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short gamma-ray bursts (sgrbs) are generally thought to result from the merger of two neutron stars or the merger of a neutron star with a black hole. it is becoming standard practice to model these mergers with hydrodynamical simulations that employ equations of state that are derived, for example, for determining the behavior of matter in core-collapse supernovae, and which therefore make use of the assumption that the matter is hot and in nuclear statistical equilibrium (nse). in this letter we draw attention to the fact that the hydrodynamical timescale (roughly the gravitational timescale of the neutron star) may be several orders of magnitude shorter than the timescale on which such equilibrium can be reestablished in the tidal debris ejected during an sgrb, and that on the initial decompression timescales the unshocked tidal ejecta may remain sufficiently cool that the employed equations of state are not appropriate for modeling the dynamics of this part of the flow. on timescales short compared with the timescale on which nse can be (re)established, the equation of state can remain relatively stiff and thus the stream of tidal debris can remain narrow and vulnerable to gravitational instability, as has recently been suggested. these findings suggest that estimates of the type and abundances of heavy elements formed in short gamma-ray bursts need to be revisited. we suggest that the most direct method of testing the physical and dynamical properties of tidal ejecta in sgrbs will come from modeling of their lightcurves, which provides the cleanest source of information on the system dynamics. | short gamma-ray bursts and the decompression of neutron star matter in tidal streams |
recent research on the classical pulsar b0950+08 demonstrates that the explanation of its high surface temperature by roto-chemical heating encounters some difficulties. we assume that there is a fall-back disc around the newborn neutron star, which originates from the supernova ejecta and influences the spin and magnetic evolution of the star. by taking into account disc accretion and magnetic field evolution simultaneously, the effect of the fall-back disc accretion process on the roto-chemical heating in the neutron star is studied. the results show that there are two roto-chemical deviation phases (spin-up deviation and spin-down deviation), but that only the spin-down deviation leads to heating. the specific cooling curve depends on the accretion disc mass, the initial magnetic field and the magnetic field decay rate. most importantly, the observations of surface temperature, magnetic field strength and spin period of the classical pulsar b0950+08 are well explained by the accretion roto-chemical heating model. the fall-back accretion process is important in roto-chemical heating for explanations of classical pulsars with high temperature. given the absence of any evidence of fall-back accretion on to b0950+08, our study is purely hypothetical. | roto-chemical heating in a neutron star with fall-back disc accretion |
black hole-torus systems from compact binary mergers are possible engines for gamma-ray bursts (grbs). during the early evolution of the post-merger remnant, the state of the torus is determined by a combination of neutrino cooling and magnetically-driven heating processes, so realistic models must include both effects. in this paper, we study the post-merger evolution of a magnetized black hole-neutron star binary system using the spectral einstein code (spec) from an initial post-merger state provided by previous numerical relativity simulations. we use a finite-temperature nuclear equation of state and incorporate neutrino effects in a leakage approximation. to achieve the needed accuracy, we introduce improvements to spec's implementation of general-relativistic magnetohydrodynamics (mhd), including the use of cubed-sphere multipatch grids and an improved method for dealing with supersonic accretion flows where primitive variable recovery is difficult. we find that a seed magnetic field triggers a sustained source of heating, but its thermal effects are largely cancelled by the accretion and spreading of the torus from mhd-related angular momentum transport. the neutrino luminosity peaks at the start of the simulation, and then drops significantly over the first 20\,ms but in roughly the same way for magnetized and nonmagnetized disks. the heating rate and disk's luminosity decrease much more slowly thereafter. these features of the evolution are insensitive to grid structure and resolution, formulation of the mhd equations, and seed field strength, although turbulent effects are not fully converged | evolution of the magnetized, neutrino-cooled accretion disk in the aftermath of a black hole neutron star binary merger |
accreting neutron stars host a number of astronomical observables which can be used to infer the properties of the underlying dense matter. these observables are sensitive to the heating and cooling processes taking place in the accreted neutron star (ns) crust. within the past few years it has become apparent that electron-capture/beta-decay (urca) cycles can operate within the ns crust at high temperatures. layers of nuclei undergoing urca cycling can create a thermal barrier, or great wall, between heating occurring deep in the crust and the regions above the urca layers. this paper briefly reviews the urca process and the implications for observables from accreting neutron stars. | the great wall: urca cooling layers in the accreted ns crust |
pulsed non-thermal quiescent emission between 10 kev and around 150 kev has been observed in $\sim10$ magnetars. for inner magnetospheric models of such hard x-ray signals, resonant compton upscattering of soft thermal photons from the neutron star surface is the most efficient radiative process. we present angle-dependent hard x-ray upscattering model spectra for uncooled monoenergetic relativistic electrons. the spectral cut-off energies are critically dependent on the observer viewing angles and electron lorentz factor. we find that electrons with energies less than around 15 mev will emit most of their radiation below 250 kev, consistent with the observed turnovers in magnetar hard x-ray tails. moreover, electrons of higher energy still emit most of the radiation below around 1 mev, except for quasi-equatorial emission locales for select pulses phases. our spectral computations use new state-of-the-art, spin-dependent formalism for the qed compton scattering cross section in strong magnetic fields. | magnetar hard spectral tails |
pairing gaps in neutron matter need to be computed in a wide range of densities to address open questions in neutron star phenomenology. traditionally, the bardeen-cooper-schrieffer approach has been used to compute gaps from bare nucleon-nucleon interactions. here, we incorporate the influence of short- and long-range correlations into pairing properties. short-range correlations are treated including the appropriate fragmentation of single-particle states, and they suppress the gaps substantially. long-range correlations dress the pairing interaction via density and spin modes, and provide a relatively small correction. we use three different interactions as a starting point to control for any systematic effects. results are relevant for neutron-star cooling scenarios, in particular in view of the recent observational data on cassiopeia a. | beyond bcs pairing in high-density neutron matter |
the role of magnetic field decay in normal radio pulsars is still debated. in this paper, we present results which demonstrate that an episode of magnetic field decay in hot young neutron stars can explain anomalous values of braking indices recently measured for more than a dozen of sources. it is enough to have few tens of per cent of such hot neutron stars in the total population to explain observables. relatively rapid decay operates at ages ≲ few ×100 kyrs with a characteristic timescale of a similar value. we speculate that this decay can be related to electron scattering off phonons in neutron star crusts. this type of decay saturates as a neutron star cools down. later on, a much slower decay due to crustal impurities dominates. finally, we demonstrate that this result is in agreement with our early studies. | magnetic field decay in young radio pulsars |
the accretion behaviour in low-mass x-ray binaries (lmxbs) at low luminosities, especially at <e34 erg/s, 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, civ. the line is relatively broad (>3500 km/s), 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. | erratum: uv and x-ray observations of the neutron star lmxb exo 0748-676 in its quiescent state |
two serious problems caused by a hyperon (y)-mixing in neutron stars (nss), (i) too-soft eos incompatible with 2m ⊙ -ns observations and (ii) too-rapid cooling inconsistent with surface temperature observations, are discussed. with a brief review as to the works to solve (i), it is stressed that the universal 3-body force u3b acting on all the baryon species is a promising candidate to solve (i). it is also stressed that in a framework to include explicitly quark degrees of freedom, a hadron-quark crossover transition can generate a stiff eos fully compatible with massive stars and provides another promising solution. the eos calculations are made by focusing an effective interaction approach with u3b(sjm) from a string-junction model (sjm), which shows that this sjm-eos can sustain massive nss with the mass m ≳ 2m ⊙ . it is remarked that the ns-matter with this sjm-eos serves as a solution for both (i) and (ii); by delaying the onset of y-mixing and thereby avoiding too-rapid y-cooling. under the same sjm-eos and including an enhancement of λ λ pairing attraction by a pauli-blocking effects on λ λ - ξn coupling, the occurrence of λ superfluidity is examined and is found to be possible in a limited density region. the result would be useful for an issue in (ii), i.e., giving an explanation for a colder class nss such as vela and 3c58 requiring a rapid y-cooling but with a moderate suppression by y-superfluidity. | hyperon mixing and two serious problems in neutron stars |
11 of the gogny forces available in literature have been studied in order to evaluate their isovector properties and to comment on their viability for beyond 1s0 pairing channels, and neutron star cooling calculations. i find that, even within a relatively narrow set of gogny functionals, there is a large variation in isospin properties. in particular, i find that the density dependence of the symmetry energy provided by gogny forces is too soft and lies outside of currently accepted values. this points to poor constraints in the isovector sector, that should be improved in future fitting protocols. in addition to this, the pairing properties of the gogny forces have been examined for the 1s0, 3s1, 3p2 and 1p1 channels. although most forces are in keeping with literature for the 1s0 gaps, the remaining channels are not particularly constrained and many of the gogny forces produce unphysical results. the pairing gaps generated for the d1p parametrisation have been used to calculate neutron star cooling curves as a proof-of-concept for the gogny force. successful cooling curves incorporating superfluid effects have been produced using the nscool software package. although most gogny forces perform poorly in both the isovector and pairing sectors, it is shown that consistent neutron star models cooling models can be generated with the gogny force. suggestions are made as to the fitting considerations for a future force. | isovector and pairing properties of the gogny interaction in the context of neutron stars. |
with the observations from rossi x-ray timing explorer, we search and study the x-ray bursts of accreting millisecond x-ray pulsar sax j1748.9-2021 during its 2010 outburst. we find 13 x-ray bursts, including 12 standard type-i x-ray bursts and an irregular x-ray burst which lacks cooling tail. during the outburst, the persistent emission occurred at ∼ (1-5)%{\dot{m}_{edd}}. we use a combination model of a blackbody (bb), a powerlaw, and a line component to fit the persistent emission spectra. another bb is added into the combination model to account for the emission of the x-ray bursts due to the thermonuclear burning on the surface of the neutron star. finally, we modify the combination model with a multiplicative factor fa, plus a bb to fit the spectra during the x-ray bursts. it is found that the fa is inversely correlated with the burst flux in some cases. our analysis suggests that the ignition depth of the irregular x-ray burst is obviously smaller than those of the type-i x-ray bursts. we argue that the detected type-i x-ray bursts originate from helium-rich or pure-helium environment, while the irregular x-ray burst originates from the thermonuclear flash in a shallow ocean. | the x-ray bursts within the 2010 outburst of the accreting millisecond x-ray pulsar sax j1748.9-2021 |
context. the recent discovery of long-period, low magnetic field pulsars in low-mass x-ray binaries (lmxbs) represents a challenge for the standard evolutionary scenario. these pulsars have a magnetic field strength comparable to that of millisecond pulsars (~108-109 g), but their period is at least an order of magnitude longer.aims: we discuss the origin of this new class of pulsars within the standard picture of lmxbs formation and apply our results to the case of igr j17480-2446.methods: the magnetothermal evolution of the binary system is studied numerically by taking into account the effect of different accretion rates during the roche-lobe overflow in the framework of the minimal cooling scenario.results: we show that, in addition to standard millisecond pulsars, long-period low magnetic field pulsars should also be expected as a possible outcome of the binary evolution, depending on the strength of the accretion rate during the roche-lobe overflow. in particular, we argue that igr j17480-2446 belongs to this new class of objects. | igr j17480-2446: a new class of accreting binaries? |
the binary neutron star (bns) merger event gw170817, followed by a low luminosity burst of γ-rays and an exceptional broadband afterglow with late onset and early brightening, has raised a heated debate concerning the post-merger outflow structure: a mildly relativistic quasi-spherical outflow or a relativistic structured jet viewed off-axis.to solve this problem, i develop the gamma-ray burst (grb) afterglow light curve analysis tool jetfit. it is capable of efficiently generating synthetic light curves, performing markov chain monte carlo (mcmc) analysis and constraining outflow structures from bns mergers and other relativistic explosions. first, i use jetfit to fit the observational data of gw170817 up to 260 days post merger. the fitting results support the relativistic structured jet with a jet opening angle θ0 ≈ 0.1 radians and lorentz factor γ ≈ 150, viewed from an off-axis angle θobs ≈ 0.55 radians. the quasi-spherical outflow is ruled out due to a significantly larger reduced χ2. the latest observations at around 350 days match the prediction from jetfit remarkably well. furthermore, i apply jetfit to a population of cosmological short grbs with enough observational data. the results indicate that they all have relativistic jet-like outflow structures, which is consistent with gw170817. i find that gw170817 is viewed off-axis and the cosmological short grbs are viewed on-axis, which explains the exceptional properties of gw170817. i demonstrate that the light curves of gw170817, if viewed on-axis, are consistent with those of cosmological short grbs. finally, i present an analysis of rayleigh-taylor (rt) instability in relativistic afterglow jets with synchrotron cooling. i demonstrate rt turbulence cannot only disturb the reverse shock and contact discontinuity, but also can corrugate the forward shock with strong synchrotron cooling, which could potentially affect the light curves. | investigating outflow structures of gamma-ray bursts |
