. | light curves of a shock-breakout material and a relativistic off-axis jet from a binary neutron star system |
we examine the influence of the density dependence of the symmetry energy on several properties of neutron stars. in particular, we study the constraints set on the nuclear matter equation of state by the values of the tidal deformability and neutron star radius, using a diverse set of relativistic and non-relativistic mean field models consistent with bulk properties of finite nuclei and the observed lower bound on the maximum mass of neutron star. the tidal deformability and radius show a strong correlation with specific linear combinations of the isoscalar and isovector nuclear matter parameters associated with the eos. such correlations suggest that a precise value of the radius or the tidal deformability can put tight bounds on several eos parameters, in particular, on the slope of the incompressibility and the curvature of the symmetry energy. we show that the density dependence of the symmetry energy has a direct influence on the amount of strangeness inside cold dense matter and, consequently, on the direct urca process and cooling of neutron stars. we explain the low luminosity of sax 1808.4-3658 as a result of hyperonic direct urca processes. finally, we discuss the strong influence of the density dependence of the symmetry energy on the extension of the crust-core transition region of a magnetized neutron star. the increase of the crust and its of complexity, due to the magnetic field effect, may have a role on the glitch mechanism or on the magnetic field decay. | neutron star properties: constraining the nuclear matter eos |
we monitored the neutron star low-mass x-ray binary sax j1750.8-2900 after the end of its 2015/2016 outburst using the x-ray telescope (xrt) aboard swift to detect possible post-outburst 'rebrightenings', similar to those seen after its 2008 outburst. we did not detect any rebrightening behaviour, suggesting that the physical mechanism behind the rebrightening events is not always active after each outburst of the source. any model attempting to explain these rebrightenings should thus be able to reproduce the different outburst profiles of the source at different times. surprisingly, our swift/xrt observations were unable to detect the source, contrary to previous swift/xrt observations in quiescence. we determined a temperature upper limit of ≤106 ev, much colder than the post 2008 outburst value of ∼145 ev. we also report on an archival chandra observation of the source after its 2011 outburst and found a temperature of ∼126 ev. these different temperatures, including the non-detection very close after the end of the 2015/2016 outburst, are difficult to explain in any model assuming we observe the cooling emission from a neutron star core or an accretion-heated crust. we discuss our observations in the context of a change in envelope (the outer ∼100 m of the crust) composition and (possibly in combination with) a cooling crust. both hypotheses cannot explain our results unless potentially unrealistic assumptions are made. irrespective of what causes the temperature variability, it is clear that the neutron star in sax j1750.8-2900 may not be as hot as previously assumed. | variable quiescent state for the neutron-star x-ray transient sax j1750.8-2900: not such a hot neutron star after all? |
recent long-term radio follow-up observations of gw170817 reveal a simple power-law rising light curve, with a slope of {t}0.78, up to 93 days after the merger. the latest x-ray detection at 109 days is also consistent with such a temporal slope. such a shallow rise behavior requires a mildly relativistic outflow with a steep velocity gradient profile, so that slower material with larger energy catches up with the decelerating ejecta and re-energizes it. it has been suggested that this mildly relativistic outflow may represent a cocoon of material. we suggest that the velocity gradient profile may form during the stage that the cocoon is breaking out of the merger ejecta, resulting from shock propagation down a density gradient. the cooling of the hot relativistic cocoon material immediately after it breaks out should have produced soft x-ray to uv radiation at tens of seconds to hours after the merger. the soft x-ray emission has a luminosity of {l}{{x}}∼ {10}45 {erg} {{{s}}}-1 over a period of tens of seconds for a merger event like gw170817. the uv emission shows a rise initially and peaks at about a few hours with a luminosity of {l}{uv}∼ {10}42 {erg} {{{s}}}-1. the soft x-ray transients could be detected by future wide-angle x-ray detectors, such as the chinese mission einstein probe. this soft x-ray/uv emission would serve as one of the earliest electromagnetic counterparts of gravitation waves from double neutron star mergers and could provide the earliest localization of the sources. | early soft x-ray to uv emission from double neutron star mergers: implications from the long-term observations of gw170817 |
we have derived analytic expressions that describe cooling of isolated neutron stars with nucleon cores after reaching the state of internal thermal relaxation. the results are valid for a wide class of equations of state of nucleonic matter and, in this sense, are universal. moreover, they accurately reproduce the evolution of neutron stars at the neutrino and photon cooling stages as well as during transition from one stage to the other. these results greatly simplify theoretical analysis of internal structure of cooling neutron stars. for illustration, we analyse the thermal state of the bright nearby neutron star rx j1856.5-3754 and present arguments that this star has already left the neutrino cooling stage and contains superfluidity of neutrons and protons inside. we discuss possible efficiency of its neutrino cooling and heat capacity of its core. | analytic description of neutron star cooling |
the light curve of gw170817 is surprisingly blue and bright. assuming that the event is a binary neutron star merger, we argue that blueness and brightness of the light curve is the result of ejecta that contains an substantial amount of thermal energy. to achieve this, the ejecta must be reheated at a substantial distance (1-2000 solar radii) from the merger to avoid losing the energy to adiabatic cooling. we show that this reheating can occur if the merger occurs in a hierarchical triple system where the outer star has evolved and filled its roche lobe. the outer star feeds mass to the inner binary, forming a circumbinary disc, driving the inner binary to merge. because the outer star fills its roche lobe, a substantial fraction of the dynamical ejecta collides with the evolved star, reheating the ejecta in the process. we suggest that the process of mass transfer in hierarchical triples tends to form coplanar triple systems such as psr j0337+1715, and may provide electromagnetic counterparts to binary black hole mergers. | gw170817: a neutron star merger in a mass-transferring triple system |
