Cep X-4中回旋线能量和光谱硬度随亮度的变化
Luminosity-dependent changes of the cyclotron line energy and spectral hardness in Cep X-4
摘要：
吸积双星的X射线光谱通常被观测到随其X射线光度而变化。特别是，硬度的X射线连续被发现取决于光度。在一些源中，回旋共振散射特征（CRSF）的能量和光度之间的相关性是明确的。不同类型（符号）的相关性被认为反映了不同的吸积模式。我们分析了NuSTAR在2014年爆发期间对瞬态吸积脉冲星Cep X-4的两次观测。我们的分析集中在CRSF能量和X射线光度的光谱硬度的依赖关系的详细调查，特别是在短时间尺度上。为了研究两次观测中光谱变化随光度的变化，我们使用了光源在单个脉冲周期（数十秒）时间尺度上的固有变化，即所谓的脉冲间变化。我们发现Cep X-4的NuSTAR光谱包含两个CRSF：在~30 keV的基本线和在~55 keV的谐波。我们第一次发现，基本CRSF的能量增加，连续变得更难与增加X射线光度不仅在两个观测之间，也就是说，在长时间尺度上，而且在一个单独的观察，在几十秒的时间尺度上。我们调查这些依赖关系的详细情况，包括其非线性。我们讨论了一个可能的物理解释所观察到的行为的框架内的一个简单的一维模型的极性发射区与碰撞冲击形成的下落等离子体附近的中子星表面。利用这个模型，我们能够再现所观察到的连续硬度比和CRSF能量随光度的变化。
英文摘要：
X-ray spectra of accreting pulsars are generally observed to vary with their X-ray luminosity. In particular, the hardness of the X-ray continuum is found to depend on luminosity. In a few sources, the correlation between the energy of the cyclotron resonance scattering feature (CRSF) and the luminosity is clear. Different types (signs) of the correlation are believed to reflect different accretion modes. We analyse two NuSTAR observations of the transient accreting pulsar Cep X-4 during its 2014 outburst. Our analysis is focused on a detailed investigation of the dependence of the CRSF energy and of the spectral hardness on X-ray luminosity, especially on short timescales. To investigate the spectral changes as a function of luminosity within each of the two observations, we used the intrinsic variability of the source on the timescale of individual pulse cycles (tens of seconds), the so-called pulse-to-pulse variability. We find that the NuSTAR spectrum of Cep X-4 contains two CRSFs: the fundamental line at ~30 keV and its harmonic at ~55 keV. We find for the first time that the energy of the fundamental CRSF increases and the continuum becomes harder with increasing X-ray luminosity not only between the two observations, that is, on the long timescale, but also within an individual observation, on the timescale of a few tens of seconds. We investigate these dependencies in detail including their non-linearity. We discuss a possible physical interpretation of the observed behaviour in the frame of a simple one-dimensional model of the polar emitting region with a collisionless shock formed in the infalling plasma near the neutron star surface. With this model, we are able to reproduce the observed variations of the continuum hardness ratio and of the CRSF energy with luminosity.
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