we have studied shock in magnetized accretion flow/funnel flow in case of neutron star with bremsstrahlung cooling and cyclotron cooling. all accretion solutions terminate with a shock close to the neutron star surface, but at some regions of the parameter space, it also harbours a second shock away from the star surface. we have found that cyclotron cooling is necessary for correct accretion solutions which match the surface boundary conditions. | study of magnetized accretion flow with cooling processes |
by the relativistic mean field theory and relevant weak-interactional cooling theory, the relativistic cooling properties in the conventional and hyperonic neutron star matter are studied. also a comparison between the relativistic and non-relativistic results after taking consideration of the gravity correction is performed. the results show that the relativistic effect of neutrino emission reduces the neutrino emissivity, luminosity, and the cooling rate of stellar objects, in comparison with the non-relativistic case. in the neutron star matter without hyperon, the amplitude of the cooling rate reduction caused by the relativistic effect is maximal after taking the gravity correction into consideration, it attains 56% for a 2 m⊙ neutron star composed of conventional neutron star matter, and in the hyperonic matter the amplitude of reduction is minimal, about 38%. | relativistic correction of neutrino emission in neutron stars |
we show a scenario for the cooling of compact stars considering the central source of cassiopeia a (cas a).the cas a observation shows that the central source is a compact star with a high effective temperature, and it is consistent with the cooling without exotic phases. the cas a observation also gives the mass range of m ≥ 1.5 m_⊙.it may conflict with the current cooling scenarios of compact stars that heavy stars show rapid cooling. we include the effect of the color superconducting (csc) quark matter phase on the thermal evolution of compact stars.we assume the gap energy of csc quark phase is large (δ ≳ 10 mev),and we simulate the cooling of compact stars. we present cooling curves obtained from the evolutionary calculations of compact stars: while heavier stars cool slowly, and lighter ones indicate the opposite tendency. | cooling of compact stars with color superconducting quark matter |
the recent measurements of the masses of the pulsar j00737-3039b and of the companion j1756-2251 and pulsars psr j1614-2230, psr j0348-0432 demonstrate the existence of compact stars with masses in a broad range from 1.2 to 2 $m_\odot$. to fulfill the constraint $m_{\rm max}>2m_{\odot}$ and to demonstrate the possibility of cooling scenarios for purely hadronic and further for hybrid stars we exploit the stiff dd2 hadronic equation of state producing a maximum neutron star mass $m\simeq 2.43 m_{\odot}$. we show that the "nuclear medium cooling" scenario for neutron stars comfortably explains the whole set of cooling curves just by a variation of the star masses without the necessity for the occurrence of the direct urca reaction. to describe the cooling data with the very stiff dd2 equation of state we select a proton gap profile from those exploited in the literature and allow for a variation of the effective pion gap controlling the efficiency of the medium modified urca process. fast cooling of young neutron stars like it is seen in the data for cas a is explained with the dd2 equation of state when the following conditions are provided: the presence of an efficient medium modified urca process, and a large proton gap at densities $n\le 2n_0$ vanishing for $n\ge (2.5 - 3) n_0$, where $n_0$ is the saturation nuclear density. | proton gaps and cooling of neutron stars with a stiff hadronic eos |
in this work we consider the effect that the appearance of pseudoscalar condensates in a neutron star can have on its cooling rate. we make no particular assumption on the origin and characteristics of these possible condensates and only assume that in regions where the pseudoscalar density varies the propagation of photons is governed by modified maxwell-chern-simons electrodynamics. we find that this gives non-trivial reflection coefficients between regions of different pseudoscalar density and may affect very substantially the star cooling rate. while quantitative results do depend on precise details that can only be answered once a proper equation of state is determined, the general trend is quite universal and serious consideration should be given to this possibility. | effects of pseudoscalar condensation on the cooling of neutron stars |
we model the pulse profiles and the phase-resolved spectra of the anomalous x-ray pulsar 1e 1048.1-5937 obtained with xmm-newton to map its surface temperature distribution during an active and a quiescent epoch. we develop and apply a model that takes into account the relevant physical and geometrical effects on the neutron star surface, magnetosphere, and spacetime. using this model, we determine the observables at infinity as a function of pulse phase for different numbers and sizes of hot spots on the surface. we show that the pulse profiles extracted from both observations can be modeled with a single hot spot and an antipodal cool component. the size of the hot spot changes from ≈80° in 2007, three months after the onset of a dramatic flux increase, to ≈30° during the quiescent observation in 2011, when the pulsed fraction returned to the pre-outburst ≈65% level. for the 2007 observation, we also find that a model consisting of a single 0.4 kev hot spot with a magnetic field strength of 1.8 × 1014 g accounts for the spectra obtained at three different pulse phases but underpredicts the flux at the pulse minimum, where the contribution to the emission from the cooler component is non-negligible. the inferred temperature of the spot stays approximately constant between different pulse phases, in agreement with a uniform temperature, single hot spot model. these results suggest that the emitting area grows significantly during outbursts but returns to its persistent and significantly smaller size within a timescale of a few years. | mapping the surface of the magnetar 1e 1048.1-5937 in outburst and quiescence through phase-resolved x-ray spectroscopy |
the galactic bulge survey (gbs) is a broad, shallow survey of bulge x-ray sources with extensive multiwavelength support. the limiting sensitivity, about 2×1032 erg/s at the bulge distance, is well suited to finding symbiotic x-ray binaries (syxbs) containing neutron stars accreting from a cool giant wind, as well as x-ray bright white dwarf systems. giant counterparts can be securely detected in ir photometry, allowing us to estimate the total number of symbiotics detected by the gbs, and identify a good number of promising candidates. such an x-ray selected symbiotic sample may be quite different to the traditional symbiotic star population which is usually selected by optical spectroscopy, and consequently biased towards systems with rich line emission. of the 1640 unique x-ray sources identified by the gbs we find 91 significant matches with candidate bulge giants. we expect 68 coincidences, so estimate a total sample of about 23 x-ray emitting cool giants detected by the gbs. most of these are likely to be syxbs or symbiotics of some type. narrowing our search to sources coincident to 1", we find 23 matches, with only 8 coincidences expected, so this subsample has a relatively high purity, and likely includes most of the gbs symbiotics. the properties of this subsample are mostly consistent with cool giants, with typical seds, long-term lightcurves, and spectra. the sources are inconsistent in color with nearby m dwarfs and show small proper motions, so the foreground contamination is likely small. we present a selection of the best studied objects, focusing on one extremely variable x-ray source coincident with a carbon giant. this is quite an unusual object as carbon stars are rare in the bulge. the scientific results reported in this article are based on observations made by the chandra x-ray observatory and data obtained from the chandra data archive. support for this work was provided by the national aeronautics and space administration through chandra award numbers go4-15047x and ar5-16004x issued by the chandra x-ray observatory center, which is operated by the smithsonian astrophysical observatory for and on behalf of the national aeronautics space administration under contract nas8-03060. | x-ray selected symbiotic candidates in the galactic bulge survey |
the neutron capture cross section of several key unstable isotopes acting as branching points in the s-process are crucial for stellar nucleosynthesis studies, but they are very challenging to measure due to the difficult production of sufficient sample material, the high activity of the resulting samples, and the actual (n, γ) measurement, for which high neutron fluxes and effective background rejection capabilities are required. as part of a new program to measure some of these important branching points, radioactive targets of 147pm, 171tm, and 204tl have been produced by irradiation of stable isotopes (146nd, 170er, and 203tl) at the institut laue-langevin (ill) high flux reactor. after breeding in the reactor and a certain cooling period, the resulting mixed 204tl/203tl sample was used directly while 147pm and 171tm were radiochemically separated in non-carrier-added quality at the paul scherrer institut (psi), then prepared as targets. a set of theses samples has been used for time-of-flight measurements at the cern n_tof facility using the 19 and 185 m beam lines, during 2014 and 2015. the capture cascades were detected with a set of four c6d6 scintillators, allowing to observe the associated neutron capture resonances. the results presented in this work are the first ever determination of the resonance capture cross sections of 147pm, 171tm, and 204tl. activation experiments on the same 147pm and 171tm targets with a high-intensity quasi-maxwellian flux of neutrons have been performed using the saraf accelerator and the liquid-lithium target (lilit) in order to extract the corresponding maxwellian average cross section (macs). the experimental setups are here described together with the first, preliminary results of the n_tof measurement. | neutron capture cross sections of the s-process branching points 147pm, 171tm, and 204tl |
considering the baryon octet in the framework of relativistic mean field (rmf) theory, and the entropy per baryon is selected to be 1 or 2, we investigate the entropy effect on the protostar properties of the massive neutron star psr j0348+0432. one set of coupling constants gl85 in the rmf are selected to reproduce the mass of psr j0348+0432 at zero temperature, and then the parameter set is extended to calculate the properties of the massive protoneutron star with the entropy per baryon s = 1 or s = 2. it is found that more hyperons will occur in the protoneutron star than that in the zero-temperature neutron star, the temperature in the protoneutron star increases with the increase of density from surface to interior, and the occurrence of hyperons will reduce the interior temperature. meanwhile, the entropy causes the increase of the mass of massive protoneutron star, and this effect exceeds that of hyperons to decrease the mass of massive neutron star. the entropy also brings on the increase of the radius of protoneutron star. in other words, the cooling of protoneutron star is a contraction process of stellar object. | composition and structure of massive protoneutron star of psr j0348+0432 |
nuclear mass measurement has maintained an important position in the field of nuclear physics for a little over a century. nuclear masses provide key evidence of the structural transformation of nuclei away from the valley of beta-stability and are essential input for many simulations of extreme astrophysical environments. however, obtaining these masses is often a challenging endeavor due to the low production cross sections and short half-lives of the exotic nuclei which are of particular interest. to this end, the time-of-flight mass measurement technique has been developed to obtain the masses of several nuclei at once to precisions of 1 part in 105 with virtually no half-life limitation. this dissertation contains a description of the experiment, analysis, and results of the second implementation of the time-of-flight nuclear mass measurement technique at the national superconducting cyclotron laboratory. 18 masses were obtained for neutron-rich isotopes of argon through iron, where the masses of 48ar, 49ar, 56sc, 57sc, 64cr, 67mn, and 69fe were measured for the first time. these newly obtained masses were applied to outstanding problems in nuclear structure and nuclear astrophysics, resulting in significant scientific advances. the measurement results for 48ar and 49ar, which were found to have atomic mass excesses of -22.28(31) mev and -17.8(1.1) mev, respectively, provide strong evidence for the closed shell nature of neutron number n = 28 in argon. it follows that argon is therefore the lowest even-z element exhibiting the n = 28 closed shell. the masses of 64cr, 67 mn, and 69fe, which were found to have atomic mass excesses of -33.48(44) mev, -34.09(62) mev, and -39.35(60) mev, respectively, show signs of nuclear deformation occurring around the n = 40 subshell. in addition, we found 64cr is substantially less bound than predicted by global mass models that are commonly used in nuclear astrophysics simulations, resulting in a significant reduction in the predicted strength and depth of electron capture heating in the accreted neutron star crust due to the rather abundant a = 64 mass-chain. the reported value for the atomic mass excess of 56sc, -24.85(59)(+0,-54) mev, which contains an asymmetric systematic uncertainty due to potential isomeric contamination, results in a smaller than expected odd-even mass staggering in the a = 56 mass chain. depending on the choice of theoretical models for electron capture transition strengths and energies, this could lead to strong urca cooling in accreted neutron star crusts, due to the large amount of a = 56 material predicted to be present on the surface of accreted neutron stars. | extension of the nuclear mass surface for neutron-rich isotopes of argon through iron |
neutrino production is the dominant cooling process in neutron stars. after the rapid cooling during the protoneutron star stage, the neutron star crust is formed. neutrinos continue to be produced in the crust, which can escape from the surface. this neutrino production is an important process to the final cooling phase of the star. they are produced in the crystalline lattice formed by nuclei permeated by an electronic gas. quantum oscillations of the electron density with respect to the lattice of nuclei generates plasmons, which decay in a pair of neutrinos. many works have studied the plasma of the nucleus in the star, however, without incorporating the effect of the lattice in the crust of the neutron star. the objective of this work is to include the lattice in the calculation of the plasmon decay rate using quantum field theory at finite temperature. it is not common to find studies on the crystalline lattice of the star using quantum field theory and the calculations generally used for neutrino emissivity consider a homogeneous and isotropic medium, which can not be directly applied in the context of neutron star crust. | neutrino emission from neutron star crusts |
