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arXiv 1206.0691astro-ph.HEastro-ph.SR
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14颗超紧X射线双星的长期光度行为

Long-term luminosity behavior of 14 ultracompact X-ray binaries

L. M. van Haaften, R. Voss, G. Nelemans

摘要:

上下文X射线双星通常分为持久源和瞬态源。对于超紧凑X射线双星(UCXB),质量转移率预计是一个强大的功能的轨道周期,预测持久源在短周期和瞬态在长周期。目标。对于14个UCXB,包括两个候选人,我们调查的长期变化和平均光度的目的是学习多久一个源可以预期是可见的高于一个给定的光度,我们比较推导出的光度与理论预测。方法.我们使用来自RXTE全天监测器的数据,因为它是长期的,无偏的观测。许多UCXB是微弱的,即,它们在大多数时间具有处于噪声水平的计数率。尽管如此,信息可以从数据中提取,或者通过只使用可靠的数据点,或者通过将亮端变化行为与时间平均光度相结合。结果亮度概率分布显示了源在给定亮度以上发射的时间分数。所有UCXB显示出显着的变化和相对相似的行为,虽然时间平均光度意味着更高的可变性,在系统的周期超过40分钟。我们通常称之为持久源和瞬变源的统计光度行为没有很大的差异。周期低于~30 min的UCXB具有与基于理论传质速率的估计值合理一致的时间平均测辐射热光度。在40分钟左右,时间平均光度的下限与基于理论质量传递速率的光度相似,这表明这些源在未被检测到时确实是微弱的。超过50分钟,一些系统比理论传质速率预测的要亮得多,除非这些系统具有氦燃烧供体或失去额外的角动量。

英文摘要:

Context. X-ray binaries are usually divided in persistent and transient sources. For ultracompact X-ray binaries (UCXBs), the mass transfer rate is expected to be a strong function of orbital period, predicting persistent sources at short periods and transients at long periods. Aims. For 14 UCXBs including two candidates, we investigate the long-term variability and average luminosity with the purpose of learning how often a source can be expected to be visible above a given luminosity, and we compare the derived luminosities with the theoretical predictions. Methods. We use data from the RXTE All-Sky Monitor because of its long-term, unbiased observations. Many UCXBs are faint, i.e., they have a count rate at the noise level for most of the time. Still, information can be extracted from the data, either by using only reliable data points or by combining the bright-end variability behavior with the time-averaged luminosity. Results. Luminosity probability distributions show the fraction of time that a source emits above a given luminosity. All UCXBs show significant variability and relatively similar behavior, though the time-averaged luminosity implies higher variability in systems with a period longer than 40 min. Conclusions. There is no large difference in the statistical luminosity behavior of what we usually call persistent and transient sources. UCXBs with a period below ~30 min have a time-averaged bolometric luminosity that is in reasonable agreement with estimates based on the theoretical mass transfer rate. Around 40 min the lower bound on the time-averaged luminosity is similar to the luminosity based on the theoretical mass transfer rate, suggesting these sources are indeed faint when not detected. Above 50 min some systems are much brighter than the theoretical mass transfer rate predicts, unless these systems have helium burning donors or lose additional angular momentum.

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