Quasi-periodic oscillations in accreting magnetic white dwarfs ? I. Observational constraints in X-ray and optical
Quasi-periodic oscillations in accreting magnetic white dwarfs ? I. Observational constraints in X-ray and optical
复制标题
吸积磁性白矮星的准周期振荡？
DOI：
10.1051/0004-6361/201425482
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发表时间：
2015
影响因子：
6.5
通讯作者：
CEA
中科院分区：
文献类型：
--
作者：
J. Bonnet;M. Mouchet;C. Busschaert;E. Falize;C. Aim;CEADSMIrfu;LUTHOdPCNRSU Paris;CEA
在S周期为1-3的某些极子的光通量中观察到了准周期振荡(QPO)，但在释放了相当大一部分吸积能量的X射线中还没有观测到准周期振荡。从激波振荡中可以预料到和预测到QPO。大多数极地是由XMM-牛顿卫星观测到的。我们利用XMM-牛顿对极点的均匀观测，在(0.5-10keV)能量范围内寻找QPO的存在，并为最亮的X射线极点设定显著的上限。我们利用EPIC-PN相机的0.07 S分辨率提取了高时间分辨率的X射线光曲线。在1998年至2012年用XMM-牛顿法观测到的65个极点中，根据其在PN仪器中的计数速率选择了24个源的样本，以确保显著的限值。我们使用快速傅立叶变换(FFT)方法搜索QPO，并使用统计工具定义探测极限。在接受调查的样本中，没有一个样本的QPO达到显著水平。QPO中分数通量的上限在7%到71%之间。这些负的结果与文献II中提出的基于2D流体动力学程序的数值模拟的详细理论预测进行了比较。吸积柱中的冷却不稳定性预计将产生激波准振荡，其最大幅度在韧致辐射(0.5-10keV)X射线发射中达到40%，在光学回旋发射中达到20%。没有X射线QPO时，比吸积速率的上限为(5-10)g cm2 S 1，但这一条件与解释观测到的光学QPO的幅度和频率所需的值不一致。这一矛盾勾勒出激波不稳定模型可能存在的缺陷。
Quasi-periodic oscillations (QPOs) are observed in the optical flux of some polars with typical periods of 1 to 3 s but none have been observed yet in X-rays where a significant part of the accreting energy is released. QPOs are expected and predicted from shock oscillations. Most of the polars have been observed by the XMM-Newton satellite. We made use of the homogeneous set of observations of the polars by XMM-Newton to search for the presence of QPOs in the (0.5‐10 keV) energy range and to set significant upper limits for the brightest X-ray polars. We extracted high time-resolution X-ray light curves by taking advantage of the 0.07 s resolution of the EPIC-PN camera. Among the 65 polars observed with XMM-Newton from 1998 to 2012, a sample of 24 sources was selected on the basis of their counting rate in the PN instrument to secure significant limits. We searched for QPOs using Fast Fourier Transform (FFT) methods and defined limits of detection using statistical tools. Among the sample surveyed, none shows QPOs at a significant level. Upper limits to the fractional flux in QPOs range from 7% to 71%. These negative results are compared to the detailed theoretical predictions of numerical simulations based on a 2D hydrodynamical code presented in Paper II. Cooling instabilities in the accretion column are expected to produce shock quasi-oscillations with a maximum amplitude reaching 40% in the bremsstrahlung (0.5‐10 keV) X-ray emission and 20% in the optical cyclotron emission. The absence of X-ray QPOs imposes an upper limit of (5‐10) g cm 2 s 1 on the specific accretion rate but this condition is found inconsistent with the value required to account for the amplitudes and frequencies of the observed optical QPOs. This contradiction outlines probable shortcomings with the shock instability model.