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Register a new userImpact of accretion on the statistics of neutron star masses
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We have collected the parameter of 38 neutron stars (NSs) in binary systems with spin periods and measured masses. By adopting the Boot-strap method, we reproduced the procedure of mass calculated for each system separately, to determine the truly mass distribution of the NS that obtained from observation. We also applied the Monte-Carlo simulation and introduce the characteristic spin period 20 ms, in order to distinguish between millisecond pulsars (MSPs) and less recycled pulsars. The mass distributions of MSPs and the less recycled pulsars could be fitted by a Gaussian function as $rm 1.45pm0.42 M_{odot}$ and $rm 1.31pm0.17 M_{odot} (rm with ~ 1sigma)$ respectively. As such, the MSP masses are heavier than those in less recycled systems by factor of $rm sim 0.13M_{odot}$, since the accretion effect during the recycling process.
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Neutron star merger accretion discs can launch neutron-rich winds of $>10^{-2},mathrm{M}_odot$. This ejecta is a prime site for r-process nucleosynthesis, which will produce a range of radioactive heavy nuclei. The decay of these nuclei releases enough energy to accelerate portions of the wind by ~0.1c. Here, we investigate the effect of r-process heating on the dynamical evolution of disc winds. We extract the wind from a 3D general relativistic magnetohydrodynamic simulation of a disc from a post-merger system. This is used to create inner boundary conditions for 2D hydrodynamic simulations that continue the original 3D simulation. We perform two such simulations: one that includes the r-process heating, and another one that does not. We follow the hydrodynamic simulations until the winds reach homology (60 seconds). Using time-dependent multi-frequency multi-dimensional Monte Carlo radiation transport simulations, we then calculate the kilonova light curves from the winds with and without dynamical r-process heating. We find that the r-process heating can substantially alter the velocity distribution of the wind, shifting the mass-weighted median velocity from 0.06c to 0.12c. The inclusion of the dynamical r-process heating makes the light curve brighter and bluer at ~1 d post-merger. However, the high-velocity tail of the ejecta distribution and the early light curves are largely unaffected.
Precise and reliable measurements of the masses and radii of neutron stars with a variety of masses would provide valuable guidance for improving models of the properties of cold matter with densities above the saturation density of nuclear matter. Several different approaches for measuring the masses and radii of neutron stars have been tried or proposed, including analyzing the X-ray fluxes and spectra of the emission from neutron stars in quiescent low-mass X-ray binary systems and thermonuclear burst sources; fitting the energy-dependent X-ray waveforms of rotation-powered millisecond pulsars, burst oscillations with millisecond periods, and accretion-powered millisecond pulsars; and modeling the gravitational radiation waveforms of coalescing double neutron star and neutron star -- black hole binary systems. We describe the strengths and weaknesses of these approaches, most of which currently have substantial systematic errors, and discuss the prospects for decreasing the systematic errors in each method.
Gravitational waves (GWs) from inspiralling neutron stars afford us a unique opportunity to infer the as-of-yet unknown equation of state of cold hadronic matter at supranuclear densities. The dominant matter effects are due to the stars response to their companions tidal field, leaving a characteristic imprint in the emitted GW signal. This unique signature allows us to constrain the neutron star equation of state. At GW frequencies above $gtrsim 800$Hz, however, subdominant tidal effects known as dynamical tides become important. In this letter, we demonstrate that neglecting dynamical tidal effects associated with the fundamental ($f$-) mode leads to large systematic biases in the measured tidal deformability of the stars and hence in the inferred neutron star equation of state. Importantly, we find that $f$-mode dynamical tides will already be relevant for Advanced LIGOs and Virgos fifth observing run ($sim 2025$) -- neglecting dynamical tides can lead to errors on the neutron radius of $mathcal{O}(1{rm km})$, with dramatic implications for the measurement of the equation of state. Our results demonstrate that the accurate modelling of subdominant tidal effects beyond the adiabatic limit will be crucial to perform accurate measurements of the neutron star equation of state in upcoming GW observations.
In accreting neutron star X-ray transients, the neutron star crust can be substantially heated out of thermal equilibrium with the core during an accretion outburst. The observed subsequent cooling in quiescence (when accretion has halted) offers a unique opportunity to study the structure and thermal properties of the crust. Initially crust cooling modelling studies focussed on transient X-ray binaries with prolonged accretion outbursts (> 1 year) such that the crust would be significantly heated for the cooling to be detectable. Here we present the results of applying a theoretical model to the observed cooling curve after a short accretion outburst of only ~10 weeks. In our study we use the 2010 outburst of the transiently accreting 11 Hz X-ray pulsar in the globular cluster Terzan 5. Observationally it was found that the crust in this source was still hot more than 4 years after the end of its short accretion outburst. From our modelling we found that such a long-lived hot crust implies some unusual crustal properties such as a very low thermal conductivity (> 10 times lower than determined for the other crust cooling sources). In addition, we present our preliminary results of the modelling of the ongoing cooling of the neutron star in MXB 1659-298. This transient X-ray source went back into quiescence in March 2017 after an accretion phase of ~1.8 years. We compare our predictions for the cooling curve after this outburst with the cooling curve of the same source obtained after its previous outburst which ended in 2001.
The dependence of the spin frequency derivative $dot{ u}$ of accreting neutron stars with a strong magnetic field (X-ray pulsars) on the mass accretion rate (bolometric luminosity, $L_{bol}$) has been investigated for eight transient pulsars in binary systems with Be stars. Using data from the Fermi/GBM and Swift/BAT telescopes, we have shown that for seven of the eight systems the dependence $dot{ u}$ can be fitted by the model of angular momentum transfer through an accretion disk, which predicts the relation $dot{ u}sim L^{6/7}_{bol}$. Hysteresis in the dependence $dot{ u}(L_{bol})$ has been confirmed in the system V 0332+53 and has been detected for the first time in the systems KS 1947+300, GRO J1008-57, and 1A 0535+26. The radius of the neutron star magnetosphere in all of the investigated systems have been estimated. We show that this quantity varies from pulsar to pulsar and depends strongly on the analytical model and the estimates for the neutron star and binary system parameters.
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