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Register a new userAccretion heated atmospheres of X-ray bursting neutron stars
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Some thermonuclear (type I) X-ray bursts at the neutron star surfaces in low-mass X-ray binaries take place during hard persistent states of the systems. Spectral evolution of these bursts is well described by the atmosphere model of a passively cooling neutron star when the burst luminosity is high enough. The observed spectral evolution deviates from the model predictions when the burst luminosity drops below a critical value of 20-70% of the maximum luminosity. We suggest that these deviations are induced by the additional heating of the accreted particles. We present a method for computation of the neutron star atmosphere models heated by accreted particles assuming that their energy is released via Coulomb interactions with electrons. We compute the temperature structures and the emergent spectra of the atmospheres of various chemical compositions and investigate the dependence of the results on the other model parameters. We show that the heated atmosphere develops the hot (20--100 keV) corona-like surface layer cooled by Compton scattering, and the deeper, almost isothermal optically thick region with a temperature of a few keV. The emergent spectra deviate strongly from those of undisturbed neutron star atmospheres, with the main differences being the presence of a high-energy tail and a strong excess in the low-energy part of the spectrum. They also lack the iron absorption edge, which is visible in the spectra of undisturbed low-luminosity atmospheres with solar chemical composition. Using the computed spectra, we obtained the dependences of the dilution and color-correction factors as functions of relative luminosities for pure helium and solar abundance atmospheres. We show that the helium model atmosphere heated by accretion corresponding to 5% of the Eddington luminosity describes well the late stages of the X-ray bursts in 4U 1820-30.
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The hydrogen and helium accreted by X-ray bursting neutron stars is periodically consumed in runaway thermonuclear reactions that cause the entire surface to glow brightly in X-rays for a few seconds. With models of the emission, the mass and radius of the neutron star can be inferred from the observations. By simultaneously probing neutron star masses and radii, X-ray bursts are one of the strongest diagnostics of the nature of matter at extremely high densities. Accurate determinations of these parameters are difficult, however, due to the highly non-ideal nature of the atmospheres where X-ray bursts occur. Observations from X-ray telescopes such as RXTE and NuStar can potentially place strong constraints on nuclear matter once uncertainties in atmosphere models have been reduced. Here we discuss current progress on modeling atmospheres of X-ray bursting neutron stars and some of the challenges still to be overcome.
Thermal dominated X-ray spectra of neutron stars in quiescent transient X-ray binaries and neutron stars that undergo thermonuclear bursts are sensitive to mass and radius. The mass-radius relation of neutron stars depends on the equation of state that governs their interior. Constraining this relation accurately is thus of fundamental importance to understand the nature of dense matter. In this context we introduce a pipeline to calculate realistic model spectra of rotating neutron stars with hydrogen and helium atmospheres. An arbitrarily fast rotating neutron star with a given equation of state generates the spacetime in which the atmosphere emits radiation. We use the Lorene/nrotstar code to compute the spacetime numerically and the ATM24 code to solve the radiative transfer equations self-consistently. Emerging specific intensity spectra are then ray-traced through the neutron stars spacetime from the atmosphere to a distant observer with the Gyoto code. Here, we present and test our fully relativistic numerical pipeline. To discuss and illustrate the importance of realistic atmosphere models we compare our model spectra to simpler models like the commonly used isotropic color-corrected blackbody emission. We highlight the importance of considering realistic model-atmosphere spectra together with relativistic ray tracing to obtain accurate predictions. We also insist on the crucial impact of the stars rotation on the observables. Finally, we close a controversy that has been appearing in the literature in the recent years regarding the validity of the ATM24 code.
We infer the collapse times of long-lived neutron stars into black holes using the X-ray afterglows of 18 short gamma-ray bursts. We then apply hierarchical inference to infer properties of the neutron star equation of state and dominant spin-down mechanism. We measure the maximum non-rotating neutron star mass $M_mathrm{TOV} = 2.31 ^{+0.36}_{-0.21} M_{odot}$ and constrain the fraction of remnants spinning down predominantly through gravitational-wave emission to $eta = 0.69 ^{+0.21}_{-0.39}$ with $68 %$ uncertainties. In principle, this method can determine the difference between hadronic and quark equation of states. In practice, however, the data is not yet informative with indications that these neutron stars do not have hadronic equation of states at the $1sigma$ level. These inferences all depend on the underlying progenitor mass distribution for short gamma-ray bursts produced by binary neutron star mergers. The recently announced gravitational-wave detection of GW190425 suggests this underlying distribution is different from the locally-measured population of double neutron stars. We show that $M_mathrm{TOV}$ and $eta$ constraints depend on the fraction of binary mergers that form through a distribution consistent with the locally-measured population and a distribution that can explain GW190425. The more binaries that form from the latter distribution, the larger $M_mathrm{TOV}$ needs to be to satisfy the X-ray observations. Our measurements above are marginalised over this unknown fraction. If instead, we assume GW190425 is not a binary neutron star merger, i.e the underlying mass distribution of double neutron stars is the same as observed locally, we measure $M_mathrm{TOV} = 2.26 ^{+0.31}_{-0.17} M_{odot}$.
The equation of state (EoS) of the neutron star (NS) matter remains an enigma. In this work we perform the Bayesian parameter inference with the gravitational wave data (GW170817) and mass-radius observations of some NSs (PSR J0030+0451, PSR J0437-4715, and 4U 1702-429) using the phenomenologically constructed EoS models to search for a potential first-order phase transition. Our phenomenological EoS models take the advantages of current widely used parametrizing methods, which are flexible enough to resemble various theoretical EoS models. We find that the current observation data are still not informative enough to support/rule out phase transition, due to the comparable evidences for models with and without phase transition. However, the bulk properties of the canonical $1.4,M_odot$ NS and the pressure at around $2rho_{rm sat}$ are well constrained by the data, where $rho_{rm sat}$ is the nuclear saturation density. Moreover, strong phase transition at low densities is disfavored, and the $1sigma$ lower bound of transition density is constrained to $1.84rho_{rm sat}$.
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