nuclear burning near the surface of an accreting neutron star produces ashes that, when compressed deeper by further accretion, alter the star's thermal and compositional structure. bygone nucleosynthesis can be constrained by the impact of compressed ashes on the thermal relaxation of quiescent neutron star transients. in particular, urca cooling nuclei pairs in nuclear burning ashes, which cool the neutron star crust via neutrino emission from e- / β-decay cycles, provide signatures of prior nuclear burning over the century timescales it takes to accrete to the e--capture depth of the strongest cooling pairs. this talk will present crust cooling models of the accreting neutron star transient maxi j0556-332 used to show that this source likely lacked type i x-ray bursts and superbursts >=120 years ago. we also identify the key nuclear physics uncertainties in rp-process reaction rates and e--capture weak-transition strengths for low-lying transitions whose reduction will improve nucleosynthesis constraints using this technique. this work was supported in part by the u.s. department of energy under grant no. de-fg02-88er40387 and national science foundation under grants no. ast-1516969 and phy-1430152. | constraints on bygone neutron star nucleosynthesis using urca coolers in the crust |
fission fragments emitted in a fissile solution create tiny gas bubbles, the size of which is determined by the linear energy transfer (let) of the particles. the let of fission fragments of 235u in aqueous solutions of uranyl nitrate has been determined, and using methods adapted from the literature, the size of gas bubbles generated along the tracks of these particles has been estimated, revealing important variations with respect to particle let and solution properties. empirical correlations are presented for the maximum radius of radiolytic gas bubbles in unsaturated solutions of uranyl nitrate as a function of solution temperature and concentration. these can be used to predict the critical concentration of dissolved hydrogen necessary for the appearance of gas voids during nuclear criticality transients. the findings are intended for use in a future model of nuclear criticality transients in aqueous fissile solutions for the purposes of nuclear criticality safety assessment. | linear energy transfer of fission fragments of 235u and nucleation of gas bubbles in aqueous solutions of uranyl nitrate |
a detailed study of magnetized astrophysical flows has been carried out in the magnetohydrodynamic and special relativistic magneto- hydrodynamic regime. we have considered the thermodynamics of the flow to be described with a fixed as well as, a variable adiabatic index equation of state (eos). as examples of mhd flow, we have studied (i) funnel accretion onto neutron stars and white dwarfs, (ii) magnetized equatorial outflows from around a compact magnetized star, and (iii) magnetized relativistic outflows about the axis of symmetry from compact objects like black holes. possibly for the first time, we obtained semi-analytical magnetized accretion solutions onto compact objects with a hard surfaces such as neutron stars which satisfies the inner boundary condition, where the accreting matter gently settles onto the surface of the star. we also compared these solutions in newtonian & pseudo-newtonian regime. we assumed that neutron star has a strong dipole magnetic field whose dipole moment is aligned along the rotation axis of the star. we have included cooling processes like bremsstrahlung and cyclotron. depending on the bernoulli parameter of the flow and the rotation period of the star, we obtain various solutions which may possess a single sonic point or multiple sonic points. we have also studied the dependence of accretion solutions on plasma compositions. all types of accretion solutions undergo a very strong primary shock which forms near the star surface. the strength of the primary shock increases with the rotation period of the star, but the shock location is weakly dependent on the period. due to the presence of multiple sonic points, we can find a secondary shock within a very small region of the parameter space but this shock is weak and the shock location, the shock strength, and the compression ratio depends significantly on the rotation period of the star and the total energy of the flow. we also calculate the total luminosity of the magnetized accretion solution which is in good agreement with observations. we have also studied a case of white dwarf where our results match with the observations. we have found that cyclotron cooling and bremsstrahlung cooling are necessary to obtain a consistent accretion solution i.e., a solution which connects the flow from the accretion disk to the star surface. we studied the effect of plasma composition on the equatorial wind outflow with variable adiabatic index eos. we have found that terminal velocity depends upon the plasma composition. lepton dominated winds with higher values of bernoulli parameter have high terminal speeds. we have also studied solutions for different energies, angular momenta and in different gravity i.e., newtonian & pseudo-newtonian potential. for the same values of the bernoulli parameter (energy) and the total angular momentum, a wind in strong gravity is more accelerated, compared to wind in newtonian gravity. we showed that flow variables like the radial, azimuthal velocity components, temperature, etc all depend on the composition of the flow. we continue our outflow study in case of collimated outflows or jets in special relativistic magnetohydrodynamic regime with variable adiabatic index eos. we found that plasma composition mainly affects the velocity and the temperature of the jet but the collimation of jet and fast critical point location appears to have no dependence on plasma composition. we explore all outflow solutions and found that the solution depends on the current distribution parameter, the magnetization parameter, the inclination angle of field lines with respect to the disk plane, and alfv`en point radius. fast point location can be related to collimation shock location because the superfast flow is causally disconnected from the flow which is behind. | the study of astrophysical magnetized flows |
after a period of active accretion, neutron stars can enter a phase where their x-ray emission is dominated by thermal emission from their surface. the rate of cooling of this emission can yield insight into neutron star structure. furthermore, emission models may help determine the neutron star radius. a number of questions arise when modeling such x-ray spectra as observed by chandra or xmm-newton. is there ongoing, low level active accretion that is contributing to the observed soft x-ray emission? in a number of cases, a hard x-ray tail is also observed. what is the origin of this hard tail? the quiescent neutron star system 4u 2129+47 presents a unique opportunity to study these questions. this system is viewed nearly edge on, as evidenced by a periodic, total eclipse that lasts 1585 seconds out of the 5.24 hour orbit. as we are viewing this system edge on, both observed neutral column variations and an observed hard x-ray tail in year 2000 chandra observations indicated ongoing active accretion. subsequent xmm and chandra observations over the next 15 years showed that both the neutral column variability and the hard x-ray tail vanished. thus, these later observations may represent a true quiescent, cooling neutron star state. we assess the evidence for cooling in the 4u 2129+47 system. furthermore, we use the timing of the x-ray eclipses to discuss evidence for a third body in the system, and derive likely orbital periods. finally, we discuss how future x-ray missions, e.g., athena and lynx, could place more stringent limits on neutron star cooling and the presence of a hard tail (i.e., active accretion) in this system. | the quiescent neutron star and hierarchical triple: 4u 2129+47 |
we use the smoothed particle hydrodynamics (sph) method to simulate the behavior of inviscid flow accreting on a neutron star. for black holes, the lack of any hard surface allows matter to advect in supersonically. for neutron stars, however, the presence of a hard surface requires matter to settle down on the surface with zero radial velocity, essentially making the flow subsonic. apart from this, near the compact object, the flow should be similar in both the cases. during the hard spectral state, the incoming flow has a negligible viscosity, leading to the absence of any disk blackbody emission. in such a state, the flow can be assumed to be purely inviscid for the computational purpose. we show that for a purely inviscid advective flow having angular momentum, the solution allows two shocks in the flow. the outer one, forms due to strong centrifugal barrier and is called centrifugal pressure dominated boundary layer or cenbol, which is a common feature for both neutron stars and black holes. the inner shock forms very close to the surface of a neutron star, due to the presence of the physical boundary. we study 1) the formation of steady shocks in 2d, in absence of any cooling process, 2) the formation and variation of winds from the post-shock region when the angular momentum of the flow varied, 3) shock formation in strong winds, 4) the radial oscillation of shocks when cooling effects are added, 5) formation of asymmetries w.r.t. the z=0 plane and instabilities due to the interaction of inflow and outgoing strong winds which leads to vertical oscillations, 6) the oscillation of nbol and the variation of effective surface temperature when stronger cooling effects are present and accretion rate is varied. our results capture both the low and high-frequency quasi-periodic oscillations observed in the power density spectra for the case of accretion around a neutron star. | formation and stability of oscillating shocks in inviscid advective flows around neutron stars in presence of cooling using smoothed particle hydrodynamic simulations |
a multispecies hydrodynamic model based on moments of the born-bogolyubov-green-kirkwood-yvon (bbgky) hierarchy is developed for physical conditions relevant to astrophysical plasmas. the modified transport equations incorporate strong correlations through a density functional theory closure, while fluctuations enters through a mixture bgk operator. this model extends the usual burgers equations for a dilute gas to strongly coupled and isothermal plasmas mixtures. the diffusive currents for these strongly coupled plasmas is self-consistently derived. the settling of impurities and its impact on cooling of white dwarfs and neutron stars can be greatly affected by strong coulomb coupling, which we show can be quantified using the direct-correlation function. this work was supported by the air force office of scientific research (grant no. fa9550-12-1-0344). | diffusive mixing in strongly coupled plasmas |
the effective λ -λ interaction energies in s-shell double-λ hypernuclei, λλ6he , λλ5he and λλ5h have been investigated by solving three-body system (λ + λ + core) within the framework of coupled rearrangement channel gaussian basis treatment. the nijmegen soft-core potential models, nsc97d, nsc97e and nsc97f, have been applied for free-space λ -λ interaction. by taking into account the medium effects, it was adjusted to fit the experimental λ -λ interaction energies of λλ6he (0.67 ± 0.17 mev). the effective λ -λ interaction energies of λλ5he are 0.92 mev, 0.88 mev, 0.86 mev for each potential model and that of λλ5h is 0.6 mev for all potential modes. moreover, the λ -superfludity in the neutron star cores which is related to cooling process has also been investigated by applying our constructed effective λ -λ nsc97e potential of λλ6he . it is found that λ superfludity begins to appear when the interaction strength is 1.5 times more attractive than our constructed interaction. the λ -superfluidity in the neutron star cores might occur at the hyperon percentage yλ (1%-5%), and vanish at yλ (15%- 20%). | medium effect in s-shell double-λ hypernuclei and hyperon superfluidity in neutron star cores |
i will present a new general relativistic magnetohydrodynamic (grmhd) code devoted to the study of compact binary mergers with finite temperature equations of state and neutrino emission. numerical modeling of neutron stars binaries, black holes binaries, and neutron star-black hole binaries has now become one of the most important fields of study in theoretical astrophysics because it allows extracting physical information from the gravitational wave and electromagnetic signals by comparing simulated data with observations. focusing on the ns-ns and ns-bh cases, in particular, it has been shown many times that only a fully general relativistic treatment taking also into account magnetic fields may give a complete picture of this scenario and this requires to solve the equations of grmhd. in order to let the so-called divergence-free condition be automatically satisfied, i considered the magnetic field coming out from a vector potential. in addition, i also consider a general treatment for the ns equation of state (eos) allowing for the use of finite temperature tabulated eos. this new code will also implement neutrino cooling in order to provide a more accurate study of the post-merger phase. | spritz: a new fully general relativistic magnetohydrodynamic code |
most neutron stars cool predominately via the modified urca process, in which emitted neutrinos carry away energy. the traditional treatment for the in-medium nucleon propagator in the modified urca process uses crude approximations. we reformulate the propagator by including the nucleon self-energy and examine the effect of this new propagator on the neutrino emissivity due to the modified urca process. this work was partly supported by the u.s. department of energy, office of science, office of nuclear physics, under award no. #de-fg02-05er41375. | improved treatment of neutron star cooling via modified urca process |
accreting neutron stars (ns) can exhibit high-frequency modulations in their lightcurves during thermonuclear x-ray bursts, known as burst oscillations. their frequencies can be offset from the spin frequency of the ns by several hz (known independently), and can drift by 1-3 hz. new x-ray missions with improved sensitivity (such as extp and strobe-x) aim to use the burst oscillation phenomenon to measure ns parameters, an effort that would be helped by a thorough understanding of the underlying mechanism. one plausible explanation is that a wave is present in the bursting ocean that decreases in frequency (in the rotating frame) as the burst cools, hence explaining the drifts. the strongest candidate is the buoyant r-mode, however, models for the ocean background used in previous studies over-predict frequency drifts by several hz. using new background models that were developed to explain the short recurrence times of some bursts (which include shallow heating, and burning in the tail of the burst), the evolution of the buoyant r-mode was calculated. the resulting frequency drifts are smaller, in line with observations. | new burning physics and burst oscillations |
using a model for the equation of state and composition of dense matter and the magnitude of singlet proton superconductivity and triplet neutron superfluidity, we perform the first simultaneous fit of neutron star masses and radii with neutron star luminosities and ages determined from observations of isolated neutron stars. we also simultaneously fit the charge radii and binding energies of heavy nuclei and find agreement with the tidal deformability obtained in gw 170817. we find that the threshold density for the direct urca process lies between the central density of 1.7 and 2 solar mass neutron stars, but that 2 solar mass stars are unlikely to cool principally by the direct urca process because of the suppression by neutron triplet superfluidity. this is the first quantitative constraint on the composition of matter above the saturation density. this work was supported by nsf grant phy 1554876, xsede allocation phy170048, and by the u.s. doe office of nuclear physics. s.h. is also supported by chandra grant tm8-19002x, nsf grant phy-1630782, and the heising-simons foundation, grant 2017-228. | multi-messenger neutron star astrophysics and determining the composition of the neutron star core |
