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arXiv 0711.2579astro-ph
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利用静止低质量X射线双星约束中子星状态方程

Constraining the neutron star equation of state using quiescent low-mass X-ray binaries

P. G. Jonker

摘要:

对静止低质量X射线双星的钱德拉或XMM-牛顿观测可以对中子星的状态方程提供重要的约束。中子星的质量和半径可以通过将中子星大气模型拟合到观测到的X射线光谱来确定。对于半径测量,到源的距离受到很好的约束是至关重要的,因为半径的分数不确定性至少与距离的分数不确定性一样大。中子星的大气层仍然存在。在这个阶段,它还不清楚,如果软的热成分在许多静态的X射线双星的光谱是可变的时间尺度太短,以适应冷却中子星的情况。这可以通过对静止状态下中子星X射线瞬态CenX-4的长时间XMM-牛顿观测来检验。通过这样的观测,人们可以使用反射光栅光谱仪光谱来将星际消光限制在光源上。这从EPIC pn和MOS谱的X射线谱拟合中去除了该参数,并允许研究在该源的静态X射线谱中观察到的变化是否是由于软热谱分量的变化或与N_H的变化耦合的幂律谱分量的变化。这将测试软热成分是否确实是由于中子星的热辉光。不管这种研究的结果如何,在地壳由于长时间的爆发而与地核严重热平衡的源(如KS 1731-260)中观察到的静止冷却似乎是通过用中子星大气模型模拟观察到的X射线来确定质量和半径的极好候选者。

英文摘要:

Chandra or XMM-Newton observations of quiescent low-mass X-ray binaries can provide important constraints on the equation of state of neutron stars. The mass and radius of the neutron star can potentially be determined from fitting a neutron star atmosphere model to the observed X-ray spectrum. For a radius measurement it is of critical importance that the distance to the source is well constrained since the fractional uncertainty in the radius is at least as large as the fractional uncertainty in the distance. Uncertainties in modelling the neutron star atmosphere remain. At this stage it is not yet clear if the soft thermal component in the spectra of many quiescent X-ray binaries is variable on timescales too short to be accommodated by the cooling neutron star scenario. This can be tested with a long XMM-Newton observation of the neutron star X-ray transient CenX-4 in quiescence. With such an observation one can use the Reflection Grating Spectrometer spectrum to constrain the interstellar extinction to the source. This removes this parameter from the X-ray spectral fitting of the EPIC pn and MOS spectra and allows one to investigate whether the variability observed in the quiescent X-ray spectrum of this source is due to variations in the soft thermal spectral component or variations in the power law spectral component coupled with variations in N_H. This will test whether the soft thermal component can indeed be due to the hot thermal glow of the neutron star. Irrespective of the outcome of such a study, the observed cooling in quiescence in sources for which the crust is significantly out of thermal equilibrium with the core due to a prolonged outburst, such as KS 1731-260, seem excellent candidates for mass and radius determinations through modelling the observed X-rays with a neutron star atmosphere model.

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