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arXiv 1001.0900astro-ph.HE
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光球超膨胀热核X射线爆发中重元素灰烬的证据

Evidence of heavy-element ashes in thermonuclear X-ray bursts with photospheric superexpansion

J. J. M. in 't Zand, N. N. Weinberg

备注：
Accepted for publication in Astronomy & Astrophysics
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摘要：

中子星上的一小部分热核X射线爆发表现出如此强烈的光球膨胀，以致在几秒钟内光球位于半径r_ph >~ 1000 km处。这种“超膨胀”意味着一个大的和快速的能量释放，一个纯He爆发模型的特征。以前的计算表明，在纯He爆发期间，新合成的重元素燃烧灰烬可以在强烈的辐射风中喷出，并产生显着的光谱吸收特征。我们检索了爆发数据目录和文献，发现了32个超膨胀爆发。我们发现，这些爆发表现出以下有趣的特点：（1）至少有31个来自（候选）超紧凑X射线双星，其中的中子星吸积氢缺乏的燃料，这表明这些爆发确实在富氦层点燃。(2)在2个突发中，我们在膨胀阶段检测到强吸收边缘。边缘能量和深度与铁峰元素的类H或类He边缘一致，丰度大于太阳的100倍，这表明我们看到了核燃烧的暴露灰烬。(3)在超膨胀阶段之后总是有一个中等膨胀阶段，在此阶段中，r_ph ~ 30 km，光度接近爱丁顿极限。(4)爆发的衰减时间td从短（约10 s）到中（>~ 1000 s）不等。然而，尽管t_d的范围很大，超膨胀的持续时间总是几秒钟，与t_d无关。相比之下，适度膨胀的持续时间总是t_d阶。(5)光球半径r_ph在适度膨胀阶段比稳定状态风模型预测的要小得多。我们表明，这可能是进一步表明，风中含有高度非太阳的重元素丰度。

英文摘要：

A small subset of thermonuclear X-ray bursts on neutron stars exhibit such a strong photospheric expansion that for a few seconds the photosphere is located at a radius r_ph >~ 1000 km. Such `superexpansions' imply a large and rapid energy release, a feature characteristic of pure He burst models. Previous calculations have shown that during a pure He burst, the freshly synthesized heavy-element ashes of burning can be ejected in a strong radiative wind and produce significant spectral absorption features. We search the burst data catalogs and literature and find 32 superexpansion bursts. We find that these bursts exhibit the following interesting features: (1) At least 31 are from (candidate) ultracompact X-ray binaries in which the neutron star accretes hydrogen-deficient fuel, suggesting that these bursts indeed ignite in a helium-rich layer. (2) In 2 bursts we detect strong absorption edges during the expansion phase. The edge energies and depths are consistent with the H-like or He-like edge of iron-peak elements with abundances greater than 100 times solar, suggesting that we are seeing the exposed ashes of nuclear burning. (3) The superexpansion phase is always followed by a moderate expansion phase during which r_ph ~ 30 km and the luminosity is near the Eddington limit. (4) The decay time of the bursts, t_d, ranges from short (approximately 10 s) to intermediate (>~ 1000 s). However, despite the large range of t_d, the duration of the superexpansion is always a few seconds, independent of t_d. By contrast, the duration of the moderate expansion is always of order t_d. (5) The photospheric radii r_ph during the moderate expansion phase are much smaller than steady state wind models predict. We show that this may be further indication that the wind contains highly non-solar abundances of heavy elements.

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