we show a cooling scenario of compact stars to satisfy recent observations of compact stars. the central density of compact stars can exceed the nuclear density, and it is considered that many hadronic phases appear at such a density. it is discussed that neutron superfluidity (1s0 for lower density, and 3p2 for higher density) and proton superfluidity/superconductivity (1s0) appears in all compact stars. and some "exotic" states are considered to appear in compact stars, such as meson condensation, hyperon mixing, deconfinement of quarks and quark colour superconductivity. these exotic states appear at the density region above the threshold densities of each state. we demonstrate the thermal evolution of isolated compact stars, adopting the effects of nucleon superfluidity and quark colour superconductivity. we assume large gap energy (δ > 10 mev) for colour superconducting quark phase, and include the effects of nucleon superfluidity with parametrised models. we simulate the cooling history of compact stars, and shows that the heavier star does not always cool faster than lighter one, which is determined by the parameters of neutron 3p2 superfluidity. | cooling of compact stars with nucleon superfluidity and quark superconductivity |
x-ray burst oscillations are coherent signals which are detected in neutron star low-mass x-ray binaries during their type-i x-ray bursts. the frequency of x-ray burst oscillation is close to the neutron star spin rate. in recent years, about 30 neutron star low-mass x-ray binaries have been detected x-ray burst oscillations from rxte, swift and nicer observations. they are important to probe the dense matter equation of state and the effects of strong gravity, to study the high magnetic field environment on the neutron star surface. the studies of x-ray burst oscillations are also crucial to understand the spinup process of neutron star low-mass x-ray binaries through accretion and the evolution into millisecond pulsars. the different characteristics of burst oscillation signals between persistent accretion-powered pulsars, intermittent accretion-powered pulsars and sources without detectable accretion-powered pulsations can help us to study more about it. we briefly introduce type i x-ray bursts, and then specifically explain the methods to search for burst oscillations and provide the observational results of x-ray burst oscillations. the theoretical burst oscillation model, i.e., the hot spot model, the surface mode model and the cooling wake model, are discussed. at present, we note that these models are difficult to explain all the observed phenomena of x-ray burst oscillations. in future, the china-led mission, the enhanced x-ray timing and polarimetry (extp) observatory will observe neutron star low-mass x-ray binaries and search x-ray burst oscillations. by modeling the stacking pulse profile of x-ray burst oscillation, the mass and radius of neutron star will be measured accurately, and the neutron star equation of state will be tightly constrained. we also expect more neutron stars will be measured their spin frequency, if luck, sub-millisecond pulsar will be discovered. | research progress of x-ray burst oscillation |
axions are a generic expectation in many extensions of the standard model. neutron stars have long been a stringent probe of axions through observations of their cooling. axions can be created within the hot core of the neutron star and escape the star due to their weak interactions with matter. however, the emitted axions can then be detected in x-ray observations if they convert into an x-ray photon on the way to earth, for example in the magnetosphere of the star. here i present a summary of recent works searching for evidence of these particles from x-ray observations of nearby neutron stars knonwn as the magnificent seven. in particular, i focus on the recent discovery of an x-ray excess from the magnificent seven. this excess is consistent with the axion-nucleon bremsstrahlung expectation for an axion with the product of photon and nucleon couplings gaγγ ×gann ∈(2 ×10-21 ,10-18) gev-1 and mass ma < 2 ×10-5 ev. furthermore, i discuss a new axion production channel inside neutron stars via synchrotron emission off of neutrons and muons, and discuss the implications of this channel for the magnificent seven excess. | searching for axions with neutron stars |
type i x-ray bursts are thermonuclear explosions observed in many accreting neutron stars (nss) that result from rapid unstable burning of hydrogen and helium accreted onto the surface of the star. during an x-ray burst the x-ray flux rapidly rises by a factor of 10-20 in a couple of seconds and then decays on a longer timescale as the surface of the star cools. oscillations have been detected during the rise and/or decay of some of these x-ray bursts that have frequencies within a few hz of the stellar spin frequency and must be due to nonuniform emission from the stellar surface. here i discuss the results of simulations of the rise and decay of a typical x-ray burst light curve and the evolution of their fractional oscillation amplitudes. we generate light curves using a physical model for a spreading hot spot, taking into account the effect of the coriolis force (latitude-dependent flame spreading speed), as well as relativistic effects. i will explain how the combination of the light curve and fractional amplitude evolution can constrain the properties of the flame spreading, such as ignition latitude, which would be important for measuring nss masses and radii using x-ray burst oscillations. i discuss the prospects for future x-ray missions such as esa's loft in this area. | fanning the flames: x-ray burst probes of nuclear burning |
we propose a 70 ks xmm-newton observation of the low-mass x-ray binary1rxs j180408.9-342058 in quiescence. continuing our study of how the crustof this neutron star cools after it was heated during a bright accretionoutburst in 2015, yields very valuable information about its structureand various processes occurring in a high density environment. inparticular, with an outburst duration of ~0.5 yr this source falls rightin between two regimes that we have probed in crust cooling studies sofar, offering an exciting opportunity to further our knowledge aboutheating and cooling of transiently accreting neutron stars. | the cooling crust of the neutron star in 1rxs j180408.9-342058 |
observing the cooling of neutron stars reheated by accretion providesunique insights into neutron-star structure. here we propose to continueour observations of the cooling neutron star in maxi j0556-332, atransient that had been accreting at near/super-eddington rates for16 months when it returned to quiescence in may 2012. xmm-newton andchandra observations in quiescence have revealed an extraordinarily hotand rapidly cooling neutron star, implying very strong heat sources inthe shallow outer crust. we request a 106 ks xmm-newton observation ofmaxi j0556-332 to investigate if the remarkable properties of the outercrust extend to deeper crustal layers and to constrain the possibleonset of a phase of more rapid cooling. | cooling of the super-heated neutron star in maxi j0556-332 |
we study the neutrino anti-neutrino pair synchrotron emission from electrons and protons in a relativistic quantum approach. this process occurs only under a strong magnetic field, and it is considered to be one of effective processes for neutron star cooling. in this work we calculate the luminosity of the neutrino anti-neutrino pairs emitted from neutron-star-matter with a magnetic field of about 10^{15} g. we find that the energy loss is much larger than that of the modified urca process. the neutrino anti-neutrino pair emission processes in strong magnetic fields is expected to contribute significantly to the cooling of the magnetars. | \\nu \\bar{\\nu} -pair synchrotron emission in neutron-star matter based on a relativistic quantum approach |
we report our recent study of how neutron star spin rates may affect the stabilization of nuclear burning on the star's surface in low mass x-ray binaries. unstable burning can be triggered by the ignition of accumulated fuel from the donor, resulting in thermonuclear (type i) x-ray bursts. the burning will become steady at accretion rates sufficiently high to provide a thermodynamics state that allows to burn all the fuels before it can pile up. between the two burning regimes is a transitional phase where so-called "marginally stable" burning occurs. theoretical and numerical works suggest that, in this phase, the nuclear energy generation rate is comparable to the cooling rate, resulting in an oscillatory burning behaviour with a timescale of a few hundred seconds. this special mode of burning is believed to be the origin of the observed millihertz quasi-periodic oscillations in several accreting low mass x-ray binaries. moreover, such burning is predicted to take place at accretion rates near the eddington limit; however, the millihertz quasi-periodic oscillations are observed only at about one-tenth of the eddington accretion rate. in our work, we show that one of the reasons for this discrepancy may be related to the spin rates of neutron stars. the stabilization of thermonuclear burning is enhanced by lower surface gravity at the equatorial region due to rotation. | the potentially rotational effect on the stabilization of thermonuclear burning of accreting neutron stars |
the disposal of residue forms an integral part of the alumina refining process. the refining of western australia bauxite, which is low grade ore by world standards, results in 2 dry tonnes of residue for every 1 tonne of alumina produced. the disposal of this residue contributes a significant proportion of the overall cost of producing alumina. the residue is also highly alkaline, and, if not contained in sealed impoundment areas, can impact on the local environment. it has been these two considerations, the cost of disposal and the potential impact of disposal on the environment, which have been the main driving forces behind changes to the way residue is stored. this paper traces the various residue disposal techniques adopted by alcoa of australia limited from containment in large settling ponds, to splitting the coarse and fine fractions for separate disposal, to the storage of the fine mud fraction in base drained ponds, to the more recent pre-thickening of the fine mud fraction for disposal in solar drying ponds. the reasons for change and the problems encountered are reviewed, and possible future developments are discussed. | developments in the disposal of residue from the alumina refining industry |
observing x-rays during quiescence from transiently accreting neutron stars provide unique clues about the nature of dense matter. current models of neutron star crust, however, systematically predict lower temperatures than those observed, and an artificial shallow heat source is required to account for the observations. it has been previously proposed that the shallow heat source could be of nuclear origin, particularly the fusion of lighter elements in the crust. the pycnonuclear fusion reaction rates implemented in our models have large uncertainties spanning several orders of magnitude. we present the sensitivity studies of these pycnonuclear fusion reactions in realistic network calculations and also study their impact on the neutron star cooling curves in quiescence. although we see a shallower deposition of nuclear heat when pycnonuclear fusion reaction rates are enhanced, we eliminate the possibility that the problem of shallow heating could be attributed to the uncertainties in pycnonuclear fusion reaction rates. this work was supported by the national science foundation under grant no. phy-1430152 (jina center for the evolution of the elements). | pycnonuclear fusion and the shallow heat source in accreting neutron star crusts |
it is very important to investigate the properties of the neutron superfluid that exists inside neutron stars. for example, it is known that the cooling curves of the stars strongly depend on the superfluid pairing gap. the theoretical study is very challenging due to the complexity of the physical system in such extreme conditions. cold atoms can help us shed light into physical mechanisms within the interior of the star; their unique flexibility, in fact, makes these systems laboratories to explore many-body physics, with applications in condensed matter physics, nuclear physics, and nuclear astrophysics. we perform extensive quantum monte carlo calculations of the pairing gap for a cold atomic fermi system. we present preliminary results and we discuss their significance for the cooling curves of neutron stars. | the superfluid pairing gap of a neutron star |
compact astrophysical sources like neutron stars and magnetars possess extremely strong magnetic fields. radiative cooling of plasma particles in their magnetospheres can be short compared to the plasma dynamical time. such magnetospheres are observed to produce strong flares, which indicates production of relativistic electrons and, possibly, positrons. how does the particle distribution change as these particles propagate in the magnetospheric 'magnetic bottle'? here we discuss the motion of a relativistic 'larmor particle' in a straight magnetic bottle subject to synchrotron energy loss. supported by the nsf grant phy-2010109 and the doe epscor grant de-sc0019474. | on motion of a particle in a strong-field magnetic bottle |
we propose joint chandra-vla-hst observations to continue our monitoring of the binary neutron star merger gw170817 at t~500-850 days since merger. our immediate objectives are: (i) to constrain the broad-band spectral evolution of the non-thermal emission from gw170817, and, in particular, the passage of the synchrotron cooling frequency within the chandra bandpass; (ii) to map the temporal evolution of the emission from gw170817 across the electromagnetic spectrum. these observations are designed to provide new insight into the current debate about the nature of the relativistic ejecta of compact-object mergers and their (now questioned) capability to successfully launch ultra-relativistic jets in their environment. | late-time monitoring of gw170817 across the electromagnetic spectrum |
various telescopes including rxte, integral, suzaku, and fermi have detected steady non-thermal x-ray emission in the 10 - 200 kev band from strongly magnetic neutron stars known as magnetars. magnetic inverse compton scattering is believed to be the leading candidate for the production of this intense x-ray radiation. scattering at ultra-relativistic energies leads to attractive simplifications in the analytics of the magnetic compton cross section. we have recently addressed such a case by developing compact analytic expressions using correct spin-dependent widths acquired through the implementation of sokolov & ternov basis states, focusing specifically on ground-state-ground-state scattering. compton scattering in magnetar magnetospheres can cool electrons down to mildly relativistic energies. moreover, soft gamma-ray flaring in magnetars may involve strong comptonization in expanding clouds of mildly relativistic pairs. such environs necessitate the development of more general magnetic scattering cross sections, in which the incoming photons acquire substantial incident angles relative to the field in the rest frame of the electron leading to arbitrary landau excitations of the intermediate and final states. due to the rapid transitions of the excited-state to the ground-state, the initial electron is still assumed to be in the ground state. the cross sections treat the plethora of harmonic resonances associated with various cyclotron transitions between landau states. polarization and spin dependence of the cross section for the four scattering modes is compared to the cross section obtained with spin-averaged widths. we present numerical results to show the comparisons to highlight the role of the spin-dependent widths of the resonances. the findings presented here will have applications to various neutron star problems, including computation of eddington luminosities and polarization mode-switching rates in transient magnetar fireballs. | resonant compton physics for magnetar astrophysics |