we report on a detailed study of the spectral and temporal properties of the neutron star low-mass x-ray binary slx 1737-282, which is located only ∼1° away from sgr a*. the system is expected to have a short orbital period, even within the ultracompact regime, given its persistent nature at low x-ray luminosities and the long duration thermonuclear burst that it has displayed. we have analysed a suzaku (18 ks) observation and an xmm-newton (39 ks) observation taken 7 yr apart. we infer (0.5-10 kev) x-ray luminosities in the range of 3-6 × 1035ergs-1, in agreement with previous findings. the spectra are well described by a relatively cool (ktbb = 0.5 kev) blackbody component plus a comptonized emission component with γ ∼ 1.5-1.7. these values are consistent with the source being in a faint hard state, as confirmed by the ∼20 per cent fractional root-mean-square amplitude of the fast variability (0.1-7 hz) inferred from the xmm-newton data. the electron temperature of the corona is ≳7 kev for the suzaku observation, but it is measured to be as low as ∼2 kev in the xmm-newton data at higher flux. the latter is significantly lower than expected for systems in the hard state. we searched for x-ray pulsations and imposed an upper limit to their semi-amplitude of 2 per cent (0.001-7 hz). finally, we investigated the origin of the low-frequency variability emission present in the xmm-newton data and ruled out an absorption dip origin. this constraint the orbital inclination of the system to ≲65° unless the orbital period is longer than 11 h (i.e. the length of the xmm-newton observation). | the very faint hard state of the persistent neutron star x-ray binary slx 1737-282 near the galactic centre |
this paper consists of two related parts: in the first part we derive an expression of the moment of inertia (moi) of a neutron star as a function of observables from a hypothetical r-mode gravitational-wave detection. for a given r-mode detection we show how the value of the moi of a neutron star constrains the equation of state (eos) of the matter in the core of the neutron star. subsequently, for each candidate eos, we derive a possible value of the saturation amplitude, α, of the r-mode oscillations on the neutron star. additionally, we argue that an r-mode detection will provide clues about the cooling rate mechanism of the neutron star. the above physics that can be derived from a hypothetical r-mode detection constitutes our motivation for the second part of the paper. in that part we present a detection strategy to efficiently search for r-modes in gravitational-wave data. r-mode signals were injected into simulated noise colored with the advanced ligo (aligo) and einstein telescope (et) sensitivity curves. the r-mode waveforms used are those predicted by early theories based on polytropic eos neutron star matter. in our best case scenario (α of order 10-1), the maximum detection distance when using the aligo sensitivity curve is ∼1 mpc (supernova event rate of 3-4 per century) while the maximum detection distance when using the et sensitivity curve is ∼10 mpc (supernova event rate of 1-2 per year). | constraining the r-mode saturation amplitude from a hypothetical detection of r-mode gravitational waves from a newborn neutron star: sensitivity study |
we investigate the impact of pairing correlations on neutrino transport in stellar matter. our analysis is extended to nuclear matter conditions where large density fluctuations may develop, associated with the onset of the liquid-vapor phase transition, and where clustering phenomena occur. within a thermodynamical treatment, we show that at moderate temperatures, where pairing effects are still active, the scattering of neutrinos in the nuclear medium is significantly affected by pairing correlations, which increase the neutrino trapping, thus modifying the cooling mechanism, by neutrino emission, of protoneutron stars. | pairing effects on neutrino transport in low-density stellar matter |
during very early age of neutron stars, the core cools down faster compared to the crust creating a large thermal gradient in the interior of the star. during 10 - 100 years, a cooling wave propagates from the core to the crust causing the interior of the star to thermalize. during this duration thermal properties of the core material is of great importance to understand the dynamics of the interior of the star. the heat capacity and thermal conductivity of the core depends on the behaviour of matter inside the core. we investigate these two properties in case of magnetars. due to presence of large magnetic field, the proton superconductivity is quenched partially inside the magnetars depending upon the comparative values of upper critical field and the strength of the magnetic field present. this produces non-uniformity in the behaviour of matter throughout the star. moreover, such non-uniformity arises from the variation of nature of the pairing and values of the pairing gap energy. we find that the heat capacity is substantially reduced due to the presence of superfluidity. on the other hand, the thermal conductivity of neutron is enhanced due to proton superconductivity and gets reduced due to neutron superfluidity. hence, the variation of the thermal properties due to superfluidity in presence of magnetic field is different at different radius inside the star. however, in all the cases the maximum variation is of the order one. this affects the thermal relaxation time of the star and eventually its the thermal evolution. | thermal properties of the core of magnetar |
young neutron stars cool via the emission of neutrinos from their core. a precise understanding of all the different processes producing neutrinos in the hot and degenerate matter is essential for assessing the cooling rate of such stars. the main standard model processes contributing to this effect are ν bremsstrahlung, murca among others. in this paper, we investigate another standard model process initiated by the wess-zumino-witten term, leading to the emission of neutrino pairs via nγ → nνν̅. we find that for proto-neutron stars, such processes with degenerate neutrons can be comparable and even dominate over the typical and well-known cooling mechanisms. | anomaly induced cooling of neutron stars: a standard model contribution |
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