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arXiv 0801.1761astro-ph
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吸积能X射线脉冲星4U 1626-67准周期振荡长期演化的研究

A study of the long term evolution of quasi periodic oscillations in the accretion powered X-ray pulsar 4U 1626-67

Ramanpreet Kaur, Biswajit Paul, Brijesh Kumar, Ram Sagar

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14 pages, 3 figures. Accepted for publication in ApJ
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摘要：

本文研究了一颗异常吸积X射线脉冲星4U 1626- 67的准周期振荡（QPO）的长期性质。这是一个独特的吸积动力X射线脉冲星，我们发现QPO存在于所有足够长的X射线观测与广泛的X射线观测。在目前的自旋下降时代的这个源，QPO中心频率被发现是减少。在这一辐射源的早期发展阶段，只有两份关于QPO探测的报告，分别是1983年EXOSAT和1988年GINGA探测到的QPO，而且有增加的趋势。4U 1626- 67在过去22年中的QPO频率演变从正趋势变为负趋势，这与该源的扭矩反转有些一致。在吸积动力X射线天体中，QPO频率与吸积盘的内径直接相关，如开普勒频率模型（KFM）和拍频模型（BFM）。早期的X射线光谱、通量和脉冲轮廓测量报告了吸积盘的逐渐耗尽。目前的QPO频率演化研究表明，X射线通量和质量吸积率的变化可能不是同一个因子，因此简单的KFM和BFM不能解释该源的QPO演化。这是唯一的X射线脉冲星显示持久的QPO，也是第一个吸积X射线脉冲星，其中QPO的历史是报告了很长的时间尺度与长期演化的吸积盘。

英文摘要：

We report here a study of the long term properties of Quasi Periodic Oscillations (QPO) in an unusual accreting X-ray pulsar, 4U 1626--67. This is a unique accretion powered X-ray pulsar in which we have found the QPOs to be present during all sufficiently long X-ray observations with a wide range of X-ray observatories. In the present spin-down era of this source, the QPO central frequency is found to be decreasing. In the earlier spin-up era of this source, there are only two reports of QPO detections, in 1983 with EXOSAT and 1988 with GINGA with an increasing trend. The QPO frequency evolution in 4U 1626--67 during the last 22 years changed from a positive to a negative trend, somewhat coincident with the torque reversal in this source. In the accretion powered X-ray pulsars, the QPO frequency is directly related to the inner radius of the accretion disk, as per Keplerian Frequency Model (KFM) and Beat Frequency Model (BFM). A gradual depletion of accretion disk is reported earlier from the X-ray spectral, flux and pulse profile measurements. The present QPO frequency evolution study shows that X-ray flux and mass accretion rate may not change by the same factor, hence the simple KFM and BFM are not able to explain the QPO evolution in this source. This is the only X-ray pulsar to show persistent QPOs and is also the first accreting X-ray pulsar in which the QPO history is reported for a long time scale relating it with the long term evolution of the accretion disk.

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