the multi-wavelength study of >100 myr old radio-pulsars holds the key to understanding the long-term evolution of neutron stars, including the advanced stages of the surface cooling history, and possible variations in the magnetosphere properties. near-uv observations of neutron stars are particularly important for such studies because they offer the chance to explore both thermal and non-thermal emission processes. the first aim of this proposal is to obtain a robust detection of the candidate optical counterpart (u=26.4) to the 166 myr old radio pulsar psr j0108-1431, discovered by us with the vlt, through wfc3/uvis imaging in the u and b bands. the detection of a point source at the pulsar radio position, computed from its vlbi radio coordinates and proper motion, with u and b-band fluxes compatible with those of the candidate counterpart, will firmly secure our proposed identification. the second aim is to obtain the first measurement of the pulsar flux in the optical-uv with both the wfc3/uvis and the acs/sbc. this will enable us to determine the slope of the rayleigh-jeans continuum, only hinted in the vlt data, affected by large errors, and measure the temperature of the bulk of the neutron star surface, too cold to be detected in the x-rays where only hot polar caps have been detected with chandra and xmm-newton. the measured temperature will provide the crucial information to constrain neutron star cooling curves for ages>100 myr, where theoretical predictions are highly uncertain, and from them verify different models of the neutron star interior. | the old pulsar psr j0108-1431, a key target to understand the long-term evolution of neutron stars |
we present two recent parametrizations of the equation of state (fsu2r and fsu2h models) that reproduce the properties of nuclear matter and finite nuclei, fulfill constraints on high-density matter stemming from heavy-ion collisions, produce 2m⊙ neutron stars, and generate neutron star radii below 13 km. making use of these equations of state, cooling simulations for isolated neutron stars are performed. we find that two of the models studied, fsu2r (with nucleons) and, in particular, fsu2h (with nucleons and hyperons), show very good agreement with cooling observations, even without including nucleon pairing. this indicates that cooling observations are compatible with an equation of state that produces a soft nuclear symmetry energy and, thus, generates small neutron star radii. nevertheless, both schemes produce cold isolated neutron stars with masses above 1.8m⊙. | the equation of state and cooling of hyperonic neutron stars |
we propose 6x30 ks observations of the radio/gamma-ray pulsar psr j2032+4127 and its companion be-star mt91 213. this nearby pulsar is in a 49 yr orbit and will reach periastron 2017 november 13, when it will undergo an outburst if it accretes from a disk that surrounds the be star. our proposed observations allow us to (1) track the x-ray lightcurve and measure cooling of the neutron star crust, thus probing fundamental physics in extreme regimes. irrespective of the outburst, our observations allow us to track (2) jet formation and (3) emission from the colliding winds of the two stars, thus serving as an important comparison to the only other gamma-ray pulsar in a be-binary psr b1259-63/ls 2883. these objectives require the long-term, high spatial resolution capabilities of chandra. | post-periastron behavior of psr j2032+4127/mt91 213: outburst, jet, and winds |
we propose to continue our highly successful coverage of the coolingneutron star in maxi j0556-332, which was strongly heated during anear/super-eddington outburst in 2011/12 and again during a smalleroutburst in 2016. our quiescent observations, which revealed anextraordinarily hot and rapidly cooling neutron star, have providedunique insights into neutron-star structure and in particular into thenature of heat sources in the shallow outer crust. recent observationssuggest that the ongoing cooling trend is now dominated by heat inducedduring the first outburst in deeper crustal layers. we request a 106 ksxmm-newton observation to investigate if the remarkable properties of theouter crust extend to deeper layers where matter reaches nuclear density. | cooling of the super-heated neutron star in maxi j0556-332 |
we propose to observe any be x-ray transient after its next type-iioutburst during a potential quasi-stable low-luminosity plateau phasethat has been identified in several sources. the origin of this plateauphase is still unclear and could be due to the cooling of an accretionheated neutron star crust or due to residual accretion onto the neutronstar surface at very low rates. crustal cooling has been well- studied inlow magnetic field neutron stars where the magnetic field does not playa relevant role, but not much is known about how much this process couldbe affected by the high magnetic fields found of the neutron stars in bex-ray transients. our proposed observation will be the second one done inthis enigmatic plateau state using high quality spectral and timing data. | raiders of the low luminosity state in be x-ray transients |
we study the screening of a strong magnetic field operated by an initial huge number of e± pairs (we do not discuss here their production mechanism). the background fields configuration is of crossed fields, (b→ = bz^, e→ = ey^), with e/b < 1. in this system the following series of processes occur: 1) the electric field accelerates the pairs, which radiate high-energy synchrotron photons; 2) these synchrotron photons interact with the background magnetic field via the magnetic pair production process (mpp hereafter), i.e. γ + b → e+ + e−, producing additional pairs; 3) the dynamic of all the pairs around the magnetic field lines generates a current that induces a magnetic field oriented in the opposite direction to the background one and then shielding it. we get that, for instance, for an initial number of pairs n±,0 = 1010, an initial magnetic field of 1012 g can be reduced by a few percent. the whole screening process described by the steps above, occurs in the short timescales 10−21 ≤ t ≤ 10−15 s, i.e. the time necessary before the particles acceleration timescale equals the synchrotron cooling timescale. further developments (as the study of this mechanism in different geometries of the e→ and b→ fields, quantum effects in overcritical fields, other mechanisms for the production, distribution and multiplicity of the e± pairs) are necessary in order to apply this model to specific and extreme astrophysical systems (as black hole or neutron star). | on the magnetic field screening in strong crossed electromagnetic field |
we reanalyse the x-ray spectrum of the psr b0833–45 (the vela pulsar) using the data of the chandra space observatory. in contrast to previous works, we consider a wide range of possible masses and radii of the pulsar. the derived surface temperature of the star ts∞=0.66‑0.01+0.04mk (1σ level over the entire mass and radius range of our study) is consistent with earlier results. however, the preferable values of vela's mass and radius given by the spectral analysis are different from those used previously; they are consistent with modern equation of state models of neutron star matter. in addition, we evaluate the vela's surface temperature as a function of assumed values of its mass and radius. this allows us to analyse the neutrino cooling rates consistent with the evaluated surface temperatures and explore the additional restrictions that could be set on the vela's mass and radius using different versions of the neutron star cooling theory. | thermal spectrum and neutrino cooling rate of the vela pulsar |
we use smoothed particle hydrodynamics (sph) technique to simulate the formation of two-component advective flow (tcaf) for accretion onto black holes and neutron stars in presence of cooling and viscous effects. we vary the accretion rates from sub-eddington to super-eddington for flows having different values of injected angular momentum and viscosity. we study the time-dependent variation of the accretion disk and the outflow geometry for such cases when the radiative pressure of the flow (for both bhs and nss) and of the normal boundary layer (nbol, for ns) is included. we show 1)for black holes and neutron stars, super-eddington accretion is highly likely when strong outflows are present, 2)how the value of viscosity limits the accretion rate, 3)how the limiting value is decreased for the case of ns due to the presence of nbol. we also compare our findings from the numerical simulation with the observed x-ray data for a few black holes and neutron stars. | time-dependent numerical simulation of eddington and super-eddington accretion flows onto black holes and neutron stars using smoothed particle hydrodynamics |
we propose to continue our program to use the crust cooling behaviorin transiently accreting neutron stars. those crusts are heated due tothe matter accretion onto the neutron stars during outbursts. after theoutbursts are over the crusts cool down until they are in equilibrium withthe cores again. following this cooling processes for several systemshas given us new insights in the structure of neutron stars, but manyuncertainties remain. therefore it is needed to enlarge our sample ofwell-studied sources to obtain better insights in the behavior of howneutron stars react to the accretion of matter. xmm-n plays a crucialrole in the earlier phase (<1 yr) of the crust cooling decay and probingthe behavior of shallow depths in the crusts. | crust cooling of accretion-heated neutron stars |
a significant fraction of young neutron stars can experience a strongfall-back episode after a supernova explosion. the accreted matter screensthe magnetic field which slowly diffuses out on a time scale of tens ofthousand years which is much shorter than the cooling timescale. however,no young radio pulsars with re-emerged magnetic field have been identifiedyet. the main goal of this proposal is to detect for the first time thethermal emission from selected old (spin- down age more than 1 myear)radio pulsars which allows us to confirm the field re-emergence scenario. | probing magnetic field re-emergence through x-ray observations of radio pulsars |
in this work we determine the equation of state and the population of baryons and leptons and discuss the effects of the hyperon-meson coupling constants to the formation of delta resonances in the stellar medium. we also discuss the structure of the protoneutron stars including the delta matter in their composition, and compared the results of a cooled neutron star, after escape of neutrinos. for protoneutron stars structure and composition, the neutrinos are considered trapped. | hyperon-meson and delta-meson coupling to protoneutron stars structure using the nonlinear walecka model |
we are entering a new era when first-principles computational and theoretical studies of complex collective plasma processes under extreme conditions-marked by the importance of relativistic, radiation, and pair-production effects-are becoming feasible and even routine. these studies, motivated by our desire to understand plasma behaviors around exotic astrophysical objects like neutron stars and black holes, form the new field of extreme plasma astrophysics. one of the most exciting frontiers in this emerging field is radiative turbulence, where turbulent energy injection is balanced by radiative cooling. we present the results of our radiative particle-in-cell simulations of driven kinetic turbulence in a relativistic pair plasma with optically thin inverse-compton cooling. we find that radiation quenches nonthermal particle acceleration, effectively thermalizing the plasma. the high-energy particle distributions are strongly anisotropic, leading to potentially observable intermittent beaming. the anisotropy, spatial inhomogeneity, and temporal variability of the high-energy emission are more extreme at high magnetizations, when bulk fluid motions become relativistic. these findings help us understand astrophysical gamma-ray flares and advance extreme plasma astrophysics. work supported by nasa, nsf, and doe. | radiative relativistic collisionless plasma turbulence as a frontier of extreme plasma astrophysics |
reaction network calculations predict that the crust of an accreting neutron star should host urca reactions: e--capture/β--decay cycles that cool the crust through neutrino emission. neutron star transients offer an opportunity to test this prediction. during accretion outbursts, the crust of a neutron star transient reaches temperatures above t > 2 ×108 k where urca cycling is expected to balance the accretion-driven crust heating. however, post-outburst thermal evolution models of the hottest transient maxi j0556-332 have shown that the urca cooling reactions must take place deeper in the star than predicted, or must be absent entirely, in order to fit quiescent observations. in order to reconcile the predictions from nuclear reaction networks and post-outburst thermal evolution models, we model the urca reaction layer with higher resolution than ever before. this modeling allows us to incorporate measured e--capture rates and follow the temperature evolution of the reaction layer on relatively small scales. these developments in reaction layer modeling will not only help delineate the effects of urca cooling on neutron star crusts, but will aid in incorporating urca cooling in x-ray burst models. joint institute for nuclear astrophysics - center for the evolution of the elements. | resolving urca cooling reaction layers in neutron stars |
binary neutron star mergers will be important sources of gravitational radiation for advanced ligo. understanding how different physical processes-such as magnetic and microphysical effects due to equations of state or neutrino cooling-are imprinted on the radiation is important for learning more about these systems. we perform a series of binary neutron star mergers to examine some of these effects on the gravitational radiation. we use three different realistic equations of state, ranging from soft to stiff, initially magnetized stars, and include neutrino cooling of the post-merger system using a leakage scheme. we discuss possible observational signatures for these systems. | gravitational radiation from binary neutron star mergers: magnetic and microphysical effects |
the goal of this work is to investigate the effects of chemical composition of heat blanketing envelopes of neutron stars on their thermal states and thermal evolution. to this purpose, we employ 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 “onion-like” envelope. for illustration, we apply these results to estimate the internal temperature of the vela pulsar and to study cooling of neutron stars. we show that uncertainties in the chemical composition of the envelopes can lead up to ~ 2.5 times uncertainty of the internal temperature of the star which significantly complicates theoretical reconstruction of the internal structure of cooling neutron stars from observations of their thermal surface emission. | the impact of heat blanketing envelopes on neutron stars cooling |
