The cooling phase of Type I X-ray bursts in 4U 1636-53
The cooling phase of Type I X-ray bursts in 4U 1636-53
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DOI：
10.1111/j.1365-2966.2011.18271.x
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发表时间：
2010-10
影响因子：
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通讯作者：
Guobao Zhang;M. Méndez;D. Altamirano
中科院分区：
文献类型：
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作者：
Guobao Zhang;M. Méndez;D. Altamirano
低质量X射线双星(LMXB)中热核X射线爆发冷却阶段的时间分辨谱可以用来测量中子星的半径和质量。我们使用ROSSI X射线定时探测器的数据分析了LMXB 4U 1636-53的300次爆发。根据爆发的性质和爆发时的源状态，我们将爆发分为光球半径扩展(Pre)爆发、硬非Pre爆发和软非Pre爆发三类。对于这三种类型的爆发，我们发现，在爆发的冷却阶段，测辐射热通量与温度之间的平均关系与典型的F/T4关系有很大的不同，即如果中子星表面的视发射面积在爆发衰变过程中保持不变，则通量减小。我们还发现，对于这三种类型的爆发，单一的幂函数不能很好地拟合平均通量-温度关系，并且这三种类型的爆发的通量-温度关系有很大的不同。最后，对于这三种类型的爆发，爆发衰变过程中不同通量层的温度分布有明显的差异。由此我们得出结论，硬的非预爆在富氢的大气中点火，而软的非预爆和预爆的燃料是富氦的。我们进一步得出结论，随着爆发的衰减，中子星大气中的金属丰度降低，这可能是因为重元素在大气中的下沉速度比H和He更快。
Time-resolved spectra during the cooling phase of thermonuclear X-ray bursts in low- mass X-ray binaries (LMXBs) can be used to measure the radii and masses of neutron stars. We analyzed 300 bursts of the LMXB 4U 1636–53 using data from the Rossi X-ray Timing Explorer. We divided the bursts in three groups, photospheric radius expansion (PRE), hard non-PRE and soft non-PRE bursts, based on the properties of the bursts and the state of the source at the time of the burst. For the three types of bursts, we found that the average relation between the bolometric flux and the temperature during the cooling phase of the bursts is significantly different from the canonical F / T 4 relation that is expected if the apparent emitting area on the surface of the neutron star remains constant as the flux decreases during the decay of the bursts. We also found that a single power law cannot fit the average flux-temperature relation for any of the three types of bursts, and that the flux-temperature relation for the three types of bursts is significantly different. Finally, for the three types of bursts, the temperature distribution at different flux levels during the decay of the bursts is significantly different. From the above we conclude that hard non-PRE bursts ignite in a hydrogen-rich atmosphere, whereas for soft non-PRE and PRE bursts the fuel is helium-rich. We further conclude that the metal abundance in the neutron star atmosphere decreases as the bursts decay, probably because the heavy elements sink faster in the atmosphere than H and He.