磁层边界吸积盘强非周期变化的猝灭
Quenching of the accretion disk strong aperiodic variability at the magnetospheric boundary
摘要：
研究了吸积流被磁层截断的双星系统中X射线通量变化的功率密度谱。吸积X射线天体的PDS在中子星接近与磁层边界处的吸积盘共转时，在中子星自旋频率处有明显的断裂/截止。这种中断自然可以用“扰动传播”模型来解释，该模型假定在吸积盘中的任何给定半径处，随机扰动都以该半径的频率特性引入到流中。这些扰动然后被平流到主要能量释放区域，导致X射线通量P~f^{-1. 1.5}。PDS中的断裂是在吸积盘截断半径（磁层半径）的频率特征下从盘向磁层流过渡的自然表现。PDS断裂频率与共转吸积盘中自旋频率的接近强烈地表明，吸积盘中典型的变率时间尺度接近开普勒变率时间尺度。在瞬时吸积X射线暴中，其质量吸积率变化较大，PDS突变频率随X射线通量变化，反映了磁层大小随吸积率的变化。在突变频率以上，PDS变陡到~f^{-2}定律，该定律在很宽的频率范围内成立。这些结果表明，强f^{-1... 1.5}在吸积盘中普遍存在的非周期性变化并不是磁层流动的特征。
英文摘要：
We study power density spectra (PDS) of X-ray flux variability in binary systems where the accretion flow is truncated by the magnetosphere. PDS of accreting X-ray pulsars where the neutron star is close to the corotation with the accretion disk at the magnetospheric boundary, have a distinct break/cutoff at the neutron star spin frequency. This break can naturally be explained in the "perturbation propagation" model, which assumes that at any given radius in the accretion disk stochastic perturbations are introduced to the flow with frequencies characteristic for this radius. These perturbations are then advected to the region of main energy release leading to a self-similar variability of X-ray flux P~f^{-1...-1.5}. The break in the PDS is then a natural manifestation of the transition from the disk to magnetospheric flow at the frequency characteristic for the accretion disk truncation radius (magnetospheric radius). The proximity of the PDS break frequency to the spin frequency in corotating pulsars strongly suggests that the typical variability time scale in accretion disks is close to the Keplerian one. In transient accreting X-ray pulsars characterized by large variations of the mass accretion rate during outbursts, the PDS break frequency follows the variations of the X-ray flux, reflecting the change of the magnetosphere size with the accretion rate. Above the break frequency the PDS steepens to ~f^{-2} law which holds over a broad frequency range. These results suggest that strong f^{-1...-1.5} aperiodic variability which is ubiquitous in accretion disks is not characteristic for magnetospheric flows.
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