the assembly sequence of our galactic halo is encoded in spectra of its surviving low-metallicity turnoff stars. while the investigation of the heaviest elements (z > 55) reveals they were produced in two independent time scales by rapid- and slow neutron capture (r- and s-process), the light trans-ironic elements with atomic number z from 32 to 52 show striking abundance patterns that are open to a range of interpretation. part of the mystery may be due to the need to determine their abundances in dwarfs from ultraviolet spectra, which are heavily contaminated by stronger lines whose identity is often unknown. we will provide clarification of their empirical behavior by adding high-resolution spectra near 2000a for two cool stars of low metallicity, 1/400 and 1/100 solar, and analyzing them together with similar spectra of other low-metallicity turnoff stars using all the latest tools. this includes incorporating newly-identified fe i lines as they are found, comparing theoretical spectra for each star with high-resolution observations over all available optical and ultraviolet wavelengths, and comparing the results from two independent synthetic-spectrum approaches. this analysis should clarify current systematic errors in modeling and in line transition probabilities (gf-values). in so doing it will provide reliable abundances for a range of trans-fe elements in extremely metal-poor stars, for quantitative comparison with a half-dozen viable models of their production. the results will shed light on the nature and time scales of production of these elusive elements, and their incorporation into stars that are relics of the earliest phase of our galaxy. | tracing the earliest nucleosynthesis from elements just past the iron peak in extremely metal-poor dwarfs |
at a projected separation of 0.1 pc from the supermassive black hole at the center of the milky way, sgr a*, the transient magnetar sgr j1745-2900 holds the record as the closest neutron star to a black hole ever observed. sgr j1745-2900 has been the object of an intensive monitoring campaign in the x-rays for about 1.5 years since the outburst onset, from april 2013 until september 2014. detailed analysis of the data has revealed an extremely slow flux decay compared to the other known transient magnetars, making this source rather unique. the extremely slow cooling is currently challenging the state-of-the art neutron star crustal cooling models. if the outburst evolution is indeed due to crustal cooling, as predicted and observed for all other magnetar outbursts in the past 10 years, then magnetic energy injection needs to be continuous over at least the first ∼200 days, something so far never observed for sources of the class. alternatively, heating of the star surface may result from strong magnetospheric currents confined within a gradually shrinking magnetic bundle which impact upon the surface. however detailed numerical simulations are needed to confirm this possibility. | the x-ray outburst of the galactic centre magnetar sgr 1745-2900 during the first 1.5 year |
in light of recent discoveries of pulsating ulxs and recently introduced models placing neutron stars as the central engines of ulxs, we revisit the spectra of seventeen ulxs, in search of indications that favor this hypothesis. to this end we examined the spectra from xmm-newton observations of all seventeen sources in our sample. for six sources, these were complimented with spectra from public nustar observations. we demonstrate that the notable ({>}6 kev) spectral curvature observed in most ulxs, is most likely due to thermal emission, with t{>} 1kev. more importantly, we find that a double thermal model (comprised of a 'cool' and 'hot' thermal component) - often associated with emission from neutron star x-ray binaries - describes all ulx spectra in our list. we propose that the dual thermal spectrum is the result of accretion onto highly magnetized nss, as predicted in recent theoretical models (mushtukov et al. 2017). we further argue that this finding offers an additional and compelling argument in favor of neutron stars as prime candidates for powering ulxs, as has been recently suggested (king & lasota 2016; king et al. 2017). in my talk i will discuss the implications of our interpretation along with its merits and shortcomings. | ulx spectra revisited: are accreting, highly magnetized neutron stars the engines of ultraluminous x-ray sources? |
to address open questions in neutron star phenomenology, pairing gaps of 1s0 and 3p2 -3f2 channels in a wide range of densities has been calculated using three different interactions (av18 cdbonn n3lo). traditionally, the bardeen-cooper-schrieffer(bcs) approach has been used to compute gaps from bare nucleon-nucleon interactions. here, we incorporate the influence of short- and long-range correlations in the pairing gaps. short-range correlations (src) are treated including the appropriate fragmentation of single-particle states, and they suppress the gaps substantially. long-range correlations(lrc) dress the pairing interaction via density and spin modes, and provide a relatively small correction. results are relevant and parametrized in a user friendly way for neutron-star cooling scenarios, in particular in view of the recent observational data on cassiopeia a. | pairing in high-density neutron matter including short- and long-range correlations |
neutron star is a stellar object that can provide us information on the extreme dense matter. at the central part of neutron stars, the exotic states of matter with strangeness are expected to exist due to the fermionic nature of protons and neutrons that are mainly composed of up and down quarks. strangeness, if exists, softens the neutron star equation of state and reduces the maximum mass of neutron stars. however, due to the recent observations of 2m ⊙neutron stars, the maximum mass of neutron stars has to be bigger than 2m ⊙ . these recent observations raised questions on the existence of hyperons inside neutron stars, so-called hyperon puzzle. in this talk, we discuss the role of strangeness to the neutron star equation of state and the possibilities of the existence of λ-hyperons and kaons. we also discuss the cooling behaviours of neutron stars in the presence of strangeness. | strangeness in neutron star cooling |
type i x-ray bursts are thermonuclear flashes observed from the surfaces of accreting neutron stars (nss) in low mass x-ray binaries. oscillations have been observed during the rise and/or decay of some of these x-ray bursts. those seen during the rise can be well explained by a spreading hot spot model, but large amplitude oscillations in the decay phase remain mysterious because of the absence of a clear-cut source of asymmetry. to date there have not been any quantitative studies that consistently track the oscillation amplitude both during the rise and decay (cooling tail) of bursts. in this talk i will discuss the results of our computations of the light curves and amplitudes of oscillations in x-ray burst models that realistically account for both flame spreading and subsequent cooling. i will present results for several such “cooling wake” models, a “canonical” cooling model where each patch on the ns surface heats and cools identically, or with a latitude-dependent cooling timescale set by the local effective gravity, and an “asymmetric” model where parts of the star cool at significantly different rates. we show that while the canonical cooling models can generate oscillations in the tails of bursts, they cannot easily produce the highest observed modulation amplitudes. alternatively, a simple phenomenological model with asymmetric cooling can achieve higher amplitudes consistent with the observations. i will discuss how the combination of the light curve and fractional amplitude evolution can constrain the properties of the flame spreading, such as ignition latitude, the flame spreading geometry and speed, and its latitudinal dependence which would be important for measuring nss masses and radii using x-ray burst oscillations. | x-ray burst oscillations: from flame spreading to the cooling wake |
the gravitational-wave signal of the binary neutron star merger gw170817 was followed by a firework of electromagnetic transients across the entire electromagnetic spectrum. the gamma-ray emission has provided strong evidence for the association of short gamma-ray bursts (sgrbs) with binary neutron star mergers and the ultraviolet, optical, and near-infrared emission is consistent with a kilonova indicative of the formation of heavy elements in the merger ejecta by the rapid neutron capture process (r-process). in this talk, i will discuss and review theoretical scenarios to interpret the gamma-ray, x-ray, and radio observations. i will present recent results from general-relativistic magnetohydrodynamic simulations and discuss possible scenarios and mass ejection mechanisms that can give rise to the observed kilonova features. in particular, i will argue that massive winds from neutrino-cooled post-merger accretion disks most likely synthesized the heavy r-process elements in gw170817. | the firework of electromagnetic counterparts from gw170817 |
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 |
4u 2129+47 is a quiescent, eclipsing neutron star that 35 years ago showed typical "accretion disk corona" (adc) behavior akin to the prototype of the class, x1822-371. now faded, 4u 2129+47 provides tests of neutron star quiescent emission. it has shown low temperature thermal emission (the neutron star surface), a power law tail (of unknown origin, although possibly due to a pulsar wind interacting with an incoming accretion stream; campana et al. 1998), and sinusoidally modulated absorption (the disk) as well as periodic x-ray eclipses. subsequent xmm-newton and chandra observations, taken 2007 through fall 2015, indicate that the hard tail and sinusoidal modulation disappeared, as if the accretion stream and disk have vanished. with the intiial loss of the hard tail, the soft x-ray flux also dropped, but since has remained steady, showing no signs of further neutron star cooling in the subsequent 8 years. we compare this behavior to recent nustar observations of the quiescent neutron star cen x-4, where the hard tail seems to persist over a wider range of quiescent flux, and correlate with the soft x-ray. it also has been speculated that 4u 2129+47 is part of a hierarchical triple system, with the third body in a much longer orbit. we use the chandra and xmm-newton eclipse ephemeris residuals to describe this third body orbit. | the quiescent neutron star and hierarchical triple, 4u2129+47 |
the coming decades are set to provide a much deeper understanding of nuclei and neutron stars. the recent observation of neutron star mergers with gravitational and electromagnetic messengers opens a new window to investigate hadronic matter at high densities. the atomic nucleus provides a mini laboratory to extract physical quantities relevant to our understanding of neutron star physics, such as the equation of state for neutron rich matter and also the role of hadronic degrees of freedom beyond the nucleon. experiments on atomic nuclei at the latest generation of electromagnetic beam facilities, such as the jefferson laboratory (usa) and mami (germany), can address long standing issues in the field. the spatial distribution of neutrons in nuclei is known to a much poorer accuracy than the corresponding proton distribution. however, the difference in these distributions (the ``neutron skin'' when expressed as a difference in rms radii) constrains the density dependence of the symmetry energy, a parameter which also has a significant role in constraining neutron star structure, cooling and merger physics. in the talk i will outline the current status and future plans in our programme of neutron skin measurements with the crystal ball at mami. the intense polarized electromagnetic beams, large acceptance detector systems and nucleon spin polarimeters available at modern facilities also offer the opportunity to investigate the role of non-nucleonic degrees of freedom. these may emerge at the higher densities involved in the cores of heavy neutron stars and the high matter densities experienced in mergers. our recent results obtained at mami and jefferson lab will be presented along with plans for the future. supported by stfc st/m001571, st/j00006x. | illuminating nuclei for neutron stars |
we propose to continue our program to use the crust cooling behaviorin transiently accreting neutron stars. those crusts are heated dueto the matter accretion onto the neutron stars during outbursts. afterthe outbursts the crusts cool down until they are in equilibrium withthe cores again. following this cooling processes for several systemshas given us new insights in the structure of neutron stars, but manyuncertainties remain. therefore it is needed to enlarge our sample ofwell-studied sources to obtain better insights in the behavior of howneutron stars react to the accretion of matter. xmm-n plays a crucialrole in the earliest times (<1 yr) of the crust cooling decay and probingthe behavior of shallow depths in the crusts. | crust cooling of accretion-heated neutron stars |
we present a new formulation to construct numerically equilibrium configurations of rotating stars in general relativity. having in mind the application to their quasi-static evolutions on a secular time-scale, we adopt a lagrangian formulation of our own devising, in which we solve force-balance equations to seek for the positions of fluid elements corresponding to the grid points, instead of the ordinary eulerian formulation. unlike previous works in the literature, we do not employ the first integral of the euler equation, which is not obtained analytically in general. we assign a mass, specific angular momentum and entropy to each fluid element in contrast to the previous eulerian methods, in which the spatial distribution of the angular velocity or angular momentum is specified. these distributions are determined after the positions of all fluid elements (or grid points) are derived in our formulation. we solve the large system of algebraic non-linear equations that are obtained by discretizing the time-independent euler and einstein equations in the finite-element method by using our new multidimensional root-finding scheme, named the w4 method. to demonstrate the capability of our new formulation, we construct some rotational configurations, both barotropic and baroclinic. as toy models, we also solve three evolutionary sequences that mimic the cooling, mass-loss, and mass-accretion. | a novel lagrangian formulation to construct relativistic rotating stars: towards its application to their evolution calculations |
we request a 60 ks observation of the crust cooling source mxb 1659-29. transient low-mass x-ray binaries exhibit outbursts that result in accretion onto the neutron star and thereby heat up the crust. once the outburst ceases the crust cools to reinstate thermal equilibrium with core. tracking this cooling evolution provides insights into the dense matter (that increases by a factor ~8 over the 1 km of the crust) present in the various layers of the crust. our observation, ~800-1000 d after the end of the last outburst, will probe the inner neutron-star crust of mxb 1659-29. the requested observation will allow us to constrain the cooling evolution and thus the crustal properties in great detail, including that of the low conductivity pasta layer formed by disordered nuclei. | probing the inner neutron-star crust in the lmxb mxb 1659-29 |
