A NEUTRON STAR STIFF EQUATION OF STATE DERIVED FROM COOLING PHASES OF THE X-RAY BURSTER 4U 1724−307
A NEUTRON STAR STIFF EQUATION OF STATE DERIVED FROM COOLING PHASES OF THE X-RAY BURSTER 4U 1724−307
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DOI：
10.1088/0004-637x/742/2/122
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发表时间：
2010-04
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影响因子：
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通讯作者：
V. Suleimanov;J. Poutanen;M. Revnivtsev;K. Werner
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作者：
V. Suleimanov;J. Poutanen;M. Revnivtsev;K. Werner
如果能精确测量到冷尾的爱丁顿通量和视半径，并且知道到X射线爆发的距离，那么X射线爆发的热辐射是确定中子星星质量和半径的有力工具。本文提出了一种利用光球半径膨胀（PRE）爆发的冷却相数据确定恒星基本参数的改进方法。因为在该阶段的黑体表观半径仅取决于光谱硬化因子（颜色校正），我们建议拟合的颜色校正与通量在爱丁顿单位的观测到的变化的表观黑体半径的平方根倒数与通量的理论依赖性。为此，我们使用了一个大的大气模型的爆发光度变化的三个数量级和各种化学成分和表面重力。我们发现，光谱变化观察到的长PRE从4U 1724-307爆发是完全一致的被动冷却NS大气的理论预期。我们的方法使我们能够更可靠地确定爱丁顿通量（这是发现是小于15%的着陆通量）和恒星的视半径的距离的比率。然后，我们发现一个下限的NS半径为14公里的质量低于2.3 M的潮汐，独立的化学成分。这些结果表明，在NS内部的物质的特征在于一个刚性的状态方程。我们还发现了有利于富氢吸积物质的证据，并获得了7千秒差距的距离上限。最后指出，4U 1724-307短爆发冷却尾的黑体辐射面积比长爆发小一倍，其演化不符合理论。这使得他们的使用确定NS参数值得怀疑，并严重怀疑以前的作品，使用类似的突发从其他来源进行分析的结果。
Thermal emission during X-ray bursts is a powerful tool for determining neutron star (NS) masses and radii if the Eddington flux and the apparent radius in the cooling tail can be measured accurately and distances to the sources are known. We propose here an improved method of determining the basic stellar parameters using the data from the cooling phase of photospheric radius expansion (PRE) bursts covering a large range of luminosities. Because at that phase the blackbody apparent radius depends only on the spectral hardening factor (color correction), we suggest fitting the theoretical dependences of the color correction versus flux in Eddington units to the observed variations of the inverse square root of the apparent blackbody radius with the flux. For that we use a large set of atmosphere models for burst luminosities varying by three orders of magnitude and for various chemical compositions and surface gravities. We show that spectral variations observed during a long PRE burst from 4U 1724–307 are entirely consistent with the theoretical expectations for the passively cooling NS atmospheres. Our method allows us to more reliably determine both the Eddington flux (which is found to be smaller than the touchdown flux by 15%) and the ratio of the stellar apparent radius to the distance. We then find a lower limit on the NS radius of 14 km for masses below 2.3 M☉, independently of the chemical composition. These results suggest that the matter inside NSs is characterized by a stiff equation of state. We also find evidence in favor of hydrogen-rich accreting matter and obtain an upper limit to the distance of 7 kpc. We finally show that the apparent blackbody emitting area in the cooling tails of the short bursts from 4U 1724–307 is two times smaller than that for the long burst and their evolution does not follow the theory. This makes their usage for determining the NS parameters questionable and casts serious doubt on the results of previous works that used similar bursts from other sources for analysis.