[论文解读] Doubling the number of Be/X-ray binaries in the SMC

F. Haberl, M. Sasaki|arXiv (Cornell University)|May 10, 2000

Stellar, planetary, and galactic studies被引用 3

一句话总结

本文通过交叉比对ROSAT和Einstein X射线源星表与Hα发射线星表，提出了25个大麦哲伦云（SMC）中的新Be/X射线双星候选体，使此类系统的已知数量显著翻倍。该方法利用了Be星（通过Hα发射识别）与X射线源之间的强相关性，揭示了大量低光度Be/X射线双星，提示在银河系中可能仍有众多此类系统尚未被探测到。

ABSTRACT

A correlation of X-ray source and Halpha emission-line object catalogues in the Small Magellanic Cloud (SMC) shows that more than two thirds of the optically identified Be stars in Be/X-ray binaries are found as emission-line objects in the catalogues. On the basis of this result we propose up to 25 X-ray sources mainly from recent ROSAT catalogues as new Be/X-ray binaries and give their likely optical counterparts. Also for the five yet unidentified X-ray pulsars in the SMC we propose emission-line stars as counterparts. This more than doubles the number of known high mass X-ray binary systems in this nearby galaxy. The spatial distribution of the new candidates is similar to that of the already identified Be/X-ray binaries with a strong concentration along the SMC main body and some systems in the eastern wing. The new candidates contribute mainly to the low-luminosity end of the X-ray luminosity distribution of Be/X-ray binaries. A comparison with the luminosity distribution in the Milky Way reveals no significant differences at the high-luminosity end and the large number of low-luminosity systems in the SMC suggests that many such systems may still be undetected in the Galaxy. The overall ratio of known Be to OB supergiant X-ray binaries in the SMC is an order of magnitude larger than in the Galaxy, however, might show spatial variations. While in the eastern wing the ratio is comparable to that in the Galaxy no supergiant X-ray binary is currently known in the main body of the SMC. Possible explanations include a different star formation history over the last ~15 My.

研究动机与目标

• 通过多波段交叉相关，识别大麦哲伦云（SMC）中的新Be/X射线双星候选体。
• 解决当前巡天中低光度Be/X射线双星代表性不足的问题，并评估其在银河系中的可探测性。
• 检验假设：许多Be/X射线双星因X射线光度较低而未被探测到。
• 研究SMC不同区域中Be/X射线双星与超巨星X射线双星比例的空间变化。
• 改进SMC中五个未关联X射线脉动星的光学对应体识别。

提出的方法

• 将SMC中517个ROSAT PSPC X射线源与MA93和MB99 Hα发射线星表进行交叉相关，使用√(r90² + 10²)作为位置阈值，以考虑位置误差。
• 利用额外的X射线星表（ROSAT HRI、Einstein IPC、ASCA、BeppoSAX、RXTE）扩展对与发射线星相关X射线源的搜索。
• 基于距离、X射线光度及与Be星一致的光学星等筛选候选体，排除已知的超新星遗迹和超软源。
• 通过X射线光度、光学星等（B、V、R）及光谱特征评估候选系统，以区分Be/X射线双星与其他源。
• 根据X射线光 light curves 和周期性（如可用）将候选体分类为Be/X?或带脉动的Be/X。
• 将新候选体的光度分布与已知Be/X射线双星及银河系系统进行比较，以评估可探测性及群体差异。

实验结果

研究问题

• RQ1 Hα发射线星与X射线源之间的相关性是否可用于可靠识别SMC中新的Be/X射线双星候选体？
• RQ2 新候选体对Be/X射线双星X射线光度函数的贡献如何，特别是在低光度区域？
• RQ3 SMC中Be星与超巨星X射线双星的比例为何比银河系高一个数量级，且是否在空间上存在变化？
• RQ4 基于光学对应体识别，SMC中五个未关联的X射线脉动星是否很可能与Be星相关？
• RQ5 SMC中低光度Be/X射线双星的存在在多大程度上表明，银河系中仍有大量此类系统尚未被探测到？

主要发现

• 作者基于与Hα发射线星的交叉相关，从ROSAT PSPC和HRI星表中提出25个SMC中新Be/X射线双星候选体。
• 新候选体显著增加了SMC中已知Be/X射线双星的数量，使该类系统的总数几乎翻倍。
• 新候选体的空间分布与已知Be/X射线双星高度一致，主要集中于SMC主体区域，部分位于东部翼区。
• 新候选体主要贡献于X射线光度函数的低光度端，光度范围为~3.0×10³⁵ erg s⁻¹至~5.0×10³⁷ erg s⁻¹。
• SMC中的光度分布与银河系在高光度区无显著差异，但大量低光度系统的存在提示，此类系统在我们银河系中可能仍有大量未被探测到。
• SMC中Be星与超巨星X射线双星的比例比银河系高一个数量级，尽管在东部翼区该比例相近，目前SMC主体区域尚未发现已知的超巨星X射线双星。