the discovery of 2 solar mass neutron stars, detection of gravitational waves with the concomitant detection of electromagnetic radiation from the binary neutron star merger gw170787, recent reports of additional mergers involving neutron stars, and accumulating data on the cooling of neutron star crusts have all given much impetus to ongoing theoretical investigations of the dense matter equation of state. in this talk, i will highlight recent work on the role played by nuclei, nucleons and nonnucleonic degrees of freedom (hyperons, quarks, etc.) in understanding the many observable facets of a neutron star. special emphasis will be placed on the composition of a neutron star at all layers from the surface to the core. the time is ripe now to achieve consistency between the global properties such as masses and radii with dynamical observables that include tidal deformations, rotational periods and their time derivatives, surface temperatures of isolated neutron stars and of those that undergo periodic accretion. while several puzzles await solutions, the need for updates of ligo detectors to detect gravitational radiation from the remnants of post merger events will be emphasized. research supported by doe grant no. de-fg02-93er-40756. | the significance of a neutron star's interior composition |
the dissertation focuses on the study of rotation-powered pulsars, the primary observational manifestation of neutron stars. these objects are powerful sources of electromagnetic radiation and relativistic particles whose emission is provided by the loss of pulsar rotational energy. understanding the evolution of pulsars, which happens over billion year timescales, requires detection and study of pulsars at different stages of evolution. i present detailed x-ray analyses of pulsars at four distinct stages of evolution and compare their emission behavior with that of other pulsars expected to be in similar evolutionary stages. i also show key characteristics of the pulsars that make them unique in their group. i start with a young and energetic pulsar, psr j2022+3842 (characteristic age tauc ≈ 9 kyr, spin-down power e = 3 x 1037 erg s-1), with powerful non-thermal emission. x-ray timing of the pulsar revealed double-peaked x-ray profile with a period twice the previously established value. our analysis allowed us to update the pulsar's spin-down power and x-ray efficiency using the correct timing results, which brought the pulsar more in-line with other young x-ray pulsars. i also provide the phase-dependent behavior of the pulsar's non-thermal emission. pulsars with true ages, often substituted by characteristic age, below tauc ∼100 kyr are considered young and ones with tau c ≥ 1 myr are considered old, with the 'middle-aged' pulsars in the middle. my next pulsar is a tauc = 1.8 myr old j1836+5925 (e = 1 x 1034 erg s-1), which is perhaps the brightest x-ray source among the oldest pulsars still observable in the gamma-rays. detailed timing and spectral analyses show strong evidence of an absorption feature (perhaps an electron cyclotron line) in the pulsar's spectrum. characterizing its thermal emission might have important implications for the neutron star cooling models. moving another two orders of magnitude up in tauc, we arrive at one of the oldest known non-recycled x-ray pulsars, psr j0108-3430 tauc = 166 myr and e = 5.8 x 1030 erg s -1. the pulsar's spectrum likely consists of a thermal component, emitted from a hot polar cap, and a non-thermal component, emitted from its magnetosphere. the x-ray pulse profile shows a single, asymmetric peak which could be explained by an axially-asymmetric temperature distribution at the pole or by the non-thermal emission from the outer gap. the three pulsars represent important stages in the evolutionary path that a hypothetical single young pulsar like j2022+3842 might take, as it passes through stages close to gamma-ray emission turn-off (like j1836+5925) and x-ray turn-off (similar to j0108-3430). pulsars in binaries can follow an alternative path. by accreting matter from their companions they can be 'recycled' to short millisecond periods and emit x-rays and gamma-rays for billions of years. i also present a special class of such recycled pulsars which are believed to be in the process of fatally ablating their companions. i present the x-ray analysis of psr j1446-4701, an e = 3.6 x 1034 erg s-1 pulsar in a 6.7 hr binary orbit, and psr j1311-3430, an e = 4.9 x 1034 erg s-1 pulsar in an extreme 1.6 hr binary orbit. psr j1446-4701 turned out to be a non-eclipser with possibly low (face-on) orbital inclination, with emission from both the pulsar and the intra-binary shock observable throughout the binary orbit. psr j1311-3430 is a known eclipser, in which hints of spectral variability have been found, between pulsar superior and inferior conjunction phases. i also present a comprehensive comparison of the sample of such extremely low-mass binary pulsars. we reveal the true nature of pulsars, slowly and steadily, usually one target at a time, but eventually we expect useful patterns to emerge that improves our understanding of the population of rotation powered pulsars. | rotation powered pulsars in the x-rays |
neutrino radiation transport plays an important role in the simulation of compact binary mergers involving neutron stars. neutrinos are the main source of cooling of the post-merger remnant. additionally, neutrino-matter interactions in the matter ejected during and after merger are crucial in determining the properties of the uv/optical/infrared transients following many neutron star mergers, as well as the outcome of nucleosynthesis in the ejecta. properly evolving boltzmann's equations for radiation transport remains however a difficult technical challenge. in this talk, i will review a significant step towards performing full transport in merger simulations: the implementation in the spec code of a monte-carlo algorithm. i will discuss the overall structure of a monte-carlo transport code, its advantages in the merger context, as well as the approximations that still have to be performed in order to keep simulations computationally affordable. this work is funded by the department of energy, office of science, office of nuclear physics under contract number de-ac02-05ch11231. | implementation of monte-carlo radiation transport for the treatment of neutrinos in the spec merger code |
transient low mass x-ray binaries (lmxbs) in their quiescent phase are observed to be cooling over timescales of decades. studying this cooling behavior reveals a great deal of information about the properties of neutron stars. the β-decay/e- capture cycles lead to an appreciable cooling of the crust but the strength of this urca cooling depends primarily on the ground-state to ground-state β-decay transition strengths. a = 33 mass chain is supposedly the strongest cooling agent for crusts composed of x-ray burst ashes which relies in part on the strong ground state branch in 33mg - 33al decay measured with high resolution β-delayed γ-ray spectroscopy. however, 33mg has been recently confirmed to have a negative parity ground state making 33mg - 33al a first-forbidden decay. this leads to a discrepancy with theoretical predictions and the 33mg decay experiment results are questioned in the literature, citing pandemonium effect as a possible reason. i will present the results and ongoing analysis of the re-measurement of the β-decay of 33mg experiment performed with the total absorption spectroscopy technique at the national superconducting cyclotron laboratory (nscl) using nero/bcs/sun detector systems. this work has been supported by the national science foundation under award nos. phy-1430152 (jina center for the evolution of the elements), oise-1927130 (irena), phy-1913554, and phy-2209429. | the role of a=33 mass chain in urca cooling of accreting neutron star crusts |
the crust of a neutron star is an exotic condensed matter system, rich in physics and accessible through a host of astrophysical observables. measurements of individual neutron star cooling and pulsar glitches, of long timescale temperature, magnetic field and rotational evolution evolution by analysis of neutron star populations, and potential observations of oscillations of the solid crust and even crust shattering all probe the thermal, mechanical, and superfluid properties of the crust. we discuss recent progress in understanding some of these properties and their observational impact, including insights from microscopic, quantum simulations of the soft condensed matter system predicted to lie at the crust-core boundary, known as nuclear pasta. we also discuss how the strategy of statistical inference using ensembles of equation of state that has been so successful in constraining the core equation of state can be applied to crust modeling, and how that will allow us to bring the host of crust observables to bear on the multimessenger physics of neutron stars. nasa grant 80nssc18k1019national science foundation grant 2050099. | nuclear pasta and the deep inner crust |
from flow without dissipation of energy to the formation of vortices when placed within a rotating container, the superfluid state of matter has proven to be a very interesting physical phenomenon. here we present the key mechanisms behind superfluidity in fermionic systems and apply our understanding to an exotic system found deep within the universe -- the superfluid found deep within a neutron star. a defining trait of a superfluid is the pairing gap, which the cooling curves of neutron stars depend on. the extreme conditions surrounding a neutron star prevent us from directly probing the superfluid's properties, however, we can experimentally realize conditions resembling the interior through the use of cold atoms prepared in a laboratory and simulated on a computer. experimentalists are becoming increasingly adept at realizing cold atomic systems in the lab that mimic the behavior of neutron stars and superconductors. in their turn, computational physicists are leveraging the power of supercomputers to simulate interacting atomic systems with unprecedented accuracy. this paper is intended to provide a pedagogical introduction to the underlying concepts and the possibility of using cold atoms as a tool that can help us make significant strides towards understanding exotic physical systems. | fermionic superfluidity: from cold atoms to neutron stars |
due the large neutron excess of ne, this isotope rapidly sediments in the interior of the white dwarfs. this process releases an extra amount of energy, thus delaying the cooling times of the white dwarf. furthermore, the overabundance of ne in the inner regions of the star, modifies the brunt-vaisala frequency, thus, altering its pulsational properties. in this work, we discuss the impact of ne sedimentation in white dwarfs derived from solar metallicity progenitors (). we performed evolutionary calculations of white dwarfs of, , and . these models are the result of the full evolutionary calculations of their progenitor stars, starting at the zero age main sequence. our computations show that ne sedimentation delays the cooling times of white dwarfs in about 1 gyr at low luminosities (), affecting the techniques that use white dwarfs to date stellar populations. neglecting ne sedimentation when calculating the theoretical white dwarf luminosity function, implies to underestimate the age of the stellar population. additionally, we studied the consequences of ne sedimentation on the pulsational properties of zz ceti white dwarfs. | el efecto de la difusión del ne en las propiedades evolutivas y pulsacionales de las enanas blancas |
in this work, we present a calculation of the non-fermi liquid correction to the specific heat of magnetized degenerate quark matter present at the core of the neutron star. the role of non-fermi liquid corrections to the neutrino emissivity has been calculated beyond leading order. we extend our result to the evaluation of the pulsar kick velocity and cooling of the star due to such anomalous corrections and present a comparison with the simple fermi liquid case. | role of magnetic interactions in neutron stars |
the composition and structure of the ~1 km thick, solid crust of neutron stars is responsible for many of their observable properties, and plays a fundamental role in the emission of gravitational waves and the evolution of their magnetic field. when residing in an x-ray binary, a neutron star accretes gas from a companion star. as matter accumulates on the neutron star surface, the underlying crust is compressed and heated due to nuclear reactions induced by this compression. once accretion switches off, sensitive x-ray satellites can be employed to observe how the heated crust cools. comparing these observations with theoretical simulations provides very valuable insight into the structure and composition of the crusts of neutron stars. i will present the latest observational results and challenges in this research field. | observational results from cooling neutron stars in x-ray binaries |
we propose to continue our successful program to use the observed cooling of the crusts in accreting neutron star x-ray systems to probe the properties of ultra-dense matter. those crusts are heated due to matter accretion onto the neutron stars during outburst. after the outbursts are over the crusts should cool down until they are in equilibrium with the core again. following this cooling processes for several systems has given us new insights in the structure of neutron stars, but many uncertainties remain. therefore it is needed to enlarge our sample of well studied sources to obtain better insights in the behavior of how neutron stars react to the accretion of matter. | crust cooling of accretion-heated neutron stars |
the equation of state (eos) for dense, beta-equilibrated nuclear matter is a fundamental ingredient for simulations of binary neutron star mergers, calculation of cooling curves, and other transport properties. we construct a new eos using a relativistic-mean-field theory (rmf). typically, the couplings in an rmf are fit to isospin-symmetric nuclear matter data, but the matter in neutron stars is much closer in composition to pure neutron matter. in this work, along with reproducing isospin-symmetric nuclear matter data, we also fit our rmf to pure neutron matter calculations from chiral effective field theory. these calculations are needed because it is not possible to perform experiments on pure neutron matter on earth. our eos agrees with current mass and radius measurements from nicer and ligo. parts of this work have been supported by doe grant number no. #de-fg02-05er41375. | constructing an equation of state from pure neutron matter calculations |
we aim to study the cooling emission of highly magnetized neutron starsin be/x- ray binaries. for this, we need to increase our sample withhigh-quality post-outburst spectra. we require the flexibility of swiftto monitor the behavior of the next potential target, and the exceptionalsensitivity and large effective area of xmm-newton to study the low x-rayluminosity regime of the source. these new observations will provide keyadditional data to i) better constrain the temporal evolution of the crusttemperature in highly magnetized accreting nss, and ii) use the observedcooling curves as input into our 2d crust cooling model which incorporatesmagnetic fields. this is required to understand the role of such magneticfields on the crust heating/cooling scenario of accreting nss | on the quest of bex-ray transients: how do highly magnetized neutron stars |
most neutron stars cool predominately via the modified urca process, in which emitted neutrinos carry away energy. the traditional treatment for the in-medium nucleon propagator in the modified urca process uses crude approximations. we reformulate the propagator by including the nucleon self-energy and examine the effect of this new propagator on the neutrino emissivity due to the modified urca process. this research was partly supported by the u.s. department of energy, office of science, office of nuclear physics, under award no. #de-fg02-05er41375. | improved treatment of neutron star cooling via modified urca process |
we propose xmm-newton observations of two middle-aged radio-quietfermi pulsars j0554+3107 and j0622+3749. the former is associated withthe supernova remnant g179.0+2.6 which provides a great advantagefor confident constraining its distance and evolution stage. usingchandra, erosita and swift shallow exposures we have found theirx-ray counterparts. thermal spectral components dominate their x-rayemission. measurements of the parameters of these components areimportant for comparison with neutron star cooling theories and thestudy of fundamental properties of superdense nuclear matter insidethe stars. however, any accurate conclusions are impossible due to lowcount statistics. we request for followup observations to study thermalemission from these pulsars. | thermal emission from two middle-aged fermi pulsars j0554+3107 and j0622+3749 |
weve searched for decades for concrete evidence of intermediate-mass black holes, black holes with masses between 100 and 100,000 times that of the sun. in spite of our best efforts, these monsters have remained elusive but a new study provides some hope.searching for the middle siblingillustrations of two types of accreting black holes: a stellar-mass black hole accreting from a binary companion (top) and a supermassive black hole accreting gas in a galaxys center (bottom). [top: esa/nasa/felix mirabel; bottom: eso/m. kornmesser]work over the past 25 years has well established the existence of supermassive black holes heavyweights with millions to tens of billions of solar masses lurking in the centers of galaxies. similarly, dynamical measurements and gravitational-wave observations provide compelling evidence of stellar-mass black holes, black holes with ~10 solar masses.but what about the range in between? theory predicts that intermediate-mass black holes should be the building blocks for larger supermassive black holes, but weve yet to find concrete evidence for a black hole with a mass between 100 and 100,000 solar masses.to weigh a black holewe have, however, found candidates! a number of observational clues have pointed to hidden middleweight black holes lurking both inside and outside of our own galaxy. unfortunately, confirming these candidates is challenging, since theres no simple means to weigh these black holes.our best bet for confirmation is to rule out alternative explanations. this is the approach taken by a team of scientists led by dacheng lin (university of new hampshire) in the case of intermediate-mass black hole candidate 3xmm j215022.4055108.new hubble image of the environment around j21500551. the source is located in the outskirts of a distant lenticular galaxy; j21500551 is outlined here with a green box and a zoomed-in view is shown in the inset. [lin et al. 2020]a promising candidate3xmm j215022.4055108 (j21500551 for short) is a source visible in both x-rays and optical light. recently, j21500551 exhibited a 12-year-long x-ray outburst and the the leading explanation for this temper tantrum is that were seeing a normally invisible intermediate-mass black hole thats indulging in a snack. in this picture, the x-ray outburst is due to the tidal disruption of a passing star and the subsequent accretion of the stars material onto the black hole.the catch? the outburst could also be explained by a source closer to home: the cooling crust of a galactic neutron star that was heated in a large outburst of accretion. so how can we rule out an ordinary, nearby neutron star and confirm the theory that j21500551 is instead a distant intermediate-mass black hole?evidence in favorlin and collaborators look to new observations of j21500551, both in optical light with the hubble space telescope and in x-rays with xmm-newton. from these data, they confirm two important points:the optical light coincident with j21500551 is not a point source.if this were a galactic neutron star, wed expect to see a point source; instead, the optical counterpart is an extended source consistent with a star cluster of about 10 million solar masses in the outskirts of a distant galaxy. such a star cluster would be the perfect size to host an intermediate-mass black hole at its center.the decay of the light curve for j21500551 is well fit by a simple tidal disruption event model over 12 years of data. click to enlarge. [adapted from lin et al. 2020]the light curve has continued to decay over the past 12 years.this decay is beautifully fit by a simple tidal disruption event model.together, these observations and careful modeling strongly support a picture in which a 50,000 solar-mass black hole lit up after disrupting a small main-sequence star providing some of the most compelling evidence yet for the existence of an elusive intermediate-mass black hole.citationmultiwavelength follow-up of the hyperluminous intermediate-mass black hole candidate 3xmm j215022.4055108, dacheng lin et al 2020 apjl 892 l25. doi:10.3847/2041-8213/ab745b | new evidence for a middleweight black hole |
generalised polynomial chaos (gpc) in conjunction with sparse grid stochastic collocation and high dimensional model representation (hdmr) is used to perform uncertainty and global sensitivity analysis for the neutron chain survival and extinction probabilities, with and without an intrinsic random source. starting with a lumped backward master equation formulation, uncertainty is introduced by allowing the factorial moments of the fission multiplicity distribution, the neutron lifetime, and strength of the intrinsic source to be independent and uniformly distributed random variables. a multidimensional legendre chaos representation of the random survival and extinction probabilities is used to achieve optimal numerical convergence in the stochastic dimension and the relative variance contributions from each random parameter are then quantified using high dimensional model representation (hdmr). the underlying deterministic results of the model are found to closely match analytical benchmarks and, once uncertainty is introduced, the gpc results match both monte carlo simulations and analytical results for polynomial order greater than two. the gpc method is found to require significantly less computational time to achieve a given accuracy on the survival and extinction probabilities than the monte carlo method. it is found that, the probabilities are most sensitive to χi for lower i and have a significant sensitivity to χ2 in all cases. a chain's survival probability is moderately sensitive to the neutron lifetime early in simulation. in the subcritical case this sensitivity increases as the simulation continues whilst it decreases in the supercritical case. the extinction probability is sensitive to the source strength. | uncertainty and global sensitivity analysis of neutron survival and extinction probabilities using polynomial chaos |
a superconductor of paired protons is thought to form in the core of neutron stars soon after their birth. minimum energy conditions suggest that magnetic flux is expelled from the superconducting region due to the meissner effect, such that the neutron star core retains or is largely devoid of magnetic fields for some nuclear equation of state and proton pairing models. we show via neutron star cooling simulations that the superconducting region expands faster than flux is expected to be expelled because cooling timescales are much shorter than timescales of magnetic field diffusion. thus magnetic fields remain in the bulk of the neutron star core for at least 106 - 107yr. we estimate the size of flux free regions at 107yr to be <~ 100m for a magnetic field of 1011g and possibly smaller for stronger field strengths. | onset of superconductivity and retention of magnetic fields in cooling neutron stars |
core collapse in stars at the lower range of massive stars (8-10 m⊙) have different behaviors relative to more massive counterparts where relatively lengthy neutrino driven convection is required to relaunch the stalled bounce shock. we compute the collapse, bounce, and explosive revival of the supernova shock in a zero-metal 9.6 m⊙ progenitor using the supernova neutrino radiation hydrodynamics code chimera in 1d, 2d, and 3d. during collapse, about 100 ms before bounce, explosive burning ignites at the base of the silicon shell, generating a burning front that proceeds outward, while the iron-core continues to collapse and bounce. we examine the origins and impact of this front on the explosion dynamics. like other supernovae in the borderlands between core collapse and white dwarf formation, the low density mantle around the core in this progenitor permits a rapid revival of the shock through neutrino heating. the launch of the explosion with cooler and more neutron-rich material leads to the formation of rare neutron-rich isotopes like 48ca, which we are able to track directly through the in situ use of a large (160-species) nuclear network for the ejected material. | supernovae in the borderlands: simulating the explosion of a low-mass supernova progenitor |
we report on the effects of strong magnetic fields on neutrino emission in the modified urca process. we show that the effect of landau levels on the various urca pairs affects the neutrino emission spectrum and leads to an angular asymmetry in the neutrino emission. for low magnetic fields, the landau levels have almost no effect on the cooling. however, as the field strength increases, the electron chemical potential increases resulting in a lower density at which urca pairs can exist. for intermediate field strength, there is an interesting interference between the landau level distribution and the fermi distribution. for high enough field strength, the entire electron energy spectrum is eventually confined to a single landau level producing dramatic spikes in the emission spectrum. | evolution of urca pairs in the crusts of highly magnetized neutron stars |
pulsars are rapidly rotating neutron stars with plasma-filled magnetospheres that radiate their rotational energy in the form of poynting flux, also referred to as the spin-down power. some of the young pulsars exhibit rotation-modulated gamma-ray emission, captured by the fermi observatory. the luminosity of this emission suggests that a significant fraction (0.1-10%) of the spin-down power is dissipated in the magnetosphere and reradiated as high-energy photons. this fraction is referred to as the gamma-ray efficiency of the pulsar. first-principles global plasma simulations of pulsar magnetospheres during the past decade enabled us to self-consistently model this process by capturing both the microscopic plasma physics and the global structure of the magnetosphere. these models show that the spin-down power is dissipated in the equatorial current sheet due to magnetic reconnection, and particles accelerated in the reconnection process emit synchrotron photons, providing the explanation for the observed gamma-ray emission. in this work we examine the reconnection process and its radiative signatures in detail using global 3d particle-in-cell simulations of pulsar magnetospheres with synchrotron cooling. we show that the fraction of the spin-down power dissipated in the magnetospheric current sheet is uniquely controlled at microphysical plasma scales and only depends on the pulsar inclination angle. we demonstrate that the maximum energy and the distribution function of accelerated pairs is controlled by the available magnetic energy per particle: plasma magnetization parameter. ultimately, the shape and the extent of the plasma distribution is imprinted in the observed high-energy emission. while the cutoff energy in gamma-rays is dictated by the synchrotron emission from the highest energy pairs, we show that the peak of the emission is also sensitive to the interplay between the efficiency of synchrotron cooling and the particle acceleration rate. we show that there are two separate parameter regimes applicable to young pulsars with low and high spin-down powers. in the former case the synchrotron cooling is dynamically weak, and the peak of the emission is close to the cutoff energy in 1-10 gev range (e.g., vela). in pulsars with higher spin-down power the cooling is dynamically important, resulting in broader spectral shapes which peak at lower energies (e.g., for crab pulsar the peak lies in the mev band). this picture naturally explains why pulsars with higher spin-down power have lower gamma-ray efficiency in the 0.1 to 100 gev band of the fermi satellite. | energy dissipation and gamma-ray emission in young pulsars |
we describe the results of an ongoing survey of x-ray selected symbiotics in the galactic bulge, begun with the galactic bulge survey and now extended to the wider bulge using the chandra source catalog. chandra's sensitivity and source localization are well suited to finding symbiotic x-ray binaries (syxbs) containing neutron stars accreting from a cool giant wind, as well as x-ray bright white dwarf systems. we find significantly more matches of x-ray sources with cool giants than would be expected by chance and such objects should not normally be intrinsic x-ray sources. we describe multi-wavelength follow-up of these objects using archival spectral energy distributions and lightcurves, and recent optical spectroscopy. one surprising result so far is the large fraction of spectroscopically observed candidates with carbon star counterparts (three out of about twenty), far in excess of that expected among bulge giants. the scientific results reported in this work are based on observations made by the chandra x-ray observatory and data obtained from the chandra data archive. support for this work was provided by the national aeronautics and space administration through chandra award numbers ar5-16004x and ar0-21005x issued by the chandra x-ray observatory center, which is operated by the smithsonian astrophysical observatory for and on behalf of the national aeronautics space administration under contract nas8-03060. | multiwavelength follow-up of x-ray selected symbiotic candidates in the galactic bulge |
gamma ray heating rates are thought to play a crucial role during the pre-supernova stage of high mass stars. gamma ray heating rates, due to β±-decay and electron (positron) capture on chromium isotopes, are calculated using proton-neutron quasiparticle random phase approximation theory. the electron capture significantly affects the lepton fraction (ye) and accelerates the core contraction. the gamma rays emitted as a result of weak processes heat the core and tend to hinder the cooling and contraction due to electron capture and neutrino emission. the emitted gamma rays tend to produce enormous entropy and set the convection to play its role at this stage. the gamma heating rates, on 50-60cr, are calculated for the density range 10 < ρ (g.cm-3) < 1011 and temperature range 107 < t (k) < 3.0×1010. | gamma ray heating rates due to chromium isotopes in stellar core during late stages of high mass stars (>10m⊙) |
this paper provides an overview of the capabilities of a new toolset that is being developed at canadian nuclear laboratories for modelling advanced non-water cooled small modular reactor concepts. throughout the study, a technology agnostic approach was adopted. the integrated toolset includes computational fluid dynamics, neutronics and system thermalhydraulics codes to handle a range of multi-physics capabilities. the toolset has been applied to two different non-water cooled smr concepts: molten salt and prismatic gas-cooled reactors. a step-wise approach was used in which the models were first tested using standalone codes, followed by execution of coupled-code simulations. this paper demonstrates the application of the toolset to gas-cooled and molten-salt reactor concepts, including transient simulations using the cfd code star-ccm+ coupled with the monte carlo neutronics code serpent 2 and with the system thermalhydraulics code relap5-3d. the scenarios considered include a step reactivity insertion and a pump trip for the molten salt reactors. overall, the results obtained so far demonstrates the potential of the developed methodology at cnl in simulating the non-water cooled concepts. | demonstration of a coupled computational fluid dynamics approach for modelling non-water cooled small modular reactors |
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